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Contents

1. lntroduction............................................................... 3

Early development, photoemitter and secondary-emitter development, applications development,

photomultiplier

and

solid-state detectors compared

2. Photomultiplier Design . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Photoemission, practical photocathode materials, opaque and semitransparent photocathodes, glass

transmission and spectral response, thermionic emission, secondary emission, time tag in photoemission and

secondary emission

3. Electron Optics of Photomultipliers , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Electron-optical design considerations, design methods for photomultiplier electron optics, specific

photomultiplier electron-optical configurations, anode configurations

4. Photomultiplier Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . 36

Photocathode-related characteristics, gain-related characteristics, dark current and noise, time effects, pulse

counting, scintillation counting, liquid scintillation counting, environmental effects

5. Photomultiplier Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Summary of selection criteria, applied voltage considerations, mechanical considerations, optical

considera-

tions, specific photomultiplier applications

Appendix A. Typical Photomultiplier Applications and Selection Guide . . . . . . . . . . . 119

Appendix B.Glossary of Terms Related to Photomultiplier lubes and Their

Applica-

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

125

Appendix C.

Spectral Response Designation Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

132

Appendix D. Photometric Units and Photometric-to-Radiant Conversion

.............

Appendix E.

Spectral Response and Source-Detector Matching

......... ..........I 43

Appendix F.

Radiant Energy and Sources

......................................

148

Appendix G. Statistical Theory of Noise in Photomultiplier Tubes. . . . . . . . . . . . . . .

Index........................................................................ 177

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TP-136

Photomultiplier Handbook

Introduction

The photomultiplier is a very versatile and

sensitive detector of radiant energy in the

ultraviolet, visible, and near infrared regions

of the electromagnetic spectrum. A schemat-

ic diagram of a typical photomultiplier tube

is given in Fig. 1. The basic radiation sensor

is the

photocathode

which is located inside a

vacuum envelope. Photoelectrons are emit-

ted and directed by an appropriate electric

field to an electrode or dynode within the

envelope. A number of secondary electrons

are emitted at this dynode for each imping-

ing primary photoelectron. These secondary

electrons in turn are directed to a second

dynode and so on until a final gain of

perhaps 106 is achieved. The electrons from

the last dynode are collected by an anode

which provides the signal current that is read

out.

PHOTOELECTRONS

92cs-32288

Fig.

Schematic representation of a photo-

multiplier tube and its operation

For a large number of applications, the

photomultiplier is the most practical or sen-

sitive detector available. The basic reason

for the superiority of the photomultiplier is

the secondary-emission amplification that

makes it possible for the tube to approach

“ideal” device performance limited only by

the statistics of photoemission. Amplifica-

tions ranging from 103 to as much as 108

provide output signal levels that are com-

patible with auxiliary electronic equipment

without need for additional signal amplifica-

tion. Extremely fast time response with rise

times as short as a fraction of a nanosecond

provides a measurement capability in special

applications that is unmatched by other

radiation detectors.

EARLY DEVELOPMENT

The development (history) of the photo-

multiplier is rooted in early studies of secon-

dary emission. In 1902, Austin and Starke1

reported that the metal surfaces impacted by

cathode rays emitted a larger number of elec-

trons than were incident. The use of secon-

dary emission as a means for signal

amplification was proposed as early as

1919.2 In 1935, Iams and Salzberg 3 of RCA

reported on a single-stage photomultiplier.

The device consisted of a semicylindrical

photocathode, a secondary emitter mounted

on the axis, and a collector grid surrounding

the secondary emitter. The tube had a gain

of about eight. Because of its better frequen-

cy response the single-stage photomultiplier

was intended for replacement of the

gas-

filled phototube as a sound pickup for

movies. But despite its advantages, it saw

only a brief developmental sales activity

before it became obsolete.

Multistage Devices

In 1936, Zworykin, Morton, and Malter,

all of

RCA4

reported on a multistage

photomultiplier. Again, the principal con-

templated application was sound-on-film

pickup. Their tube used a combination of

electrostatic and magnetic fields to direct

electrons from stage to stage. A photograph

of a developmental sample is given in Fig. 2.

Although the magnetic-type photomultiplier

provided high gain, it had several dif-

ficulties. The adjustment of the magnetic

field was very critical, and to change the gain

by reducing the applied voltage, the

magnetic field also had to be adjusted.

Photomultiplier Handbook

Another problem was that its rather wide

open structure resulted in high dark current

because of feedback from ions and light

developed near the output end of the device.

For these reasons, and because of the

development of electrostatically focused

photomultipliers, commercialization did not

follow.

Fig. 2

Magnetic-type multistage

photomul-

tiplier reported by Zworykin, Morton, and

Malter

in 1936.

The design of multistage electrostatically

focused photomultipliers required an

analysis of the equipotential surfaces be-

tween electrodes and of the electron trajec-

tories. Before the days of high-speed com-

puters, this problem was solved by a

mechanical analogue: a stretched rubber

membrane. By placing mechanical models of

the electrodes under the membrane, the

height of the membrane was controlled and

corresponded to the electrical potential of

the electrode. Small balls were then allowed

to roll from one electrode to the next. The

trajectories of the balls were shown to cor-

respond to those of the electrons in the cor-

responding electrostatic fields. Working

with the rubber-dam analogue, both J.R.

Pierce5

of Bell Laboratories and J.A.

Rajchman6

of RCA devices linear arrays of

electrodes that provided good focusing prop-

erties. Although commerical designs did not

result immediately from the linear dynode

array, The Rajchmann design with some

modifications eventually was, and still is,

used in photomultipliers-particularly for

high-gain wide-bandwidth requirements.

First Commercial Devices

The first commercially successful

photomultiplier was the type 931. This tube

had a compact circular array of nine

dynodes using electrostatic focusing. The

first such arrangement was described by

Zworykin and

Rajchman.7

Modifications

were later reported by Rajchmann and

Snyder8 and by Janes and

Gloverg,

all of

RCA. The basic electron-optics of the cir-

cular cage was thus well determined by 1941

and has not changed to the present time

although improvements have been made in

processing, construction, and performance

of the 931A product.

The success of the 931 type also resulted

from the development of a much improved

photocathode, Cs3Sb, reported by Gorlich 10

in 1936. The first experimental photo-

multipliers had used a Ag-O-Cs photocath-

ode having a typical peak quantum efficien-

cy of 0.4% at 800 nm. (The Ag-O-Cs layer

was also used for the dynodes.) The new

Cs3Sb

photocathode had a quantum effi-

ciency of 12% (higher today) at 400 nm. It

was used in the first 931’s, both as a

photocathode and as a secondary-emitting

material for the dynodes.

PHOTOEMITTER

AND

SECONDARY-

EMITTER DEVELOPMENT

Photocathode Materials

Much of the development work on

photomultiplier tubes has been concerned

with their physical configuration and the

related electron optics. But a very important

part of the development of photomultiplier

tubes was related to the photocathode and

secondary-emission surfaces and their pro-

cessing. RCA was very fortunate during the

1950’s and 60’s in having on its staff, prob-

ably the world’s foremost photocathode ex-

pert, Dr. A.H. Sommer. His treatise on

Photoemissive

Materials1

continues to pro-

vide a wealth of information to all

photocathode process engineers.

Sommer explored the properties of

numerous photocathode materials-par-

ticularly alkali-antimonides. Perhaps his

most noteworthy contribution was the

multialkali photocathode (S-20 spectral

response). This photocathode,

Na2KSb:Cs,

is important because of its high sensitivity in

the red and near infrared; the earlier Cs3Sb

photocathode spectral response barely ex-

tends through the visible, although it is very

sensitive in the blue where most scintillators

emit.

Bialkali photocathodes were also de-

veloped by Sommer and have proven to be

better in some applications than the

Cs3Sb

photocathode. Thus, the

Na2KSb

photo-

cathode has been found to be stable at higher

temperatures than

Cs3Sb

and, in addition,

has a very low dark (thermal) emission. It

has been particularly useful in

oil-well-

logging applications. Another bialkali

pho-

tocathode,

K2CsSb,

is more sensitive than

Cs3Sb

in the blue and is, therefore, used by

RCA to provide a better match to the

NaI:Tl crystals used in scintillation count-

ing.

Dynode Materials

The first secondary-emission material

used practically by RCA was the Ag-O-Cs

surface. But with the development of the

Cs3Sb

material for photocathodes, it was

found that this material was also an excellent

secondary emitter. Other practical secondary

emitters developed during the early years of

photomultiplier development were MgO:Cs

(often referred to as “silver-magnesium”)

and BeO:Cs (“copper-beryllium”).

In the early

1960’s,

R.E.

Simon12

while

working at the RCA Laboratories developed

his revolutionary concept of Negative Elec-

tron Affinity

(NEA).

Electron affinity is the

energy required for an electron at the

conduction-band level to escape to the

vacuum level. By suitably treating the sur-

face of a p-type semiconductor material, the

band levels at the surface can be bent

downward so that the effective electron af-

finity is actually negative. Thermalized elec-

trons in the conduction band are normally

repelled by the electron-affinity barrier; the

advantage of the NEA materials is that these

electrons can now escape into the vacuum as

they approach the surface. In the case of

secondary emission, secondary electrons can

be created at greater depths in the material

and still escape, thus providing a much

greater secondary-emission yield. In the case

of photoemission, it has been possible to

achieve extended-red and infrared sen-

sitivities greater than those obtainable with

any other known materials. The first prac-

tical application of the NEA concept was to

secondary emission.

Simon and Williams

An early paper by

described the theory

and early experimental results of

secondary-

emission yields as high as 130 at 2.5

for

GaP:Cs.

Introduction

APPLICATIONS DEVELOPMENT

Astronomy and Spectroscopy

Early applications of the photomultiplier

were in astronomy and spectroscopy.

Because the effective quantum efficiency of

the photomultiplier was at least ten times

that of photographic film, astronomers were

quick to realize the photomultiplier tube’s

advantage. Furthermore, because the output

current of the photomultiplier is linear with

incident radiation power, the tube could be

used directly in photometric and

spec-

trophotometric astronomy. The type 1P28, a

tube similar to the 931 but having an

ultraviolet-transmitting envelope was par-

ticularly useful in spectroscopy. The size and

shape of the photocathode were suitable for

the detection and measurement of line spec-

tra and the very wide range of available gain

proved very useful. 14

Radar Jammer

A totally unexpected application for the

new photomultiplier tube occurred during

World War II. The development of radar for

detecting and tracking aircraft led to the

simultaneous need for wideband

electronic-

noise sources as radar jammers. Although

other sources of noise were tried, the

photomultiplier proved to be most suc-

cessful. The advantage of the tube was its

high gain

(107)

and wide band width (several

hundred MHz). As a noise source the tube

was operated with a non-modulated input

light source and with high gain. The output

amplifier photoelectric shot noise was

“white” and thus indistinguishable from

natural noise sources. This application of

photomultiplier tubes resulted in production

of thousands per month compared with

previous production measured in only hun-

dreds per year.

Scintillation Counting

A proliferation of photomultiplier designs

followed the invention of the scintillation

counter shortly after World War II.15,16 The

photomultiplier tubes were designed with

semitransparent photocathodes deposited on

an end window which could be coupled

directly to the scintillator. The principal

scintillator used, NaI doped with thallium,

was discovered by

Hofstadter17.

Much of

the development work on photomultiplier

Photomultiplier Handbook

tubes during this period was reported by

RCA and its competitors in the biannual

meetings of the Scintillation Counter Sym-

posium.

These symposia were reported fully

in the IRE (and later the IEEE)

Transactions

on Nuclear Science beginning with the

meeting in Washington, January 1948. The

scintillation counter became the most impor-

tant measurement instrument in nuclear

physics, nuclear medicine, and radioactive

tracer applications of a wide variety.

Headlight Dimmer

During the

1950’s,

RCA collaborated with

the General Motors Company (Guide-Lamp

Division) on a successful headlight dimmer.

The photoelectric headlight dimmer-first

made available only on Cadillacs and

Oldsmobiles-basically used a tube similar

to the

931A,

but redesigned and tested to the

auto manufacturer’s particular require-

ments. The optical engineering problem was

to sense the oncoming headlights or tail-

lights being followed without responding to

street and house lights. Vertical and horizon-

tal angular sensitivity was designed to match

the spread of the high beams of the automo-

bile. A red filter was installed in the optical

path to provide a better balance between sen-

sitivity to oncoming headlights and to

tail-

lamps being followed. The device achieved a

remarkable success, probably because of the

novelty, and thousands of photomultiplier

tubes were used. But today, one rarely sees a

headlight dimmer.

Medical Diagnostic Equipment

In recent years two medical applications

have used large numbers of photomultiplier

tubes and have spurred further develop-

ments and improvements. The gamma

cam-

era18

is a sophisticated version of the scin-

tillation counter used medically for locating

tumors or other biological abnormalities. A

radioactive isotope combined in a suitable

compound is injected into the blood stream

or ingested orally by the patient. The

radioactive material disintegrates and gam-

ma rays are ejected from preferential loca-

tions such as tumors or specific organs. A

large crystal intercepts the gamma rays and

scintillates. Behind the crystal are photo-

multiplier tubes, perhaps 19, in hexagonal

array. The location of the point of scintilla-

tion origin is obtained by an algorithm

depending upon the individual signals from

each of the photomultipliers. Counting is

continued until several hundred thousand

counts are obtained and the organ in ques-

tion is satisfactorily delineated. The location

of each scintillation is represented by a point

on a cathode-ray-tube presentation.

The Computerized Axial Tomographic

(CAT) scanner was introduced to this coun-

try in 1973. The device uses a pencil or

fan-

beam of X-rays which rotates around the pa-

tient providing X-ray transmission data

from many directions. A scintillator coupled

to a photomultiplier detects the transmitted

beam-as an average photomultiplier cur-

rent-and a computer stores and computes

the cross-section density variation of the pa-

tient’s torso or skull. The photomultipliers

are1/ or 3/4-inch end-on tubes which

couple to the scintillator, commonly BGO

(bismuth germanate). Each unit is equipped

with as many as 600 photomultipliers.

PHOTOMULTIPLIERS AND

SOLID-

STATE DETECTORS COMPARED

In some applications either a photomulti-

plier or solid-state detector could be used.

The user may make his choice on the basis of

factors such as cost, size, or previous ex-

perience. In other applications, the choice

may be dictated by fundamental properties

of the photomultiplier or the solid-state

detector. A discussion follows of some of the

common applications favoring one or the

other detector with reasons for the choice. A

summary presents the principal considera-

tions the user must apply in making a choice

in an application for which he requires a

photodetector. This information should be

particularly useful to the designer who is not

well acquainted in this field.

Photomultiplier Features

The photomultiplier is unique in its ability

to interface with a scintillation crystal and

not only count the scintillations but measure

their magnitude and time their arrival. Most

scintillators emit in the blue and near ultra-

violet. This spectral output obviously favors

the photomultiplier having a photocathode

with high quantum efficiency in the short

wavelength range. On the other hand a sili-

con p-i-n diode is relatively poor in this part

of the spectrum but does best in the red and

near infrared. The most important factor,

probably, is the gain of the photomultiplier

which permits the measurement of the very

small signals from individual scintillations

with a good signal-to-noise ratio, limited

primarily by the statistics of the number of

photoelectrons per pulse. Finally, the short

rise time of the photomultiplier using fast

scintillators permits time-of-flight measure-

ments to be made in nuclear physics.

Although the CAT scanner equipment

also uses photomultipier tubes to detect the

scintillations in bismuth germanate (BGO)

crystals, the situation is somewhat different

from the scintillation counting applications

discussed above. In the CAT scanner the

X-rays produce a broad band of pulse

heights and no attempt is made to single out

and detect single scintillation events. The

photomultiplier is used in an analog mode to

detect the level of radiation incident on the

crystal. In the CAT scan operation the

typical machine scans the patient in a few

seconds and the level of irradiance from the

crystal onto the photomultiplier is relatively

high so that only a relatively low gain photo-

multiplier is required. Furthermore, the

speed of response requirement for the

photomultiplier is relatively modest-per-

haps a few hundred microseconds. Still, the

principal advantage of using a photomulti-

plier in this application for the detection of

the radiant signal is its good signal-to-noise

ratio. This ratio is very important to the pa-

tient because a reduction in its

signal-to-

noise ratio would have to be made up for

with an increased X-ray dose. Nevertheless,

there is interest and development activity

aimed at replacing the photomultiplier with

silicon p-i-n detectors. Two factors could

favor the alternate use of a silicon cell: (1) a

better scintillator (BGO is almost an order of

magnitude less sensitive than NaI:Tl; (2) a

faster scanning machine (a very desirable

technological advance because is would

minimize effects of body motions). Both of

these factors would result in a larger

photocurrent and could bring the signal level

for the silicon detector to the point where the

fundamental signal-to-noise ratio from the

X-ray source would not be degraded. Such

developments may be anticipated because

the silicon detector would also have the ad-

vantage of smaller size and perhaps lower

cost.

Introduction

As a result of increasing concern about en-

vironment, pollution monitoring is becom-

ing another important application for photo-

multiplier tubes. For example, in the moni-

toring of NOx the gas sample is mixed with

O3 in a reaction chamber. A

chemilum-

inescence results which is measured using a

near-infrared-pass filter and a photomulti-

plier having an S-20 spectral response. Al-

though the radiation level is very low, NO

can be detected down to a level of 0.1 ppm.

The advantage of the photomultiplier in this

application is again the high gain and good

signal-to-noise ratio (the photomultiplier is

cooled to 0°C to reduce dark-current noise)

even though the radiation spectrum is ob-

served near the threshold of the S-20 spectral

range.

In another pollution-monitoring applica-

tion,

SO2

is detected down to a level of 0.002

ppm. Here, the sample containing SO2 is ir-

radiated with ultraviolet and the excited SO2

molecules fluoresce with blue radiation that

is detected with a combination of a narrow-

band filter and photomultiplier. Very weak

signals are detected and again it is the high

gain, good signal-to-noise ratio and, in addi-

tion, good blue sensitivity which makes the

detection and measurement of small contam-

inations of SO

possible.

Spectroscopy is one of the very early ap-

plications for photomultipliers. The wide

range of radiation levels encountered is

readily handled by the approximately loga-

rithmic gain variation of the photomultiplier

with voltage. At very low signal levels, the

signal-to-noise capability of the photomulti-

plier is essential. Because photomultiplier

spectral response (with quartz or

ultraviolet-

transmitting-glass windows) covers the range

from ultraviolet to near infrared, the

photomultiplier is the logical choice for spec-

troscopic applications, except in the infrared

region of the spectrum.

Photocell* Features

Because of their small size and low cost,

CdSe and CdS type photocells are the logical

selection for applications such as automatic

exposure control in photographic cameras or

various inspection and counting require-

ments.

*“Photocell” is used here to indicate a photosensitive

device in which the charge transport takes place through

a solid as compared with “phototube” in which the

charge transport is through a vacuum.

Photomultiplier Handbook

Many p-i-n silicon cells are used in com-

bination with lasers or LED’s (light emitting

diodes). Here, one of the principal advantages

of the silicon cell is its good response in the

near infrared out to 1100 nm. In combina-

tion with the Nd:YAG laser emitting at 1060

nm, the silicon cell is used widely in laser

ranging and laser tracking. A similar ap-

plication utilizes an LED emitting near 900

nm with a silicon cell for automatic ranging

for special camera equipment. Size and in-

frared sensitivity are again the important

qualifications.

A rapidly growing application for photo-

cells is for fiber-optic communication

systems. LED’s are coupled to the fibers and

the detector may be a p-i-n diode or, for a

better signal-to-noise ratio, a silicon ava-

lanche diode. The qualifying attributes for

the choice of detector are size, near infrared

sensitivity, adequate speed of response, and

good signal-to-noise ratio.

Smoke detectors now use large numbers of

LED’s and p-i-n silicon cells. Again size,

cost, and infrared sensitivity are the impor-

tant qualifications.

Characteristics Comparison Summary

Spectral Response. Photomultipliers can

be obtained with good spectral sensitivity in

the range 200 to 900 nm. Silicon cells have

rather poor blue sensitivity, but are excellent

out to 1100 nm. In general, then, the photo-

multiplier is to be preferred for applications

involving the shorter wavelengths, although

other factors may override this considera-

tion.

Speed of Response.

If very fast response is

required, the photomultiplier is usually the

best choice of a detector. Photomultipliers

are available with rise times (10 to 90%) of 1

or 2 nanoseconds using a

50-ohm

load. The

inherent rise time of silicon cells may be in

the range 10 to 20 nanoseconds, depending

upon the area of the cell. However, because

of the cell’s capacitance, the effective rise

time is much longer depending upon the

choice of load resistance. For example, with

1-megohm

load resistance, the rise time

may be of the order of 20 microseconds. A

fairly large load resistance must be chosen to

maintain good signal-to-noise characteristics

for the silicon cell. Silicon avalanche photo-

diodes can have rise times as short as 2

nanoseconds. Gain for an avalanche photo-

diode can be of the order of 100, but the sen-

sitive area is small-about 0.5 square

millimeter.

Sensitive Area. Photomultiplier tubes are

made in a variety of sizes so that many dif-

ferent optical configurations can be accom-

modated. The largest photocathode area

available in commercial RCA photomulti-

plier tubes has a nominal diameter of 5 in-

ches and a minimum useful area of 97 square

centimeters. By way of contrast, the

-inch

side-on photomultiplier has a projected

pho-

tocathode area of 0.14 square centimeter.

Silicon p-i-n diodes are available with sen-

sitive areas generally not larger than 1 square

centimeter; and avalanche silicon cells, 0.005

square centimeter. In many applications, a

fairly large area is required, e.g., coupling to

a cathode-ray tube or a large scintillator.

This requirement generally indicates the use

of a photomultiplier tube. Silicon cells are at

an advantage when the source is small for

direct coupling or for lens imaging.

Temperature.

Photomultipliers are gener-

ally not rated for operation at temperatures

higher than 75° C. Exceptions are

photomul-

tipliers having a Na

KSb photocathode. This

bi-alkali photocathode can tolerate temper-

atures up to 150° C or even higher for short

cycles. In oil-well logging measurements this

consideration is important. Photocathode

sensitivities and gain change very little with

temperature, but dark current does increase

rapidly. Dark currents at room temperature

are of the order of 10

ampere at the pho-

tocathode and double about every 10° C.

Silicon cells are rated from

50 to 80° C.

Sensitivities are also relatively independent

of temperature. But dark current which may

be 10-7ampere at room temperature, also

tends to double about every 10° C.

Signal-to-Noise Ratio. At very low light

levels, the limitation to detection and

measurement is generally the signal-to-noise

ratio. One way of describing the limit to

detection is to state the Equivalent Noise In-

put

(ENI)

or the Noise Equivalent Power

(NEP).

The NEP is the power level into the

device which provides a signal just equal to

the noise. Most often the bandwidth is

specified as 1 hertz and the wavelength of the

measurement is at the peak of the spectral

responsivity. ENI is the same type of specifi-

cation except the unit instead of power may

be luminous flux.

For a photomultiplier such as one used for

spectroscopy, the NEP at room temperature

400

nm is about 7 x 10-16

watts, or the

EN1 is about 7 x 10

lumens. Both

specifications are for a l-hertz ban

dwidth.

For a p-i-n silicon photocell, the NEP at 900

nanometers may be of the order of 2 x

10- watts, or the EN1 of 1.5 x 10-

lumens. Both values are for a l-hertz band-

width. Thus, the photomultiplier is clearly

superior in this category. Also it should be

pointed out that the silicon diode must be

coupled into a load resistance of about 5

megohms in order to avoid noise domination

from the coupling resistor. Unfortunately,

this large resistance then increases the effec-

tive rise time of the silicon device to about

100 microseconds. The NEP of a silicon ava-

lanche photodiode is about 10-14 watt at

900 nanometers or the ENI is 8 x 10

lumens, both for a l-Hz bandwidth. The

lumen in these descriptions is that from a

tungsten source operating at 2856 K color

temperature.Peak emission for such a

source is near 1000 nm and thus closely

matches the spectral peak of the silicon

devices.

Gain. A photomultiplier can have a gain

factor, by which the fundamental photo-

cathode signal is multiplied, of from 103 to

108. Silicon avalanche photodiodes have a

gain of about 100. Silicon p-i-n diodes have

no gain. The high gain of the photomulti-

plier frequently eliminates the need of

special amplifiers, and its range of gain con-

trolled by the applied voltage provides flex-

ibility in operation.

Stability. Photomultiplier tubes are not

noted for great stability although for low

anode currents and careful operation they

are satisfactory. When the light level is

reasonably high, however, the very good

stability of the silicon p-i-n cell is a con-

siderable advantage. The silicon cell makes a

particularly good reference device for this

reason. In fact, the National Bureau of Stan-

dards has been conducting special calibra-

tion transfer studies using p-i-n silicon

diodes.

REFERENCES

1. H. Bruining, Physics and applications

of secondary electron emission,

(McGraw-

Hill Book Co., Inc.; 1954).

Introduction

2. J. Slepian, U.S. Patent 1, 450, 265,

April 3, 1923 (Filed 1919).

3. H. E. Iams and B. Salzberg, “The

secondary emission phototube,”

Proc.

IRE,

Vol. 23, pp. 55-64 (1935).

V.K. Zworykin, G.A. Morton, and L.

Malter,

"The secondary-emission multipli

er-a new electronic device,” Proc. IRE,

Vol. 24, pp. 351-375 (1936).

5. J.R. Pierce, “Electron-multiplier

design,”Bell Lab. Record, Vol. 16, pp.

305-309 (1938).

6. J.A. Rajchman,“Le courant residue1

dans les multiplicateurs d’electrons

elec-

trostatiques,”These L’Ecole Polytechnique

Federale (Zurich, 1938).

7. V.K. Zworykin and J.A. Rajchman,

“The electrostatic electron multiplier,

Proc.

IRE,

Vol. 27, pp. 558-566 (1939).

J.A. Rajchman and R.L. Snyder, “An

electrostatically focused multiplier

phototube,”Electronics, Vol. 13, p. 20

(1940).

R.B. Janes, and A.M. Glover, “Recent

developments in phototubes,”

RCA Review,

Vol. 6, pp. 43-54 (1941). Also, A.M. Glover,

“A review of the development of sensitive

phototubes,”

Proc.

IRE, Vol. 29, pp.

413-423 (1941).

10. P. Gorlich, “Uber zusammengesetzte,

durchsichtige Photokathoden,” 2. Physik,

Vol. 101, p. 335 (1936).

11.

A.H. Sommer, Photoemissive

materials,

John Wiley and Sons; 1968.

12. R.E. Simon, Research in electron

emission from semiconductors, Quarterly

Report, Contract DA 36-039-AMC-02221

(E) (1963).

13. R.E. Simon and B.F. Williams,

“Secondary-electron emission,” IEEE

Trans.

Nucl.

Sci., Vol. NS-15, pp. 166-170

(1968).

14. M.H. Sweet,“Logarithmic photomul-

tiplier tube photometer,” JOSA, Vol. 37, p.

432 (1947).

15. H. Kallmann, Natur u

Technik

(July

1947).

16.

J.W. Coltman and F.H. Marshall, “A

photomultiplier radiation detector,”

Phys.

Rev.,

Vol. 72, p. 582 (1947).

17. R. Hofstadter,

“Alkali halide scintilla-

tion counters,”Phys. Rev., Vol. 74, p. 100

(1948).

18. H.O. Anger,“Scintillation camera”,

Rev. Sci. Instr.,

Vol. 29, pp. 27-33 (1958).

Photomultiplier

Design

PHOTOEMISSION

The earliest observation of a photoelectric

effect was made by Becquerel in 1839. He

found that when one of a pair of electrodes

in an electrolyte was illuminated, a voltage

or current resulted. During the latter part of

the 19th century, the observation of a

photovoltaic effect in selenium led to the

development of selenium and cuprous oxide

photovoltaic cells.

The emission of electrons resulting from

the action of light on a photoemissive sur-

face was a later development. Hertz dis-

covered the photoemission phenomenon in

1887, and in 1888 Hallwachs measured the

photocurrent from a zinc plate subjected to

ultraviolet radiation. In 1890, Elster and

Geitel produced a forerunner of the vacuum

phototube which consisted of an evacuated

glass bulb containing an alkali metal and an

auxiliary electrode used to collect the

negative electrical carriers (photoelectrons)

emitted by the action of light on the alkali

metal.

Basic Photoelectric Theory

The modern concept of photoelectricity

stems from Einstein’s pioneer work for

which he received the Nobel Prize. The

essence of Einstein’s work is the following

equation for determining the maximum

kinetic energy E of an emitted photoelec-

tron:

(1)

Eq. (1) shows that the maximum energy of

the emitted photoelectron

mv2/2

is propor-

tional to the energy of the light quanta

must be given to an electron to allow it to

escape the surface of a metal. For each

metal, the photoelectric effect is character-

ed in electron-volts.

In the energy diagram for a metal shown

in Fig. 3, the work function represents the

energy which must be given to an electron at

the top of the energy distribution to raise it

to the level of the potential barrier at the

metal-vacuum interface.

METAL

FERMI

ENERGY

92CS

32289

Fig. 3

Energy mode/ for a metal showing

the relationship of the work function and the

Fermi level.

According to the quantum theory, only

one electron can occupy a particular quan-

tum state of an atom. In a single atom, these

states are separated in distinct “shells”; nor-

mally only the lower energy states are filled.

In an agglomeration of atoms, these states

are modified by interaction with neighboring

atoms, particularly for the outermost elec-

trons of the atom. As a result, the outer

energy levels tend to overlap and produce a

continuous band of possible energy levels, as

shown in Fig. 3.

The diagram shown in Fig. 3 is for a

temperature of absolute zero; all lower

energy levels are filled. As the temperature is

increased, some of the electrons absorb ther-

mal energy which permits them to occupy

scattered states above the maximum level for

absolute zero. The energy distribution of

electrons in a particular metal is shown in

Fig. 4 for several different temperatures. At

absolute zero, all the lower states are oc-

cupied up to the Fermi level. At higher

temperatures, there is some excitation to up-

per levels. The electron density at a par-

ticular temperature is described by the

Fermi-Dirac energy-distribution function,

which indicates the probability of occupa-

tion f for a quantum state having energy E:

f = 1

(2)

When E is equal to E

, the value of f is 1/2.

It is customary to refer to the energy of level

Ef,

for which there is a

50-per-cent

prob-

ability of occupancy, as the Fermi level. At

absolute zero, the Fermi level corresponds to

the top of the filled energy distribution.

If the energy derived from the radiant

energy is just sufficient to eject an electron at

the Fermi level, the following relation exists:

(3)

radiation, is related to the long-wavelength

(4)

The relationship may be rewritten to relate

the long-wavelength limit to the work func-

tion, as follows:

(5)

Because some of the electrons occupy

states slightly higher than the Fermi level, as

shown in Fig. 4, excitation of these electrons

produces an extended response at the red

threshold of the spectral-response character-

istic. As a result, there is no abrupt red

threshold at normal temperatures, and the

true work function cannot be obtained in a

simple manner from the spectral-response

measurement. However, a universal func-

tion devised by Fowler can be used to predict

the shape of the spectral-response curve near

the threshold; the work function can then be

calculated from these data.

Photomultiplier Design

fig. 4

Energy distribution of conduction

electrons in potassium at temperatures of 0,

200, and 1033 degrees Kelvin based on

elementary Sommerfeld theory. (ref. 19)

Work Function and

Spectral

Response

Measurement of the work function and

spectral response for clean metal surfaces

has been of considerable importance in the

development of photoelectric theory.

Work functions for pure metals are in the

range 2 to 5 electron-volts. (See A.H.

Som-

mer, Ref. 11, Table 3.)

Fig. 5 shows spectral-response curves for

the alkali metals. The curves indicate a

regular progression of the wavelength for

maximum response with atomic number.

The most red-sensitive of these metals is

cesium, which is widely used in the activa-

tion of most commercial phototubes.

The energy distribution of emitted photo-

electrons has been measured for a number of

metals and photosurfaces. Typical results

are shown in Fig. 6 for a potassium film of

20 molecular layers on a base of

silver.20

The

maximum emission energy corresponds to

that predicted by the Einstein photoelectric

equation.

Quantum Efficiency

Because a quantum of radiation is neces-

sary to release an electron, the photoelectric

current is proportional to the intensity of the

radiation. This first law of photoelectricity

has been verified experimentally over a wide

range of light intensities. For most materials,

the quantum efficiency is very low; on the

best sensitized commercial photosurfaces,

the maximum yield reported is as high as one

electron for three light quanta.

Photomultiplier Handbook

WAVELENGTH-NANOMETERS

92CS-32291

Fig. 5

Spectral-response characteristics for the alkali metals showing

gression in the order of the periodic table. (ref. 19)

An ideal photocathode has a quantum ef-

ficiency of 100 per cent; i.e., every incident

photon releases one photoelectron from the

material into the vacuum. All practical

photoemitters have quantum efficiencies

below 100 per cent. To obtain a qualitative

understanding of the variations in quantum

efficiency for different materials and for dif-

ferent wavelengths or photon energies, it is

useful to consider photoemission as a pro-

cess involving three steps: (1) absorption of a

photon resulting in the transfer of energy

Fig. 6

Energy distribution of

photoelec-

trons from a potassium film. (ref.

20)

regular

pro-

from photon to electron, (2) motion of the

electron toward the material-vacuum inter-

face, and (3) escape of the electron over the

potential barrier at the surface into the

vacuum.

Energy losses occur in each of these steps.

In the first step, only the absorbed portion

of the incident light is effective and thus

losses by transmission and reflection reduce

the quantum efficiency. In the second step,

the photoelectrons may lose energy by col-

lision with other electrons (electron scat-

tering) or with the lattice (phonon

scattering). Finally, the potential barrier at

the surface prevents the escape of some elec-

trons.

Metallic and Semiconductor Materials

The energy losses described vary from

material to material, but a major difference

between metallic and semiconducting

materials makes separate consideration of

each of these two groups useful. In metals, a

large fraction of the incident visible light is

reflected and thus lost to the photoemission

process. Further losses occur as the photo-

electrons rapidly lose energy in collisions with

the large number of free electrons in the

metal through electron-electron scattering.

As a result, the escape depth, the distance

from the surface from which electrons can

reach the surface with sufficient energy to

overcome the surface barrier, is small, and is

typically a few nanometers. Finally, the

work function of most metals is greater than

three electron-volts, so that visible photons

which have energies less than three

electron-

volts are prevented from producing electron

emission. Only a few metals, particularly the

alkali ones, have work-function values low

enough to make them sensitive to visible

light. Because of the large energy losses in

absorption of the photon and in the motion

of the photoelectron toward the vacuum (the

first and second steps described above), even

the alkali metals exhibit very low quantum

efficiency in the visible region, usually below

0.1 per cent (one electron per 1000 incident

photons)l

As expected, higher quantum effi-

ciencies are obtained with higher photon

energies. For

12-electron-volt

photons,

quantum yields as high as 10 per cent have

been reported.

Fig. 7

Simplified semiconductor energy

band model.

The concept of the energy-band models

that describe semiconductor photoemitters is

illustrated in its simplest form in Fig. 7. Elec-

trons can have energy values only within well

defined energy bands which are separated by

forbidden-band gaps. At 0 K, the electrons

of highest energy are in the so-called valence

band and are separated from the empty con-

duction band by the bandgap energy

EG.

The probability that a given energy level may

be occupied by an electron is described by

Fermi-Dirac statistics and depends primarily

on the difference in energy between the level

under consideration and Fermi level. As a

first approximation, it may be said that any

energy levels which are below the Fermi level

will be filled with electrons, and any levels

which are above the Fermi level will be emp-

Photomultiplier Design

ty. At temperatures higher than 0 K, some

electrons in the valence band have sufficient

energy to be raised to the conduction band,

and these electrons, as well as the holes in the

valence band created by the loss of electrons,

produce electrical conductivity. Because the

number of electrons raised to the conduction

band increases with temperature, the con-

ductivity of semiconductors also increases

with temperature. Light can be absorbed by

valence-band electrons only if the energy of

the photon is at least equal to the band-gap

energy

EG.

If, as a result of light absorption,

electrons are raised from the valence band

into the conduction band, photoconductivity

is achieved. For photoemission, an electron

in the conduction band must have energy

greater than the electron affinity EA. The

additional energy

is needed to overcome

the forces that bind the electron to the solid,

or, in other words, to convert a “free” elec-

tron within the material into a free electron

in the vacuum. Thus, in terms of the model

of Fig. 7, radiant energy can convert an elec-

tron into an internal photoelectron

(photo-

conductivity) if the photon energy exceeds

and into an external photoelectron

(photoemission) if the photon energy ex-

ceeds (EG

+ EA

). As a result, photons with

total energies Ep

less than

(EG

EA)

cannot

produce photoemission.

The following statements can therefore be

made concerning photoemission in semicon-

ductors. First, light absorption is efficient if

the photon energy exceeds

EG.

Second,

energy loss by electron-electron scattering is

low because very few free electrons are pres-

ent; thus, energy loss by phonon scattering is

the predominant loss mechanism. The

escape depth in semiconductors is therefore

much greater than in metals, typically of the

order of tens of nanometers. Third, the

threshold wavelength, which is determined

by the work function in metals, is given by

the value of (EG + EA) in semiconductors.

Synthesis of materials with values of

EG + EA) below 2 electron-volts has

demonstrated that threshold wavelengths

longer than those of any metal can be ob-

tained in a semiconductor. Semiconductors,

therefore, are superior to metals in all three

steps of the photoemissive process: they ab-

sorb a much higher fraction of the incident

light, photoelectrons can escape from a

Photomultiplier Handbook

greater distance from the vacuum interface,

and the threshold wavelengths can be made

longer than those of a metal. Thus, it is not

surprising that all photoemitters of practical

importance are semiconducting materials.

Negative-Electron-Affinity Materials

20a

In recent years, remarkable improvements

in the photoemission from semiconductors

have been obtained through deliberate

modification of the energy-band structure.

The approach has been to reduce the elec-

tron affinity,

EA,

and thus to permit the

escape of electrons which have been excited

into the conduction band at greater depths

within the material. Indeed, if the electron

affinity is made less than zero (the vacuum

level lower than the bottom of the conduc-

tion band, a condition described as

“negative electron affinity” and illustrated

in Fig.

8),

the escape depth may be as much

92CS-32294

Fig. 8

Semiconductor energy-band model

showing

negative

electron affinity.

as 100 times greater than for the normal

material. The escape depth of a photoelec-

tron is limited by the energy loss suffered in

phonon scattering. Within a certain period

of time, of the order of 10-12

second, the

electron energy drops from a level above the

vacuum level to the bottom of the conduc-

tion band from which it is not able to escape

into the vacuum. On the other hand, the

electron can stay in the conduction band in

the order of 10-10 second without further

loss of energy, i.e.,without dropping into

the valence band. If the vacuum level is

below the bottom of the conduction band,

the electron will be in an energy state from

which it can escape into the vacuum for a

period of time that is approximately 100

times longer than if an energy above the bot-

tom of the conduction band is required for

escape, as in the materials represented by

Fig. 7. Therefore, a material conforming to

the conditions of Fig. 8 has greatly increased

escape depth. Under such circumstances, the

photosensitivity is significantly enhanced.

Substantial response is observed even for

photons with energies close to that of the

band gap where the absorption is weak. Effi-

cient photoemission in this case results only

because of the greater escape depth.

The reduction of the electron affinity is

accomplished through two steps. First, the

semiconductor is made strongly p-type by

the addition of the proper “doping” agent.

For example, if gallium arsenide is the host

material, zinc may be incorporated into the

crystal lattice to a concentration of perhaps

1,000 parts per million. The zinc produces

isolated energy states within the forbidden

gap, near the top of the valence band, which

are normally empty, but which will accept

electrons under the proper circumstances.

The p-doped material has its Fermi level just

above the top of the valence band. The se-

cond step is to apply to a semiconductor a

surface film of an electropositive material

such as cesium. Each cesium atom becomes

ionized through loss of an electron to a

p-type energy level near the surface of the

semiconductor, and is held to the surface by

electrostatic attraction.

The changes which result in the

energy-

band structure are two-fold. In the first

place, the acceptance of electrons by the

p-type impurity levels is accompanied by a

downward bending of the energy bands.

This bending can be understood by observ-

ing that a filled state must be, in general,

below the Fermi level; the whole structure

near the surface is bent downward to ac-

complish this result. In the second place, the

potential difference between the charged

electropositive layer (cesium) and the body

charge (filled zinc levels) results in a further

depression of the vacuum level as a result of

a dipole moment right at the surface.

Another way to describe the reduction of

the electron affinity is to consider the surface

of the semiconductor as a capacitor. The

charge on one side of the capacitor is

represented by the surface layer of cesium

ions; the other charge is represented by the

region of filled acceptor levels. The reduc-

tion in the electron affinity is exactly equal

to the potential difference developed across

the capacitor.

In a more rigorous analysis, the amount

by which the energy bands are bent is found

to be approximately equal to the band-gap,

and the vacuum level is lowered until the ab-

sorption level of the electropositive material

is essentially at the top of the valence band.

PRACTICAL PHOTOCATHODE

MATERIALS

Research on commercially useful photo-

emitters has been directed primarily toward

developing devices sensitive to visible radia-

tion. The first important commercial photo-

surface was silver-oxygen-cesium. This sur-

face, which provides a spectral response

designated S-l, is sensitive throughout the

entire visible spectrum and into the infrared.

Although it has rather low sensitivity and

high dark emission, the good response in the

red and near-infrared still recommends its

use in special applications although other

photocathodes are more generally used in

photomultipliers today.

200 400 600 800 1000

WAVELENGTH

NANOMETERS

92CM-32295

Fig. 9

Typical spectra/-response curves,

with 0080 lime-glass window for (a)

silver-

oxygen-cesium (Ag-O-Cs), (b) cesium-

antimony (Cs3Sb), (c) multialkali or trialkali

(Na2KSb:Cs

*The terminology

“:Cs”

indicates trace quantities of

the element.

Photomultiplier Design

The photocathodes most commonly used

photomultipliers are cesium-antimony

(Cs3Sb), multialkali or trialkali (Na2KSb:

Cs),*

and bialkali (K2CsSb). Another

bialkali photocathode

(Na2KSb)

is par-

ticularly useful at higher operating tempera-

tures because of its stability. Recently, the

rubidium-cesium-antimony (probably

Rb2

Cs Sb) photocathode has been introduced

because of its favorable blue sensitivity.

Typical spectral response curves for these

materials are shown in Figs. 9 and 10. Addi-

tional information about these and other

photocathodes of practical importance is

shown in Table I.

300 400 500 600 700

WAVELENGTH-NANOMETERS

92CM-32296

Fig.

Typical spectral-response curves

for various photocathodes useful in scintilla-

tion counting applications. The variation in

the cutoff at the low end is due to the use of

different envelope materials.

The long-wavelength response of the

mul-

tialkali photocathode has been extended by

processing changes including the use of an

increased photocathode-film thickness at the

expense of the short-wavelength response.

Fig. 11 shows two typical spectral-response

curves of the ERMA types (Extended Red

Multi-Alkali) II and III compared with the

S-20 response of the conventionally pro-

cessed multialkali photocathode.

Photomultiplier Handbook

Table I

Nominal Composition and Characteristics of

Various Photocathodes

Nominal

Composition

3Sb

KSb

CsSb

Na2KSb:Cs 0

KSb:Cs

Na2KSb:Cs

GaAs: Cs-0 Oe

Photo-

cathode

JEDEC

Envelope

MaterialaResponse

Designation

Conversion Luminous

Factorb Respon-

sivitv

(lumen/

watt)

(uA/lumen) Response (mA/watt) (percent)

Wave-

length of

Respon-

Maximum sivitv

Quantum

Efficiency

Dark

Emission

at 25° C

(fA/cm2)

0080 S-4

950 40

380

38 12

9741

S-5

1244 40

340

0080

S-11

857 70 400 60 19

7056

S-24

1250 40 380

Sapphire

1400 43 380 60 19

0080

938 60 380 56 18

7740 1083 60 400 65 20

B270

1111 90 380 100 33

0080 1120 80 380 90 29

7740 1140

420 82 24

9741 1240 56 380 70 23

0080 948 100 420 95 28

9741

510

75 380 38 12

7740

ERMA

IIIc

160 180 575 29

0080

ERMA

IIc

250 200 550 50

0080

S-20

480

135 390 65

0080 230 300 530 70 16

7056 432

117

420

9741

115

720 800 80

Numbers refer to the following glasses:

0080

Corning Lime Glass

9741

Corning Ultraviolet Transmitting Glass

7056

Coming Borosilicate Glass

7740

Corning Pyrex Glass

B270

Schott BK270

These conversion factors are the ratio of the radiant

responsivity at the peak of the spectral response char-

acteristic in amperes per watt to the luminous

respon-

sivity in amperes per lumen for a tungsten lamp oper-

ated at a color temperature of 2856 K.

0.2

0.3

.0003

.02

.08

0.4

1.2

92.

c A BURLE designation for “Extended-Red Multial-

kali.”

d Reflecting substrate.

e Single crystal.

0 = Opaque

S = Semitransparent

Photomultiplier Design

The negative-electron-affinity materials

described earlier are used in both opaque

and semitransparent photocathodes. Spec-

tral response curves for GaAs:Cs-O and

InGaAs:Cs-O are shown in Fig. 12. There

has been considerable interest in detectors

92CS-32298

Fig. 12

Spectra/ response characteristics Fig. 13

Spectral absorptance of the

Ag-O-

Cs and the

K2CsSb

semitransparent photo-

pared with the S-1 characteristic (Ag-O-Cs).

cathodes.

for 1060 nm, the wavelength of the Nd:YAG

laser. For comparison of sensitivities at this

wavelength, the spectral response of the

Ag-

0-Cs photocathode is also shown. The

InGaAs:Cs-O photocathode has the higher

responsivity at 1060 nm. The NEA photo-

cathodes are generally fairly small compared

with the large semitransparent photocath-

odes used for scintillation counting. Stabili-

ty, especially for the longer-wave-length

NEA photocathodes, is a problem unless the

photomultiplier output current are kept low.

OPAQUE AND SEMITRANSPARENT

PHOTOCATHODES

Photocathodes may be classified as

opaque or semitransparent. In the opaque

photocathode, the light is incident on a thick

photoemissive material and the electrons are

emitted from the same side as that struck by

the radiant energy. In the second type, the

semitransparent photocathode, the photo-

emissive material is deposited on a

transparent medium so that the electrons are

emitted from the side of the photocathode

opposite the incident radiation.

Because of the limited escape depth of

photoelectrons, the thickness of the

semitransparent photocathode film is

0 200 400 600 800 1000 1200

WAVELENGTH -NANOMETERS

92cs -32299

Photomultiplier Handbook

critical. If the film is too thick, much of the

incident radiant energy is absorbed at a

distance from the vacuum interface greater

than the escape depth; if the film is too thin,

much of the incident radiant energy is lost by

transmission.

200 300 400 500 600 700 800 900

WAVELENGTH- NANOMETERS

92CS-32300

Fig. 14 - Spectral absorption coefficient data

tocathodes.

The radiant spectral flux absorption of a

semitransparent photocathode varies with

wavelength as illustrated in Figs. 13 and 14.

The ordinate in Fig. 13 is

absorptance

which

is defined as the ratio of the radiant flux ab-

sorbed by the layer to that incident upon it.

(Absorptance

plus

reflectance

plus

transmit-

tance add to unity.) The data shown in Fig.

14 are spectral absorption coefficients. The

of the photocathode layer is given by

(6)

that the data in Fig. 14 are given in units of

micrometers - l so that in Eq. 6, d must be

given in micrometers. Typical thickness of a

Na2KSb:Cs

photocathode is about 0.030

Thus, at 400 nm the photocathode ab-

sorbs 87% of the flux which is not reflected.

The

spectral response of semitransparent

photocathodes can be controlled to some ex-

tent by thickness variation. With increasing

thickness in the case of alkali antimonides,

blue response decreases and red response in-

creases.

WAVELENGTH - NANOMETERS

92CS-32301

Fig. 15 - Ultraviolet transmittance cut off of various glasses and crystals used in

photomultiplier photocathode windows. Data are all for 1 mm thickness.

GLASS TRANSMISSION AND

SPECTRAL RESPONSE

Although photocathode spectral response

is determined primarily by the nature of the

photocathode surface, especially in the visi-

ble and long wavelength cut-off regions, the

short-wave length cut-off characteristic of

all photocathodes is determined by the

transmission of the window to the photo-

cathode.

Utraviolet

cut-off characteristics

of a number of glasses or crystals which have

been used in photomultiplier fabrication are

shown in Fig. 15.

tion. Photomultiplier tubes made with fused

quartz windows are useful in Cerenkov

counting applications where the spectral

energy distribution increases with decreasing

terval. Fused quartz is also a useful material

in liquid scintillation counting because of its

minimum contamination with

K which can

cause unwanted background counts.

The data presented in this figure are all for

1 mm thickness. The data also include losses

from reflection. For most glasses with an in-

dex of refraction of about 1.5, the reflection

loss is about 4% at each surface. The loss is

higher in high index-of-refraction material

such as sapphire. Although some photo-

multipliers, especially those of small size

may have window thickness of 1 mm or less,

larger face plates are generally thicker in

order to provide adequate strength. The

transmittance varies with thickness accord-

ing to the following relationship

Less expensive than synthetic fused quartz

is Corning 9741 glass, which is frequently

used in photomultipliers designed for the

near ultraviolet. It is a Kovar-sealing glass

but has the disadvantage of possible

weathering over long periods of exposure to

the atmosphere.

Another glass for the near ultraviolet is

Corning 9823, which seals to 0120 lead glass

and which can be used with Dumet metal

leads. This glass is somewhat inferior to 9741

at the shortest wavelengths, but is better for

wavelengths longer than 240 nm.

(7)

where k is a factor (approximately 0.92 for

most glasses) dependent upon the surface

tion, and t is the thickness.

Glass type 7056 is selected for its good op-

tical quality. It is a hard glass which seals to

7052 and Kovar. Pyrex type 7740 is selected

primarily for its low content of 40K and is

used in liquid scintillation-counting applica-

tions. Lime glass is the least expensive; it is a

soft glass which seals to lead glass, type

0120.

The window extending the furthest into

the ultraviolet is

LiF.

A few tubes are made

with this material but, because its fabrica-

tion is difficult, LiF face plates are only used

for special applications of spectroscopy.

Transmission extends to wavelengths shorter

than the Lyman-alpha limit of 121.5 nm.

Corning 9025 is a special non-browning

glass. It is doped with

cerium

and resists

darkening from exposure to ionizing radia-

tion. One application is in satellites which

must pass through space regions of

high-

intensity ionizing radiation.

THERMIONIC EMISSION

Sapphire windows (ultraviolet grade) are

good down to about 150 nm and are easier to

use than

LiF.

Sapphire can be sealed to

Kovar by a metalizing and brazing tech-

nique. Special photomultiplier tubes having

sapphire windows were used in the HEAO

program (High Energy Astronomical Obser-

vatory).

Current flows in the anode circuit of a

photomultiplier tube even when it is

operated in complete darkness. The dc com-

ponent of this current is called the anode

dark current, or simply the dark current.

This current and its resulting noise compo-

nent usually limit the lower level of

photomultiplier light detection. As a result,

the anode dark-current value is nearly

always given as part of the data for any tube.

Except for the extended short wavelength There are several sources of dark current

cutoff of sapphire, Suprasil, an ultraviolet in a photomultiplier tube: ohmic leakage,

grade of synthetic fused quartz, has a better thermionic emission, and regenerative ef-

transmission characteristic. Sapphire suffers fects. Ohmic leakage may result from con-

some loss in transmission by reflection taminations on the insulators within the

because of its relatively high index of refrac-

tube, on the outside of the tube envelope, or

Photomultiplier

Design

Photomultiplier Handbook

on the base.

Thermionic emission

generally

originates from the photocathode itself and

is amplified by the gain of the multiplier sec-

tion. Some emission may also come from the

secondary-emission dynode surfaces.

Regen-

erative effects can occur in the tube par-

ticularly if it is operated with high voltage

and high gain. (Regenerative effects are dis-

cussed in more detail in a later section on

Dark Current and

Noise.) The following dis-

cussion relates to the origin of thermionic

emission current and gives practical values

for this current in photomultiplier photo-

cathodes.

In a metal, the electrons which escape as

thermionic emission are generally from the

top of the conduction band (see Fig. 3).

Thus, the work functions for photoemission

and for thermionic emission are the same.

Thermionic emission as a function of work

ture T in degrees Kelvin is given by the

familiar Richardson equation:

where j is the thermionic current density; e,

the electron charge; m, the electron mass; k,

Boltzman’s constant; and h, Planck’s con-

stant.

If the constants before the exponential

expression are given in mks units, the equa-

tion (8) expressed in amperes per meter2

becomes

For semiconductor photocathodes, the

work functions of photoemission and

ther-

mionic emission may be quite different. The

work function for photoemission (see Fig. 7)

is the electron energy corresponding to the

height from the top of the valence band to

the vacuum level, or

(the electron affini-

ty) plus

(the forbidden gap, i.e., the

*An electron volt is the energy acquired by an electron

in being accelerated through a drop in potential of one

volt. In equations such as

(8),

the value of

in the ex-

ponent must also be expressed in electron volts. It is the

in volts must be multiplied by the electron charge, e, in

coulombs (1.6 x 10

19)

to obtain the work function in

joules. The value of

may then also be expressed in

joules.

separation of valence and conduction

bands). For an intrinsic semiconductor, ther-

mionic emission originates from the valence

band, as does photoemission, but the “work

function” is not the same as for

photoemis-

sion. In the case of an intrinsic semiconduc-

tor, thermionic-emission density can be ex-

pressed as

For an impurity semiconductor where

thermionic emission originates from the

im-

Fig. 16

Variation of thermionic-emission

current density from various photocathodes

used in photomultiplier tubes as a function of

reciprocal temperature. Thermionic emission

multiplied by the gain of the photomultiplier

is a principal source of anode dark current.

purity centers, the equation for thermionic

emission may be written as

follows21:

where

is the Fermi level energy referenced

from the top of the valence band, and

the impurity concentration.

Typical dark-emission current-density

characteristics for various photocathodes are

shown in Fig. 16 as a function of reciprocal

temperature. The current density is plotted

on a logarithmic scale to show the

exponential-like character of the emission.

Anode dark current of the photomultiplier

results from the cathode emission multiplied

by the gain of the tube. Thermionic dark

emission varies from tube to tube of the

same type, probably because of the variation

no.

Note that some of the curves show

substantial curvature at the low temperature

end. Some of this curvature is explained by

the variation of T in the T2/T3/4

= T5/4

term. But an explanation for the greater part

of the curvature may be the presence of

patches of different impurity level or con-

centration, or it may be that the impurity

concentration itself is a function of tempera-

ture. Exposure to temperatures above the

normal operating range sometimes results in

permanent reduction in dark current. Most

photocathodes are p-type semiconductors

one result of which is a lower dark emission

than for n-type semiconductors because of

the reduced Fermi-level energy.

SECONDARY EMISSION

When electrons having sufficient kinetic

energy strike the surface of a material,

secondary electrons are emitted. The

fined as follows:

(12)

where N

is the average number of secondary

electrons emitted for Ne primary electrons

incident upon the surface.

Photomultiplier Design

Fundamentals of Secondary

Emission

The physical processes involved in secon-

dary emission are in many respects similar to

those already described under

Photoemis-

sion.

The main difference is that the impact

of primary electrons rather than incident

photons causes the emission of electrons.

The steps involved in secondary emission can

be stated briefly as follows:

1. The incident electrons interact with

electrons in the material and excite them to

higher energy states.

2. Some of these excited electrons move

toward the vacuum-solid interface,

3. Those electrons which arrive at the sur-

face with energy greater than that repre-

sented by the surface barrier are emitted into

the vacuum.

When a primary beam of electrons im-

pacts a secondary-emitting material, the

primary-beam energy is dissipated within the

material and a number of excited electrons

are produced within the material. The

numbers of excited electrons produced are

indicated in Fig. 17 for primary energies

varying from 400 to 2200 electron-volts. The

total number of excited electrons produced

by a primary is indicated by the area of the

individual rectangles in the figure. These ap-

proximate data are based on experimental

data assuming that the range of primary

electrons varies as the 1.35 power of the

primary energy and that the number of elec-

trons excited is uniform throughout the

primary range.

0.5

Fig.

The

processes of secondary

sion.

See

textfor explanation.

-NANOMETERS

92cs-32303

emis-

Photomultiplier Handbook

As an excited electron in the bulk of the

material moves toward the vacuum-solid in-

terface, it loses energy as a result of colli-

sions with other electrons and optical

phonons. The energy of the electron is very

rapidly dissipated as a result of these

col-

lisons, and it is estimated that the energy of

such an electron will decay to within a few

times the mean thermal energy above the

bottom of the conduction band within

10-12 second. If the electron arrives at the

vacuum-solid interface with energy below

that required to traverse the potential bar-

rier, it cannot escape as a secondary elec-

tron. Therefore, only those electrons excited

near the surface of the material are likely to

escape as secondary electrons. The probabil-

ity of escape for an excited electron is as-

sumed to vary exponentially with the excita-

tion depth, as indicated in Fig. 17. If the pro-

duct of the escape function and the number

of excited electrons (which is a function of

primary energy and depth) is integrated, a

obtained, as indicated in the insert at the top

of Fig. 17. The model which has been as-

sumed thus explains the general

characteristics of secondary emission as a

function of primary energy.

Secondary-

emission yield increases with primary

energy, provided the excited electrons are

produced near the surface where the escape

probability is high. As the primary-electron

energy increases, the number of excited elec-

trons also increases, but the excitation oc-

curs at greater depths in the material where

escape is much less probable. Consequently,

the secondary-emission yield eventually

reaches a maximum and then decreases with

primary energy.

Secondary Emitter Materials

Experimental secondary-emission-yield

values are shown as a function of

primary-

electron energy in Fig. 18 for

MgO,

a tradi-

tional secondary-emission material, and for

GaP:Cs, a recently developed

negative-

electron-affinity material. Also shown is a

calculated curve from Simon and Williams13

based on their model of the GaP:Cs emitter.

Although both

MgO

and GaP:Cs display the

general characteristics of secondary emission

as a function of primary energy, as expected

from the model illustrated in Fig. 17, the

secondary-emission yield for GaP:Cs

in-

creases with voltage to much higher values.

Even though electrons are excited rather

deep in the negative-electron-affinity

material and lose most of their excess energy

as a result of collisions, many still escape in-

to the vacuum because of the nature of the

surface barrier.

PRIMARYENERGY

keV

92cs

-32304

Fig. 18

Typical experimental curve of sec-

ondary-emission yield as a function of .

primary-electron energy in GaP:Cs and

MgO.

Also shown in calculated curve for GaP:Cs

from Simon and Williams.

Because the GaP:Cs material is more dif-

ficult to handle than more conventional

secondary emitters, and, therefore, results in

higher cost, its use as a dynode has been

restricted to applications where the very high

secondary emission is particularly advan-

tageous. It is used as, for example, in

photomultiplier applications benefitting by

the reduction in statistical noise, or in the

design of photomultipliers having fewer

stages for a given amplification. The use of

fewer stages also reduces the variation of

gain with voltage changes.

In the development of photomultiplier

tubes it has been found that the photocath-

ode material may also be useful as a secon-

dary emitter

22.

Such was the case in some of

the first photomultipliers developed which

used a Ag-O-Cs photocathode and Ag-O-Cs

dynodes. Because of its high dark-emission

current and its instability, especially at

moderate current-density

material is no longer used. levels,this

Other photocathode materials which also

serve as secondary emitters are Cs3Sb,

Rb-

Cs-Sb, K2CsSb, and

Na2KSb:Cs.

Because

the processing of these secondary emitters is

Photomultiplier Design

not identical in most cases to that of the cor-

responding

photocathode, the particular

chemical

formulations specified here may

not be accurate. Secondary emission ratios

for these and other materials are shown in

Fig. 19.

Fig. 19

Secondary emission ratios for a

number of

materials

which have been used

dynodes in photomultipliers as a function of

accelerating voltage of the primary electrons.

Very high secondary-emission yields have

been reported23,24 for Na2KSb:Cs, the

multi-alkali photocathode (S-20 response).

Some photomultiplier with this

material as the secondary emitter although

its processing is complex. The very high

yields, particularly at high primary energies,

would suggest that the material has an effec-

tive negative electron affinity similar to that

of GaP:Cs. This explanation may also hold

true for K-Cs-Sb, which is used is some tubes

having this type of photocathode. A material

that has been very commonly used is

Cs3Sb

corresponding to the photocathodes with S-4

or S-l1 spectral responses. It has good

secondary emission in the practical working

range near 100 volts. Rb-Cs-Sb is a rather

new material which is just coming into use

because the corresponding photocathode has

good properties.All of the alkali

an-

timonides mentioned here have limitations.

They cannot tolerate exposure to air and

they are damaged by temperatures in excess

of 75 degrees C. In addition, stability suffers

cm-2.

A very practical secondary emitter can be

made from an oxidized silver-magnesium

alloy25,26 containing approximately 2 per

cent of magnesium. Although silver-

magnesium dynodes do not have as high a

secondary-emission ratio as some of the

materials mentioned above (see Fig.

18),

the

material is easily processed and is more

stable at relatively high currents. In addi-

tion, it can tolerate higher temperatures.

This surface has a low thermionic

background emission which is important in

applications requiring detection of low-level

light. When it is activated with cesium, gain

is somewhat higher. Without the cesium ac-

tivation,the oxygen-activated silver-

magnesium layer has been used effectively in

demountable systems for detecting ions and

other particles.

A material having characteristics very

similar to those of silver-magnesium is an

oxidized layerof copper-beryllium

alloy27,28,29in which the beryllium compo-

nent is about 2 per cent of the alloy. Secon-

dary emission is usually enhanced by the

bake-out in cesium vapor. A

secondary-

emission characteristic of the cesium-

activated copper-beryllium material is shown

in Fig, 19. Because of the advantages in

handling and the manufacturing cost, the

copper-beryllium is largely taking the place

of silver-magnesium in applications requir-

ing low dark emission and stability at

relatively high current densities. The lower

secondary-emission yield is usually compen-

sated for in photomultiplier design by ap-

plication of higher voltage or by an increase

in the number of dynode stages.

92CS

32306

Fig. 20

Typical secondary-electron energy

distribution; peak at right is caused by re-

flected primary electrons.

Photomultiplier Handbook

When secondary electrons are emitted into

the vacuum, the spread of emission energies

may be quite large, as illustrated in the curve

of Fig. 20 for a positive-electron-affinity

emitter. The peak at the right of the curve

does not represent a true secondary, but

rather a reflected primary. Data are not

available for the emission energies from a

negative-electron-affinity material, but they

are expected to be considerably less than for

positive-affinity materials.

TIME LAG IN PHOTOEMISSION AND

SECONDARY EMISSION

Because both photoemission and secon-

dary emission can be described in terms of

the excitation of electrons within the volume

of the solid and the subsequent diffusion of

these electrons to the surface, a finite time

interval occurs between the instant that a

primary (photon or electron) strikes a sur-

face and the emergence of electrons from the

surface. Furthermore, in the case of secon-

dary emission, the secondaries can be ex-

pected to reach the surface over a period of

time. Within the limitations of a mechanistic

approach to a quantum phenomenon, time

intervals for metals or insulators of the order

of 10-13 to 10-14

second may be estimated

from the known energy of the primaries,

their approximately known range, and the

approximately known diffusion velocities of

the internal electrons. In

negative-electron-

affinity semiconductors, it is known that the

lifetime of internal “free” electrons having

quasi-thermal energies (i.e., electrons near

the bottom of the conduction band) can be

of the order of 10-10 second.

Thus far, experiments have provided only

upper limits for the time lag of emission. In

the case of secondary emission, a variety of

experiments have established limits. Several

investigators

30,31,32

have deduced limits

from the measured performance of electron

tubes using secondary emitters. Others,

making direct measurements of these limits,

have determined the time dispersion of

secondary emission by letting short electron

bunches strike a target and comparing the

duration of the resulting secondary bunches

with the measured duration of the primary

bunch. Bythis means an upper limit of

6x10-l 2

second was determined for

platinum33

and an upper limit of 7 x 10-1

second for an

MgO

layer34

formed on the

surface of an

AgMg

alloy.

The upper limit for the time lag in photo-

emission, however, is not well established.

From careful measurements of the time per-

formance of fast photomultipliers it can be

inferred that the limit must be less than

seconds.

While these limits are a useful guide to the

type of time performance to be expected in

present photomultipliers, they will probably

have less significance as photomultipliers us-

ing new semiconducting photoemitters and

secondary emitters are developed. Semicon-

ductors having minority-carrier lifetimes of

the order of microseconds are now available.

Probably, by combination of this character-

istic with negative electron affinity, higher

gains and quantum efficiencies can be

achieved, but at a sacrifice of time response

or band width. However, the first generation

of negative-electron-affinity emitters (e.g.,

Gap) has actually resulted in photomulti-

pliers having better time performance

because a smaller number of stages

operating at higher voltage can be used. At

this time it can only be concluded that in the

future photomultipliers will probably be

designed to match in more detail the re-

quirements of a particular use.

REFERENCES

19. V.K. Zworykin, and E.G.

Ramberg,

Photoelectricity and its Application, John

Wiley and Sons, Inc., New York, (1949).

20a. H. Rougeot and C. Baud, “Negative

Electron Affinity Photoemitters,” Advances

in Electronics and Electron Physics,

Vol. 48,

Edited by L.

Marton,

Academic Press, 1979.

20. J.J. Brady,“Energy Distribution of

Photoelectrons as a Function of the

Thickness of a Potassium Film,” Phys.

Rev.,

Vol. 46, (1934).

21. D.A. Wright, Semi-conductors,

Methuen and Co., New York (1955).

22. A.H. Sommer,“Relationship between

photoelectric and secondary electron emis-

sion, with special reference to the Ag-O-Cs

(S-l) photocathode,” J. Appl. Phys., Vol.

42,

pp. 567-569, (1971).

23.

A.A. Mostovskii, G.B. Vorobeva, and

G.B. Struchinskii,Soviet Physics-Solid

State,

Vol. 5, p. 2436, (1964).

24. C. Ghosh and B.P. Varma, “Secon-

dary emissionfrom multialkali photo-

cathodes,”J. Appl. Phys., Vol. 49, pp.

4554-4555, (1978).

25.

V.K. Zworykin, J.E. Ruedy, and E.W.

Pike,“Silver-magnesium alloy as a secon-

dary emitting material,” J. Appl.

Phys.;

Vol. 12, (1941).

26. P. Rappaport,

“Methods of processing

silver-magnesium secondary emitters for

electron tubes,”J. Appl. Phys., Vol. 25, pp

288-292, (1954).

27. J.S. Allen,“An improved electron

multiplier particle counter,”

Rev. Sci. Instr.,

Vol. 18, (1947).

28.

A.M. Tyutikov, “Structure and secon-

dary emission of emitters of activated

beryllium bronze, "

Radio Engineering and

Electronic Physics, (Translated from the

Russian and published by the IEEE), Vol. 8

No. 4, pp. 725-734, (1963).

Photomultiplier Design

29. A.H. Sommer,“Activation of

silver-

magnesium and copper-beryllium dynodes,”

J. Appl. Phys.,

Vol. 29, pp. 598-599, (1958).

30. G. Diemer and J.L.H. Jonker, “On

the time delay of secondary emission,”

Philips Research Repts., Vol. 5, p. 161

(1950).

M.H. Greenblatt and P.A. Miller, Jr.,

“A microwave secondary-electron multi-

plier,” Phys. Rev., Vol. 72, p. 160 (1947).

32. C.G. Wang,

"Reflex Oscillators using.

secondary-emission current,” Phys. Rev.,

Vol. 68, p. 284 (1945).

33. I.A.D. Lewis and F.H. Wells,

Milli-

Microsecond Pulse Techniques, Pergamon

Press (1959).

34. K.C. Schmidt and C.F. Hendee,

“Continuous-channel electron multiplier

operated in the pulse saturated mode,”

IEEE Trans.

Nucl.

Sci.,

Vol. NS-13, No. 3,

p. loo (1966).

Photomultiplier Handbook

Electron

Optics

Photomultipliers

ELECTRON-OPTICAL DESIGN

CONSIDERATIONS

One of the primary design considerations

in a photomultiplier tube is the shaping and

positioning of the dynodes (usually in a re-

current geometrical pattern) so that all the

stages are properly utilized and no electrons

are lost to support structures in the tube or

deflected in other ways. Although it is not

necessary that the electrons come to a sharp

focus on each succeeding stage, the shape of

the fields should be such that electrons tend

to return to a center location on the next

dynode, even though the emission point is

not at the optimum location of the preceding

dynode. If this requirement is not met, the

electrons increasingly diverge from the

center of the dynode in each successive

dynode stage. This effect in turn can lead to

the skipping of stages and loss of gain.

Magnetic fields may be combined with elec-

trostatic fields to provide the required elec-

tron optics, although today most photomul-

tipliers are electrostatically focused. In

addition to providing good collection of

secondary electrons from stage-to-stage, it is

important for some applications to minimize

the time spread of electron trajectories. For

this purpose is is useful to provide strong

electric fields at the surfaces of the dynodes

to assure high initial acceleration of the elec-

trons. Also important may be the design of

configurations which provide nearly equal

transit times between dynodes regardless of

the point of emission on the dynode.

In scintillation counting applications, fair-

ly large photocathode areas are required for

efficient scintillator coupling. Ideally, the

photocathode should be semitransparent

and located on the flat window of the tube.

This requirement poses a special problem in

design of efficient photocathode-to-first-

dynode electron optics. If all of the emitted

photoelectrons are not guided properly to

the first dynode, the signal-to-noise ratio of

the photomultiplier is degraded and poorer

pulse-height-resolution characteristics result.

The region between the last dynode and

the anode is also of special electron-optical

concern. The withdrawal field at the last

dynode should be large to minimize space

charge effects which limit the linearity and

magnitude of output current pulses. Another

consideration in high-speed photomultipliers

is to provide a structure which is matched to

appropriate transmission lines.

The complete electron-optical configura-

tion of the photomultiplier must also be such

as to avoid regenerative effects. For exam-

ple, there should not be an open path in the

tube through which occasional ions or light

could feed back from the output end to the

photocathode.

DESIGN

METHODS FOR

PHOTO-

MULTIPLIER ELECTRON OPTICS

Before the days of high-speed computers,

photomultiplier electron-optical problems

were often solved by means of mechanical

analogues such as a stretched rubber mem-

brane. When mechanical models of the elec-

trodes were placed under such a membrane

and their height adjusted to correspond to

the desired electrical potential, the height of

the membrane in the spaces between the elec-

trodes corresponded to the equivalent elec-

trical potential. Small balls were then al-

lowed to roll from one electrode to the next.

The trajectories of the balls corresponded to

those of the electrons in the photomultiplier

structure. With appropriate model design,

friction and depression of the membrane by

Electron Optics of Photomultipliers

the ball were made negligible. This model,

however, was only valid for geometries in

which the electrodes could be assumed to be

cylindrical surfaces (generated by a line

parallel to a fixed direction and moving

along a fixed curve) sufficiently long to be

considered infinite in extent. Application

was made to numerous dynode configura-

tions.

Another useful analogue was the resis-

tance network such as a two-dimensional

array of connectors having equal spacing

vertically and horizontally. This array repre-

sented a cross-section plane through cylin-

drical surfaces again assumed to be infinite

in length. Resistors of equal value were con-

nected between adjacent connectors both

vertically and horizontally. Points in the ar-

ray corresponding to an electrode were all

connected to the same potential. Equipoten-

tial lines between electrodes could then be

determined by observing the potential values

on the connectors between the electrodes.

When the equipotential lines had been deter-

mined, electron trajectories could be readily

calculated between closely spaced equipoten-

tial surfaces.

In another variation of the resistance net-

work, the vertical distribution of resistance

values was made logarithmic instead of uni-

form. This array then corresponded to an

axially symmetrical system with the axis ap-

proximated across the top of the board. An

example of a relevant problem is the region

between the photocathode and first dynode

for end-on-type photomultipliers where the

axis passes through the center of the photo-

cathode.

Only very simple electron-optical systems

can be solved in closed form, which requires

However, by using relaxation techniques

with a computer, the potential distribution

on a set of points confined within defined

boundary conditions can be determined.

Secondly, using the force equations, electron

paths can be traced. Computer programs

ex-

ist for solving various cases with or without

symmetry and with irregular potential boun-

daries.

An electron-optical design for a photo-

multiplier may be arrived at from computer-

developed equipotential lines and electron

trajectories plotted on a plan showing the

electrode configurations. Collection efficien-

cy and time response may be predicted from

an analysis of the electron trajectories. Col-

lection efficiency at the first dynode is de-

fined as the ratio of the number of photo-

electrons which land upon a useful area of

the dynode to the number of emitted photo-

electrons. If all the photoelectrons begin

their trajectories at the surface of the photo-

cathode with zero velocity,

100%

collection

would be possible. Because of the finite in-

itial velocities, however, some electrons

begin their trajectories with unfavorable

angles of launch and are not collected on a

useful area.

In modern photomultiplier structures,

first-dynode collection efficiencies range

from 85 to 98 per cent. Ideally, the emitted

photoelectrons should converge to a very

small area on the first dynode. In practice,

this electron-spot diameter is usually less

than 1/4 of the cathode diameter, depending

upon the tube type and focusing structure.

SPECIFIC PHOTOMULTIPLIER

ELECTRON-OPTICAL

CONFIGURATIONS

Circular-Cage Structure with

Side-On Photocathode

The first commercially successful photo-

multiplier design was based on a circular ar-

ray of photocathode and dynodes-as in the

931A. This design is depicted schematically

in Fig. 21. The photocathode is of the

“opaque” type; i.e., electrons are emitted

from the same side as the photocathode is il-

luminated. This particular type of tube is

relatively inexpensive and is useful for ap-

plications such as spectroscopy where high

sensitivity is required but the photocathode

area need not be large. In this case the best

collection is from the side of the photocath-

ode near the first dynode. In the part of the

photocathode near the apex formed by the

grill and the photocathode, the electrostatic

fields are small and collection efficiency is

poor. The time of response of the tube is

short because of its small size and high inter-

dynode field strengths, and there is relatively

small feedback from the anode end of the

tube back to the photocathode.

Photomultiplier Handbook

Circular-Cage Structure with

End-On Photocathode

Fig. 22 illustrates the use of a circular cage

coupled to a flat semi-transparent photo-

cathode. Such a configuration is useful in

scintillation counting where the flat photo-

cathode is coupled to the scintillating crystal.

In order to improve the collection efficiency

of photoelectrons by the first dynode, the

electrode which was the photocathode in

Fig. 21 has been modified to provide a larger

l-10: DYNODES

I I : ANODE

92CS-32308

fig. 22

An end-on photomultiplier structure

utilizing a circular dynode arrangement. This

type of tube would be useful as a detector for

scintillation counting.

useful first-dynode area in the end-on con-

struction. In addition, a skewed field is pro-

vided by the focusing electrode which results

in a more favorable pattern of impacting

photoelectrons on the first dynode. This

end-on construction still has a relatively

short time response because of the focused

dynode arrangement, although the transit-

time spread of electrons traversing the

photocathode-to-first-dynode space in-

creases the time of response as compared

with that of the simple circular side-on struc-

ture of Fig. 21.

Box-and-Grid Structure with

End-On Photocathode

The box and grid dynode structure shown

in Fig. 23 is also useful in scintillation count-

ing because of the relatively large flat semi-

transparent photocathode. This configura-

tion has the advantage of providing a rather

large entrance area to collect photoelectrons

so that collection efficiency is very nearly

100%. This feature is important in providing

good pulse-height resolution in scintillation

counting. The individual dynode boxes are

open at the exit end and have a grid at the en-

trance. The grid provides an electric field

which penetrates the preceding dynode re-

gion and aids in the withdrawal of secondary

electrons. The grid also eliminates a retard-

ing field that would be caused by the poten-

tial of the preceding dynode. Because the

field penetration is rather weak, the electron

Electron Optics of Photomultipliers

INCIDENT

‘RADIATION

INCIDENT

RADIATION

Fig. 23

The box-and-grid multiplier structure.

FACEPLATE

INCIDENT

RADIATION

Fig. 24

The Venetian-blind multiplier structure.

transit time between dynodes is relatively

slow and has a rather large time spread.

Venetian-Blind Structure

Another structure which provides good

collection of photoelectrons, but again is

rather slow in response time, is the

venetian-

blind photomultiplier shown in Fig. 24.

Good collection of photoelectrons is aided

by the size of the photoelectron collecting

area which can be even larger than that of

the box-and-grid construction. The relatively

slow time response is the result of the weak

electric field at the surfaces of the dynode

vanes. The structure is very flexible as to the

number of stages. Some of the secondary

electrons are lost because of the interposition

of the grids between stages, as with the

box-

and-grid construction.

Tea-Cup Structure

A recent innovation in front-end design is

the so-called “tea-cup” photomultiplier,

named after its large first dynode. Secondary

electrons from the first dynode are directed

to an opening in the side of the tea-cup and

thence to the second dynode. Fields between

the photocathode region and the first

dynode region are separated by a very fine

grid structure. The particular advantage of

this design is that the collection efficiency of

the large first dynode is good not only for

Photomultiplier Handbook

92CM-32311

Fig. 25

Tea-cup photomultiplier showing photoelectron paths directly from the

photocathode and those initiated by light transmitted through the photocathode

and striking the side wall which also has an active photoemissive layer. Also in-

dicated are equipotential lines in the region between photocathode and first

dynode.

photoelectrons emitted from the photocath-

ode but for photoelectrons emitted from the

activated side walls between the first dynode

and the photocathode as shown in Fig. 25.

The side-wall photoemission results from

light which passes through the front semi-

transparent photocathode. The increased

photoemission and collection efficiency im-

proves the pulse-height resolution in scin-

tillation counting applications. Indicated on

the diagram are equipotential lines and pho-

toelectron paths showing the collection of

electrons from the front surface and from

the side walls.

In-Line Dynode Structure with

Curved Photocathode

The planar-photocathode design, such as

that shown in Figs. 22-25, provides excellent

coupling to a scintillation crystal, but its

time response is not as good as that of a

spherical-section-photocathode design. The

spherical-section photocathode shown in

Fig. 26 when coupled to a high-speed elec-

tron multiplier provides a photomultiplier

having a very fast time response. Some tube

types utilize a spherical-section photocath-

ode on a plano-concave faceplate to facili-

tate scintillator coupling. This design is

Electron Optics of Photomultipliers

FRONT- END REGION

92CM-32312

Fig. 26

Photomultiplier design with curved faceplate and in-line dynode structure to

provide a minimum transit time and transit-time spread.

PHOTOCATHODE

DYNODE NO. I

TYPICAL

ELECTRON

TRAJECTORIES

TYPICAL

EQUIPOTENTIAL

LINES

92CM-32313

Fig. 27

Cross section of a photomultiplier showing equipotential lines and electron

trajectories that were plotted by computer.

Photomultiplier Handbook

usually limited to faceplate diameters of two

inches or less because of the excessive thick-

ness of the glass at the edge. The

plano-

concave faceplate may also contribute to

some loss in uniformity of sensitivity be-

cause of internal reflection effects near the

thick edge of the photocathode.

The performance of the front-end struc-

ture shown in Fig. 26 has been determined by

the use of a computer to trace equipotential

lines and electron trajectories which have

then been superimposed on a schematic dia-

gram of the tube structure as shown in Fig.

27.

A time parameter of interest is the photo-

cathode transit-time difference, the time dif-

ference between the peak current outputs for

simultaneous small-spot illumination of dif-

ferent parts of the photocathode. In a

planarcathode design, the transit time is

longer for edge illumination than for center

illumination because of the longer edge tra-

jectories and the weaker electric field near

the edge of the photocathode. The

center-to-

edge transit-time difference may be as much

as 10 nanoseconds. The spherical-section

photocathode affords more uniform time re-

sponse than the planar photocathode be-

cause all the electron paths are nearly equal

in length; however, the transit time is slightly

longer for edge trajectories than for axial

trajectories because of the weaker electric

field at the edge.

The photocathode transit-time difference

is ultimately limited by the initial-velocity

distribution of the photoelectrons; this

distribution causes time-broadening of the

electron packet during its flight from the

photocathode to the first dynode. The

broadening effect can be minimized by in-

creasing the strength of the electric field at

the surface of the photocathode.

Because the energy spread of secondary

electrons is even larger than that of photo-

electrons, initial-velocity effects are the ma-

jor limitation on the time response of the

electron multiplier. Multiplier time response

is usually improved by the use of high

electric-field strengths at the dynode surfaces

and compensated design geometries. In a

compensated design, such as that shown in

Fig. 28 and as used in the type of photomul-

tiplier shown in Fig. 26, longer electron

paths and weaker fields alternate with

shorter electron paths and stronger fields

from dynode to dynode to produce nearly

equal total transit time.

Certain materials have advantages over

other materials in providing good multiplier

time performance. For example, a standard

dynode material, copper beryllium, has a

maximum gain per stage of 8 at a 600-volt

in-

terstage potential difference. In contrast,

gallium phosphide exhibits a gain of 60 or

more at a

1200-volt

interstage potential dif-

ference. The advantage in time performance

of the

GaP

dynode over one of the

CuBe

type is that the number of stages, and thus

the total transit time may be reduced, and

that the energy spread of the secondary elec-

trons is less.

92cs-32314

Fig. 28

A compensated-design multiplier.

Continuous-Channel Multiplier Structure

The continuous-channel multiplier

struc-

ture35

shown in Fig. 29 is very compact and

utilizes a resistive emitter on the inside sur-

face of a cylinder rather than a discrete

number of dynodes. The lack of discrete

dynodes causes the electron-multiplication

statistics to be poor because of the variable

path lengths and the variable associated

voltages. The gain of a continuous-channel

multiplier is determined by the ratio of the

channel length to inside diameter; a typical

value of this ratio is

but it may range from

to 100.

92cs-32315

Fig. 29

The continuous-channel multiplier

structure.

Very high gain can be achieved with such a

single-channel multiplier, provided the chan-

nel is curved or bent to avoid line-of-sight

feed-back paths. Numerous special purpose

photomultiplier tubes have been built using

this concept.

Recently,36

very-high-speed photomulti-

plier tubes have been designed utilizing

microchannel plates in proximity with the

photocathode and anode. Such a device is il-

lustrated in Fig. 30. A microchannel plate is

an array of parallel channels, each perhaps

ray is mounted close to the photocathode

with the plane of the channel plate parallel to

the photocathode. A high voltage, 1 kilo-

volt, may be applied between photocathode

and microchannel plate, and similar voltages

applied across the microchannel sandwich

and between the plate and the anode. These

voltages assure short transit times. Time

resolution for pulses initiated by single

MICROCHANNEL

PLATE

Electron Optics of Photomultipliers

photoelectrons measured at full width at half

maximum

(FWHM)

is less than 300

pico-

seconds.37

Because of the construction and

high electric fields employed, the sensitivity

to external magnetic fields is much reduced,

a fact which is important in some nuclear

physics experimentation.

Crossed-Field Multiplier Structure

As mentioned earlier, a crossed-field

photomultiplier was first reported in earl

1936 by Zworykin, Morton and Malter.

Their tube, shown schematically in Fig. 31,

used a combination of electrostatic and mag-

netic fields to direct electrons to repeated

stages of secondary emission. Above each

emitter was a field plate whose potential was

set to be equal to that of the next emitter

down the line. As a result of this configura-

tion, electrons emitted from the photocath-

ode or from one of the secondary emitters,

were caused to follow approximately cy-

cloidal paths to the next electrode. This early

development was not carried into large-scale

manufacture because of the critical

magnetic-field adjustments needed to change

the gain. Also, the rather wide open struc-

ture resulted in high dark current because of

feedback from ions and light developed near

the output end of the device.

Recently, however, similar, but improved

crossed-field photomultipliers39 have been

designed to provide perhaps the fastest rise

time of any photomultiplier. The electron

trajectories are essentially isochronous in the

crossed magnetic and electric fields so that a

very short rise time, 250

picoseconds40,

PHOTOCATHODE

/FACEPLATE

ANODE

92cs-32316

Fig. 30

Microchannel-plate photomultiplier.

Photomultiplier Handbook

achieved. Unfortunately, the photocathode

is rather small and inaccessible by the nature

of the design. A schematic drawing of the re-

cent crossed-field photomultiplier is shown

in Fig. 32. Note the single field electrode in

contrast to the multiple field the plates of the

Zworykin tube. The stepped arrangement is

required in order to provide uniform electric

field. A

50-ohm

coaxial output connector

provides coupling to the high-speed photo-

multiplier.

FIELD PLATES

CATHODE COLLECTOR

92CS

32317

Fig. 31

Schematic of the Zworykin

crossed-

field photomultiplier reported in 1936. A mag-

netic field, perpendicular to the plane of the

drawing,

and an electrostatic field, produced

by the upper fieldplates, combine to bend the

electron paths in the cycloidal trajectories il-

Iustrated.

FIRST

DYNODE

92CS-32318

Fig. 32

Schematic arrangement of a

modern static crossed-field photomultiplier.

ANODE CONFIGURATIONS

The primary function of the anode is to

collect secondary electrons from the last

dynode. The anode should exhibit a

constant-current characteristic of the type

shown in Fig. 33. The simplest anode struc-

ture, shown in Fig. 34, is a grid-like collector

used in some Venetian-blind structures. The

secondary electrons from the next-to-last

dynode pass through the grid to the last

dynode. Secondary electrons leaving the last

dynode are then collected on the grid-like

anode.

POTENTIAL BETWEEN LAST DYNODE AND ANODE-VOLTS

92cs -32319

Fig. 33

Constant-current characteristics of

a photomultiplier anode.

GRID-LIKE

TRAJECTORY

Fig. 34

The

simplest anode structure, a

grid-like collector.

In applications where large output pulse

currents are required it is important to pro-

vide a design having reasonably high with-

drawal fields to avoid space-charge develop-

ment which could limit the output current.

Space charge may actually limit the output at

the next to the last dynode, which is the case

in the circular-cage structure, Fig. 21,

because the withdrawal field at the last

dynode is significantly higher than at the

eighth dynode. In special applications a

tapered divider network may be used to pro-

vide higher inter-stage voltages in the last

several stages of the tube (see Chapter

5-Photomultiplier Applications).

Where fast time response is important, the

type of design shown in Fig. 34 may be at a

disadvantage because electrons from the last

dynode are not necessarily collected on the

first pass through the anode grid structure. It

is also important for fast time response that

the anodes be designed with

matched-

impedance transmission lines or with short

connecting support leads. Most high-speed

circuits are designed to utilize a

50-ohm

im-

Electron Optics of Photomultipliers

pedance, which requires a suitable connector

or lead geometry outside the tube to permit

proper impedance matching.

Anode configurations and internal trans-

mission lines may be analyzed by use of the

standard methods of cavity and transmis-

sion-line analysis. These methods yield ap-

proximate design

parameters41,

which are

optimized experimentally by means of

time-

domain reflectometry,

TDR42’43.

TDR pro-

vides information about the discontinuities

in the characteristic impedance of a system

as a function of electrical length and is an ex-

tremely useful approach in the design of

voltage-divider circuits and mating sockets

which do not readily lend themselves to

mathematical analysis.

Certain anode structures in fast-rise-time

photomultipliers exhibit a small-amplitude

pulse (prepulse) that can be observed a

nanosecond or two before the true signal

pulse. In grod-like anode structures this

prepulse is induced when electrons from the

next-to-last dynode pass through the anode

grid. In more sophisticated photomultiplier

designs this phenomenon may be suppressed

by auxiliary grids that shield the anode from

the effects of the impinging electron cloud.

REFERENCES

35. K.C. Schmidt and C.F. Hendee,

“Continuous-Channel Electron Multiplier

Operated in the Pulse-Saturated Mode,”

IEEE Trans.

Nucl.

Sci.,

Vol. NS-13, No. 3,

p. 100 (1966).

36. Ph. Chevalier, J.P. Boutot and G.

Pietri, “PM of new design for high speed

physics,” IEEE Trans.

Nucl.

Sci.,

Vol.

NS-17, No. 3, pp. 75-78 (June, 1970); G.

Pietri ,

“Contribution of the channel elec-

tron multiplier to the race of vacuum tubes

towards picosecond resolution time,” IEEE

Trans.

Nucl.

Sci. , Vol. NS-24, No. 1, pp.

228-232,

(Feb.

1977).

37.

C.C. Lo, P. Lecomte and B. Leskovar,

“Performance studies of prototype

micro-

channel plate photomultipliers," IEEE

Trans.

Nucl.

Sci., Vol. NS-24, No. 1 , pp.

302-311 , (Feb. , 1977).

38.

V.K. Zworykin, G.A. Morton, and L.

Malter ,

“The secondary emission multi-

plier-a new electronic device”’

Proc.

I.R.E.

Vol. 24 p. 351-375 (1936).

39. R.S. Enck and W.G. Abraham,

“Review of high speed communications

photomultiplier detectors,"

Proc.

Soc.

Photo Optical Instrumentation Engineers,

Vol. 150; Laser and Fiber Optics Communi-

cations’ San Diego CA; 28-29 Aug. , 1978

(Bellingham , WA; Soc. Photo-Optical In-

strumentation Engineers , 1978) , p 3 l-8.

40.

B. Leskovar and C.C. Lo, “Time Res-

olution Performance Studies of Contempo-

rary High Speed Photomultipliers,” IEEE

Trans.

Nucl.

Sci., Vol. NS-25, No. 1, pp.

582-590,

(Feb. 1978).

41.

I.A.D. Lewis and F.H. Wells,

Milli-

Microsecond Pulse Techniques, Pergamon

Press (1959).

42.

“Time-Domain Reflectometry,”

Hewlett Packard Application Note 62.

43.

“Time-Domain Reflectometry,”

Tektronix Publication 062-0703-00.

Photomultiplier Handbook

Photomultiplier

Characteristics

PHOTOCATHODE-RELATED

CHARACTERISTICS

Current-Voltage Characteristics

Photomultiplier tubes may be operated as

photodiodes by utilizing the first dynode and

focusing electrode (if present) tied together

as an anode. In this mode of operation the

photocurrent is linear with light flux except

that, for semitransparent photocathodes, the

resistivity of the thin photocathode layer

limits the current which can be drawn. In the

case of a resistive photocathode, when the

light is directed to the center of the sensitive

area, there is a drop in potential between the

outer conductive ring and the illuminated

area. Because the illuminated area becomes

positively charged with respect to the photo-

cathode contact, only the more energetic

photoelectrons emitted will overcome the

slightly repelling electric field near the

photocathode surface.

Fig. 35 shows a pair of current-voltage

characteristics for

75-mm-diameter

photo-

multipliers operated as photodiodes. In both

cases the illuminated area is approximately 5

mm in diameter and located at the center of

the photocathode. The multialkali photo-

cathode (S-20 response) layer is fairly con-

ductive so that even for a photocurrent of

over 200 nanoamperes, good collection effi-

ciency is achieved for a collection voltage as

low as 50 volts. On the other hand, the

bialkali photocathode ,

K2CsSb

, is very

resistive causing poor collection efficiency

over a wide range of voltage even for photo-

emission currents as low as 10 nanoamperes.

For resistive photocathodes it is thus

necessary to limit the maximum photocath-

VOLTAGE-VOLTS

92CM-32321

Fig. 35

Current-voltage characteristics for

75-mm diameter photomultiplier tubes oper-

ated as photodiodes. The poor collection effi-

ciency shown for the tube with the resistive

photocathode is caused by the voltage drop

from the edge of the photocathode to the cen-

ter illuminated spot and the resulting electro-

static field distortion between the photocath-

ode and the first dynode and focus electrode

used together as an anode.

ode current to avoid non-linear operation.

Resistivity characteristics of several types of

photocathode are shown in Fig. 36. Here,

the resistance per square is shown as a func-

tion of temperature. The increase in resistivi-

ty with decreasing temperature is expected

because of the semiconductor nature of

photocathode materials. Resistance per

square is the resistance of a surface layer be-

tween conductors at opposite sides of a

square of the layer. Note that, if the resistivi-

square of side dimension d, and thickness t,

the resistance

is given by

where d l t is the cross-section area. Thus,

the resistance per square is independent of

the side dimension. On photocathode types

that are very resistive , it is advisable to main-

tain operating temperatures above -100° C

depending upon photocathode diameter , the

light-spot diameter, and the photocathode

current. The larger the photocathode diam-

eter and the smaller the light-spot diameter,

the more severe the effect.

Fig. 36 - Resistance per square as a function

of temperature for various semitransparent

photocathodes.

These data were ob-

tained with special

tubes

having connections

to parallel conducting lines or between con-

centric conducting circular rings on the Sur-

face of the photocathode.

Photomultiplier Characteristics

Photocathode resistive effects can be

avoided at manufacture by the use of nearly

transparent conductive undercoatings.

Opaque types of photocathode, such as

Cs3Sb on a solid substrate of nickel, do not

have a resistivity problem. Table II provides

guidance as to the maximum dc current that

can be utilized with various semitransparent

photocathodes at room temperature. It

should be realized that these data are only

typical; individual photocathodes may vary

considerably from the guideline provided in

their capability of delivering current without

resistive blocking.

Table II - Maximum Recommended Pboto-

cathode DC Currents for Various Semitrans-

parent Photocathode Types as Determined

by Surface Resistivity.

Photocathode Maximum Recommended

Current for T = 22°C

In the case of pulsed photocathode cur-

rents, the peak current values may be higher

than those shown in Table II. Average cur-

rents, however, are still limited, as shown,

by the resistance of the photocathode layer.

For a current pulse, the local surface poten-

tial of the cathode is sustained for a time by

the electrical charge associated with the pho-

tocathode capacitance. Thus, a square cen-

timeter of photocathode may have a capaci-

tance of about 0.5 picofarad. A 5-volt

change in potential is about the maximum

that could occur without blocking of the

photoemission current. The equivalent

charge is therefore 2.5 picocoulombs. For a

pulse duration of t seconds, a pulse current

of 2.5/t picoamperes could be maintained.

For t = 1 microsecond, the current maximum

could be 2.5 microamperes, as determined

by stored charge alone.

Pulse current maximum can be increased

by the capacitance of the photocathode

layer. For example, if a ground plane is pro-

vided in contact with the outside of the

Photomultiplier Handbook

photocathode faceplate, the capacitance of a

square centimeter of photocathode may be

increased to about 2 picofarads (assuming a

dielectric constant 6.75 and a glass thickness

of 3 mm). The ground plane must, of course,

be reasonably transparent to the light signal.

A conducting mesh is a suitable solution.

The ground plane should also be set at or

near photocathode potential, or noisy opera-

tion and deterioration of the photocathode

may result. See the section below under

“Dark Current and Noise.”

Variations in Spectral Response with

Temperature.

Minor variations in the spectral response

characteristics of photocathodes occur with

changes in temperature. Data are presented

here on the typical variations which can be

expected. Deviations from these data may be

expected on individual photocathodes be-

cause of variations in thickness and process-

ing.

When the temperature is decreased, the

response of photocathodes usually improves

in the shorter-wavelength region and

worsens in the longer-wavelength region

near the threshold. An illustration of this

trend is provided in Fig. 37 in which the

Fig. 37

Temperature coefficient of

cesium-

antimony cathodes as a function of wave-

length at 20 “C. Note the large positive effect

near the threshold.

temperature coefficient of responsivity of a

cesium-antimony photocathode is plotted as

a function of wavelength.46 The difference

in spectral response from room to liquid-air

temperature is illustrated in Fig. 38, also for

Cs3Sb.

According to Spicer and

Wooten47,

the increase in response in the blue at low

temperatures is the result of a decrease in

energy loss from lattice scattering; the de-

crease in the red at low temperatures results

from a decrease in occupied defect levels in

the forbidden energy band because of an in-

crease in band gap and possibly because of

an unfavorable change in band bending. It

may also be the case that, in photocathodes

having significant impurity levels, the ex-

tended red sensitivity at higher temperatures

is partly the result of thermally assisted

photoemission. The change of slope at the

elevated temperature in Fig. 38 is suggestive

of this mechanism.

0.1

I I

400 500 600

WAVELENGTH

NANOMETERS

92cs-32324

Fig. 38

Dependence of responsivity of a

Cs3Sb

photocathode on temperature; taken

from Spicer and

Wooten.

The data shown in Fig. 39 is of the same

type as that in Fig. 38, but for a GaAs:Cs

photocathode. Note, again, the increase in

sensitivity with decrease in temperature ex-

cept for the shift in the long wavelength

threshold. In GaAs:Cs the photoexcitation is

from the valence band; there is essentially no

excitation from the forbidden energy band.

Note the sharp long-wavelength cutoff.

Photomultiplier Characteristics

Fig. 39

Relative spectral responsivity shift

with temperature for a

GaAs:Cs

photocath-

ode.

Variation of Photocathode Response with

Angle of Incidence and Angle of Polariza-

tion.

In a theoretical paper49

Ramberg

has

evaluated the optical factors which deter-

mine the variation of photoresponse as a

function of polarization angle and angle of

incidence. One interesting conclusion of

Ramberg’s work is that for normal incidence

and for a certain regime of cathode thickness

and escape depth, the photoemission “may

be expected to be about 1.4 times as great for

illumination from the glass side as for il-

lumination from the vacuum side.” Photo-

multiplier tubes with semitransparent

photocathodes such as are used in typical

scintillation counting applications are all

designed for radiation incident on the glass

substrate which supports the photocathode.

The two curves of Fig. 40 are calculated

from Ramberg’s formulae for incidence

through the glass faceplate, and for polariza-

tion perpendicular and parallel to the plane

of incidence. (i.e. “parallel” implies that the

electric vector lies in the plane defined by the

incident ray and the normal to the glass sur-

face.) These curves assume an index of

refraction for the glass of 1.5, for the photo-

sensitive layer of 3.25, and the absorption in-

dex for the photosensitive layer, K, of 3.25.

The photocathode layer is assumed to be

thin with respect to the wavelength of

light-

taken as 550 nanometers for Fig. 40.

Note that Ramberg’s prediction is that

response should be greater for

perpendicular

polarization, especially at large angles of

in-

cidence.

This conclusion is at odds with

some reported measurements. Experimental

measurements for a semitransparent

Cs3Sb

92CM-32326

Fig. 40

Theoretical photoexcitation deter-

mined by optical factors for different polariza-

tion orientation as a function of angle of

incidence. Radiation is incident on a semi-

transparent photocathode such as Cs3Sb

through a glass substrate of index of refrac-

tion 1.5. Data are taken from

Ramberg.

2.0

(b)

92cs-32327

Fig. 41

(a) Photoresponse for a semitrans-

parent

Cs3Sb

photocathode as a function of

(0°) and perpendicular (90°) to the plane of in-

cidence. (b) Photoresponse ratio for polariza-

sured at an angle of incidence of 70°(from

Hoenig and

Cutler50).

Photomultiplier Handbook

photocathode on Corning 9741 glass are

reported by Hoenig and Cutler50

. Their

data, are shown in Fig. 41 a and b. Their

curve, a, shows the variation of photosen-

for two different angles of polarization. The

parallel to the plane of incidence. The curve,

b, shows the ratio of the photoresponse for

polarization angle 0° to the photoresponse

both measured at 70° angle of incidence.

Note that the response is greater for

polarization parallel to the plane of in-

cidence .

Measurements at RCA support the general

conclusion that the response is higher for

parallel

polarization for Cs

Sb,

K2CsSb

and

Na2KSb:Cs

semitransparent photocathodes

at large angles of incidence. On the other

hand, Hora51

finds the response for a

“trialkali antimonide” to be higher for

perpendicular polarization.

It is likely that the discrepancies noted

above are related to the differences in

photocathode substrates (between the glass

and the photocathode), photocathode thick-

ness, and photocathode processing which

may result in non-isotropic and structured

layers, in contrast, for example, to the

assumption by

Ramberg

of a very thin and

isotropic photocathode layer.

Optical Devices to Enhance Photoresponse

Numerous optical devices have been de-

vised to enhance photoresponse by utilizing

multiple paths in the faceplate and photo-

cathode. Experimental data and theoretical

analyses have been reported by a number of

different

authors.52-56

Fig. 42 shows a

typical arrangement permitting the light

beam to enter the glass substrate so that

reflected rays approach the glass-air inter-

face at an angle greater than the critical

angle and thus do not permit a refracted ray

to escape the glass. In this manner multiple

excitations of the photosensitive layer are

possible. Enhancement of the quantum effi-

ciency for an alkali-antimonide photocath-

ode may be something less than 2:1 in the

blue where the absorption of the photocath-

ode layer is high, but may be as high as 4: 1 in

the red where the absorption is low.

An interesting series of measurements and

theoretical predictions have been reported by

92CS-32328

Fig. 42

A glass quadrant in optical contact

with the photocathode window, permitting in-

coming radiation to approach the

glass-

photocathode interface at any angle of in-

critical angle for the glass-air interface (about

42°) all of the radiation is reflected,

permit-

ting multiple excitations of the photosen-

sitive surface.

D.P. Jones

on the behavior of a semitrans-

parent

Na2KSb:Cs

photocathode with angle

of incidence, wavelength, and polarization

angle. The optical arrangement is similar to

that shown in Fig. 42 except that a full 180°

cylindrical lens is cemented to the faceplate

and half of the surface is coated with

aluminum to reflect the rays back to the

photocathode. His data and theory favor

perpendicular polarization, particularly at

longer wavelengths.

Photocathode Uniformity

The uniformity of photocathode response

over its area may be of importance in some

applications. For example, in scintillation

counting, light pulses which excite different

areas of a non-uniform photocathode would

then result in variations in measured pulse

height. Such an effect would be more ap-

parent in the case where the scintillating

crystal is thin. In flying-spot scanners, non-

uniform photocathode sensitivity would

result in picture shading unless precaution is

taken to avoid even approximate imaging on

the photocathode surface.

Photocathode non-uniformity may result

from a variety of causes. Fig. 43 shows some

tests of photocathode uniformity. These

curves represent photoresponse for a small

focused (l/16-inch) spot as it is scanned

across the diameter of a semitransparent

photocathode. Curve (a) represents the

Photomultiplier Characteristics

uniformity ( ± 3%) typical of a well-pro-

cessed photocathode in a

2-inch

photomulti-

plier designed for scintillation counting. In

curve

(b),

the variation (1) across the photo-

cathode is the result of a non-uniform

evaporation of antimony during the process-

ing of the photocathode. Curve (c) shows

some peculiar non-uniformities in a 3-inch

photocathode. The peaks (2) are the result of

light entering the rounded corner of the tube

envelope and increasing emission by reflec-

tions in the glass faceplate. The peaks (3)

result when light,which is transmited

through the photocathode, strikes the re-

entry shoulder of the glass envelope. The in-

side shoulder of the envelope is aluminized

as part of the photocathode contact. Some

light striking the shoulder may be reflected

back to the photocathode and cause

in-

92cs-32329

Fig. 43

Photocathode uniformity patterns

observed by scanning a 1/16-inch spot

across

a diameter of a semitransparent photocath-

ode in

2-inch

and

3-inch

photomultiplier tubes

used in scintillation counting. (a), scan on a

typical well-processed

2-inch

tube;

(b),

scan

on a tube with non-uniform antimony evapora-

tion on the faceplate (1); (c), scan on a typical

3-inch

tube showing peaks (2) due to entry of

light on the edge of the faceplate and (3)

peaks due to reflection or photoemission

from transmitted radiation through the photo-

cathode onto the shoulder of the bulb.

creased emission, or there may actually be

some photosensitivity on the shoulder

because of the presence of processing

materials.

Anomalies in the photocathode uniformi-

ty such as shown in Fig. 43(c) can be largely

eliminated by a fabrication technique57

creating a diffusing layer on the photocath-

ode substrate. The fabrication of a diffusing

layer can be easily accomplished by sand-

blasting the inside surface of the photomulti-

plier faceplate. The effect of the diffusing

layer is to scatter the incoming light so that

light transmitted through the photocathode

is spread out in many directions, thus

avoiding direct correlation between the posi-

tion of the incoming light spot and any inter-

nal reflections. The diffusing layer also has a

desirable effect of enhancing the photocath-

ode sensitivity.

Uniformity of photocathode response,

however,

is not sufficient to assure a

uniform output response from the photo-

multiplier. All of the emitted photoelectrons

may not be properly directed to the first

dynode by the electron optics. In the case of

a Venetian-blind dynode structure, some of

the photoelectrons may strike parts of the

dynode which do not provide good collec-

tion fields to the second dynode. Fig. 44 is a

DISTANCE ACROSS PHOTOCATHODE-INCHES

92CS-32330

Fig. 44

Relative collection efficiency for

photoelectrons in

venetian-blind type photo-

multiplier tube.

plot of collection uniformity in a 3-inch

Venetian-blind photomultiplier tube. The

structure in the pattern for the scan perpen-

dicular to the length of the dynode is related

to the individual slats in the Venetian-blind

structure. By way of contrast, Fig. 45 shows

similar collection uniformity plots for a

3-inch “teacup”design where the large

Photomultiplier Handbook

open structure of the “tea-cup” first dynode

eliminates the electron-optical effects

observed in the Venetian-blind type tube.

DISTANCE ACROSS PHOTOCATHODE- INCHES

92CS-32331

Fig. 45

Relative collection efficiency for

photoelectrons in a “teacup” type RCA

pho-

tomultiplier tube.

Uniformity of response of a photomulti-

plier may also be affected by the voltage at

which the focusing electrode is operated.

Many photomultipliers are equipped with a

focusing electrode, between the photocath-

ode and the first dynode to provide optimum

collection of the photoelectrons emitted

from the photocathode. The focusing-

electrode voltage is usually set at the point at

which maximum anode output current is ob-

tained. In some applications, spatial unifor-

mity i.e., the variation of anode current with

position of photocathode illumination, may

be more important than maximizing output

current. In such cases, however, the final ad-

justment of the focusing-electrode potential

should not differ significantly from the ad-

justment that provides optimized collection

efficiency. Fig. 46 shows a typical focusing-

electrode characteristic.

Photocathode Stability

Stability and life of a photocathode is

usually related approximately inversely to

the current drawn from it. More stable and

reliable performance results if small areas of

concentrated illumination on the cathode

surface are avoided. In normal operation of

photomultipliers, of course, the photocath-

ode current is usually small and damage to

the tube is more likely to be caused by the

amplified secondary emission currents in the

latter dynode stages.

If a photomultiplier were to contain any

significant trace of gas, ionization of the gas

could occur , depending upon the applied

voltages and level of photocurrent. Ion bom-

bardment of the photocathode can readily

cause damage and loss of sensitivity in pro-

portion to the ion current. Photomultiplier

tubes, however, are generally so well evacu-

ated that ion damage is essentially only a

theoretical possibility.

Electrolytic effects can bring about serious

fatigue in

Cs3Sb

photocathodes (Ref. 11, p.

79) and presumably in other alkali-

antimonide photocathodes. An electrolytic

effect is caused when a potential gradient is

maintained across the cathode surface.

Photocathodes generally only have one

physical contact, but a gradient can be

developed because of the resistive nature of

the photocathode layer if a large photocur-

rent is drawn, for example, from a center

spot on the cathode. Actual electrolytic

decomposition takes place, which can be

recognized by a color change in the cathode

material.

92CS-32332

Fig.46

typicalfocusing-electrode

charac-

teristic.

It is also possible to damage a photocath-

ode by maintaining a difference in potential

through the faceplate supporting the photo-

cathode, particularly at elevated tempera-

tures. For example, if the photocathode in a

photomultiplier is maintained at

1000

Photomultiplier

Characteristics

volts and a ground plane at 0 volts is

established on the outside surface of the

glass, ionic conduction takes place through

the glass. Photocathode sensitivity will be

gradually deteriorated by the ionic

Photomultiplier tubes should be stored in

the dark when not in use. Blue and ultra-

violet radiation, especially, can cause photo-

chemical changes in the photocathode which

result in changes in sensitivity. It is especially

important to avoid exposure to intense il-

lumination such as sunlight even when no

voltage is applied to the tube. Permanent

damage may also result if the tube is exposed

to radiation so intense that it causes ex-

cessive heating of the photocathode. Tubes

should not be stored for long periods at

temperatures near the maximum rating of

the tube; high temperatures almost always

result in loss of sensitivity in the photocath-

ode.

A photomultiplier having a multialkali

photocathode (S-20 spectral response) tends

to lose sensitivity especially in the red por-

tion of its spectral response upon extended

exposure to high ambient room lighting; the

change is usually permanent. Contrary to

this behavior, photocathodes of the ex-

tended red multi-alkali

(ERMA)

type ap-

parently do not exhibit this loss.

The Ag-O-Cs photocathode (S-l) spectral

response) also suffers a decrease in sensitivi-

ty, particularly during operation, when ex-

posed to high radiant-energy levels normally

not harmful to other types of photocathode

materials. The decreased sensitivity occurs

primarily in the infrared portion of the spec-

trum. Loss of infrared sensitivity may also

occur following long periods of storage.

The

GaAs:Cs

photocathode is particularly

sensitive to responsivity loss even for

Another effect related to the exposure of

photocathodes to excessive blue or ultra-

violet radiation, as from fluorescent room

lighting, is a temporary increase in photo-

cathode dark emission current. The increase

may be as much as three orders of magnitude

even from relatively short exposure. This in-

crease in dark current occurs even though

voltage is not applied to the tube and may

persist for a period of from 6 to 24 hours

after such irradiation. Fig. 47 illustrates the

recovery after exposure to fluorescent-lamp

radiation for several photocathode types.

Some of this effect is the result of phosphor-

escence in the glass faceplate of the photo-

multiplier, but a larger part of the effect is

apparently an excitation phenomena in the

Fig.

Variation of dark current following

exposure of photocathode to cool white fluo-

rescent-lamp radiation. The various photo-

cathodes are identified by their spectral-

response symbols.

When semitransparent photocathodes

such as

Cs3Sb

and Ag-Bi-O-Cs (and prob-

ably others) are kept in the dark for many

hours, they become very

resistive59,

especial-

ly at reduced temperatures. The effect may

be so great that the photomultiplier may ap-

pear to have lost sensitivity. Apparently,

passage of current through the photosen-

sitive layer as a result of normal operation

restores the photocathode conductivity. At

reduced temperatures, the process may re-

quire minutes or hours of operation to re-

turn the photocathode to a reasonable con-

ductivity.

Photomultiplier tubes have been exposed

to gamma and X-radiation to an intensity of

1010

roentgens per hour by the Naval

photocathode damage was noted except that

faceplate discoloration was observed for ex-

posure in excess of

104

roentgens. Glass

fluorescent effects were also noted during

the tests. For applications where excessive

radiation may be present, it may be noted

that Corning has developed a non-browning

glass, Ce-doped No. 9025, which has been

used for special photomultipliers.

As a result of enhanced dark count rates

observed in photomultiplier tubes used in

various earth-orbiting satellites, an in-

Photomultiplier Handbook

vestigation was made by Viehmann et al61

on fluorescent and phosphorescent effects in

windows used in photomultiplier tubes when

bombarded by beta rays. Source of the beta

rays was an 0.8 millicurie beta emitter,

to 2.23

MeV

(megaelectron volts). A number

of special windows were tested. For Coming

9741 glass, two decay constants were ob-

served in the

phosphorescense:

4.2 and 57

minutes. For an exposure of 9.5 x

1010

elec-

trons per square centimeter in 30 minutes, an

steradian)-

was observed in a

photomulti-

plier with an S-20 response, closely coupled

to the glass plate.

GAIN-RELATED CHARACTERISTICS

Gain vs. Voltage

When several secondary-emission stages

are coupled together, so that the secondary

electrons from one become the primary elec-

multiplier phototube is given by

In practice, some of the electrons may skip

stages, or become lost to

the

amplification

process by impinging upon nonproductive

secondary-emission areas.

It is customary to describe the gain of the

multiplier phototube as a function of the ap-

plied voltage. Fig. 48 shows two such curves

on a

semilog

scale. These curves illustrate the

wide range of amplification in a multiplier

phototube. They also indicate the necessity

of providing a well regulated voltage supply

for the dynode stages.

It is possible to operate a four-to-six stage

photomultiplier so that each stage is at the

voltage required for maximum secondary

emission, as shown in Fig. 19. In such cases,

the gain could be made practically indepen-

dent of voltage over a small range. However,

such a condition would require approximate-

ly 500 volts per stage; thus the total voltage

required would be very high for the amount

of gain achieved.

In the design or operation of a multiplier

phototube having a fixed supply voltage, the

number of stages can be chosen so that the

gain of the tube is maximum. For this pur-

pose, the optimum voltage per stage is that

value at which a line through the origin (uni-

ty gain on the log-gain scale) is tangent to the

curve, as shown in Fig. 48. This point is

92cs-32334

Fig. 48

Log of gain as a function of volts

per stage for a tube (1P21) with Cs-Sb

dynodes and for a tube (6342A) with Cu-Be

dynodes.

identified on the graph as the point of max-

imum gain per volt. (Note that this argument

neglects the voltage used between the last

dynode and the anode and any discrepancy

resulting from nonuniform distribution of

voltage per stage). In most applications of

multiplier phototubes, the tubes are

operated above the point of maximum gain

per volt. When both the gain and the voltage

are presented on a logarithmic scale, the

resultant curve is then closely approximated

by a straight line. Fig. 49 describes the anode

sensitivity in amperes per lumen and the

typical amplification characteristics of a

photomultiplier as a function of the applied

voltage. Curves of minimum and maximum

sensitivity are also shown.

Photomultiplier Characteristics

SUPPLY VOLTS (E) BETWEEN ANODE AND CATHODE

92cs-32335

Fig. 49

Typical anode sensitivity and ampli-

fication

characteristics of a photomultiplier

tube as a function of applied voltage. Note

the log-log scaling.

External Magnetic and Electrostatic Fields

All photomultipliers are to some extent

sensitive to the presence of external magnetic

and electrostatic fields. These fields may

deflect electrons from their normal path be-

tween stages and cause a loss of gain. Tubes

designed for scintillation counting are

generally very sensitive to magnetic fields

because of the relatively long path from the

cathode to the first dynode; consequently,

such tubes ordinarily require electrostatic

and magnetic shielding. Magnetic fields may

easily reduce the anode current by a much as

50 per cent or more of the “no-field” value.

The three curves in Fig.

show the effect

on anode current of magnetic fields parallel

to the main tube axis and perpendicular to

the main axis in the directions parallel and

perpendicular to the dynodes. The curves are

I-

D-

92CS-32336

Fig. 50

Curves for

3/4-inch

diameter type

4516 photomultiplier showing the effect on

anode current of magnetic fields parallel to

the main axis of the tube and perpendicular

to the main axis in the directions parallel and

perpendicular to

the

dynodes. (Units for mag-

netic field intensity are shown in both Sl

units, ampere turns per meter, and conven-

tional cgs units, oersteds. Note that 1 oersted

Photomultiplier Handbook

plier tubes contain some parts which have

magnetic properties, but if they are

neglected, the magnetic induction-or mag-

netic flux density B-measured in units of

gauss would be numerically the same as the

oersted values. in the case of Sl units for

magnetic induction, 1 weber per square meter

tesla =

104

gausses.)

usually provided for one or more values of

over-all applied voltage and indicate the

relative anode current in per cent as a func-

tion of magnetic field intensity. Fig. 51

shows the variation of output current of

several photomultiplier tubes as a function

of magnetic-field intensity directly parallel

to the major axis of the tube. The magnitude

of the effect depends to a great extent upon

the structure of the tube, the orientation of

the field, and the operating voltage. In

general, the higher the operating voltage, the

less the effect of these fields.

High-mu material in the form of foils or

preformed shields is available commercially

for most photomultipliers. When such a

shield is used, it must be at cathode poten-

tial. The use of an external shield may pre-

sent a safety hazard because in many ap-

plications the photomultiplier is operated

with the anode at ground potential and the

cathode at a high negative potential. Ade-

quate safeguards are therefore required to

prevent personnel from coming in contact

with the high potential of the shield.

It is possible to modulate the output cur-

rent of a photomultiplier with a magnetic

field. The application of a magnetic field

generally causes no permanent damage to a

photomultiplier although it may magnetize

those internal parts of a tube that contain

ferromagnetic materials (tubes are available

which contain practically no ferromagnetic

materials). If tube parts do become magne-

tized, the performance of the tube may be

`-2400 -1600-800 0 8001600 24003200

MAGNETIC FIELD INTENSITY

AMPERE TURNS PER METER

II I I

-30-20-10 0

203040

OERSTEDS

92CM

32337

Fig. 51- Variation of output current of several photomultiplier tubes as a function of

magnetic-field intensity directly parallel to the major axis of the tube. Positive

values of magnetic field are in the direction of the tube base. Operating voltages are

indicated. The 931A is a circular-cage, side-on type. The 4902, 8053 and 8575 are

2-inch

end-on types: teacup; Venetian-blind dynode; and in-line dynode structure,

respectively.

degraded somewhat; however, the condition

is easily corrected by degaussing, a process in

which a tube is placed in and then gradually

withdrawn from the center of a coil operated

at an alternating current of

Hz with a

maximum field strength of 8000 ampere

turns per meter (100 oersteds).

Linearity

Because the emission rate of photoelec-

trons is proportional to the incident radiant

flux, and the yield of secondary electrons for

a given primary electron energy is propor-

tional to the number of primary electrons,

the anode current of a photomultiplier is

proportional to the magnitude of the inci-

dent radiant flux. A linearity plot over a

wide range of light level is shown in Fig. 52

LIGHT FLUX -LUMENS

92CS-32338

Fig. 52

Range of anode-current linearity as

a function of light flux for a 931A photomulti-

plier.

for a type 931A. The limit of linearity occurs

when space charge begins to form.

Space-

charge-limiting effects usually occur in the

space between the last two dynodes. The

voltage gradient between anode and last

dynode is usually much higher than between

dynodes and, therefore, results in a limita-

tion at the previous stage, even though the

current is less. The maximum output cur-

rent, at the onset of space charge, is propor-

tional to the 3/2 power of the voltage gra-

dient in the critical dynode region. By use of

an unbalanced dynode-voltage distribution

increasing the interstage voltages near the

output end of the multiplier, it is possible to

increase the linear range of output current.

Photomultiplier Characteristics

Table III provides guidance as to the max-

imum pulse current which can be drawn

from the anode of various types of

photo-

multipliers before spacecharge effects limit

linearity. Note that the type 8575, which has

a focused dynode structure, provides the

highest withdrawal fields and highest linear

output current. On the other hand,

venetian-

blind or box-and-slot dynodes have relative-

ly low withdrawal fields. Higher currents can

be obtained from all types of photomulti-

pliers by unbalancing the voltage distribu-

tion to provide higher fields at the critical

last stages.

Some photomultipliers used in applica-

tions requiring high output pulse current use

an accelerating grid between the last dynode

and the anode to reduce the effects of

space-

charge limiting. The potential of such a grid

is usually between that of the last and the

next-to-last dynode and is adjusted by ob-

serving and maximizing the value of the

anode output current.

Another factor that may limit

anode-

output-current linearity is cathode

resistivi-

ty; cathode resistivity is a problem only in

tubes with semitransparent photocathodes,

particularly of the Cs-Sb or bialkali type.

Linear behavior is not always obtained

from photomultipliers even at low current

levels. For example, if the test light spot on a

931A is not directed close to the center of the

active area of the photocathode, disturbing

effects may arise from the proximity of the

ceramic end plates. Near the end plates, the

fields are not uniform and are affected by

charge patterns on the insulator spacers,

which change with the current level. An ex-

terior negative shield placed around the bulb

wall may improve tube linearity by elimi-

nating bulb charging effects. The passage of

excessive current may change the sensitivity

of the tube and cause an apparent non-

linearity.

Some photomultipliers exhibit a tem-

porary instability in anode current and

change in anode sensitivity for several

seconds after voltage and light are applied.

This instability, sometimes called

hysteresis

because of cyclic behavior, may be caused by

electrons striking and charging the dynode

support spacers and thus slightly changing

the electron optics within the tube. Sensitivi-

ty may overshoot or undershoot a few per

Tube Type

931A

(side-on, circular

cage)

4524

(3-inch, venetian-

blind dynodes)

4900

(3 -inch, Tea-cup

type)

8575

(2-inch, linear fo-

cused dynode struc-

ture)

8575

C-31059

(1 1/8-inch, box and

slot dynodes)

C-31059

Photomultiplier Handbook

cent before reaching a stable value. The time

to reach a stable value is related to the behavior may be a problem in applications

resistance of the insulator, its surface such as photometry where a photomultiplier

capacitance, and the local photomultiplier is used in a constant-anode-current mode by

current. This instability and non-linear varying the photomultiplier voltage as the

light input changes.

Table III

Maximum Anode Currents That Can be Utilized in Various Photomultiplier Types Without

Serious Loss of Linearity from Space-Charge Build-up.

Supply

Voltage Maximum

Volts Distribution*

Output Current

(mA)

with Linearity Loss Less

Than 10%

1000

1500

1200

1760

2100

1050

1150

120

*These number series represent the relative voltage applied between cathode and first dynode,

first dynode and second dynode,. . . . . ,

last dynode and anode. Note that in the case of the 8575

and the C-31059, a second voltage distribution is given with increased voltage drop in the last

stages providing higher output currents before the onset of space-charge-limiting conditions.

Photomultiplier Characteristics

Hysteresis has been eliminated in many

tubes by coating the dynode spacers with a

conductive material in the manufacturing

process and maintaining the coating at a

fixed potential. Tubes treated by this method

assume final sensitivity values almost im-

mediately upon application of light and

voltage.

Peak linear and saturation currents are

usually measured by pulsed methods. One

common method makes use of a cathode-ray

tube with a P15 or P16 phosphor. The grid is

double-pulsed with pulses of unequal ampli-

tudes but fixed amplitude ratio. As the

amplitude of the two pulses is increased, a

point is observed at which the amplitude of

the larger of the anode pulses does not in-

crease in the same proportion as the smaller

pulse. At this point the tube is assumed to

become non-linear. The current value at this

point is then measured by means of an

oscilloscope and load resistor. The max-

imum saturation current is found when a

further increase in radiation level yields no

further increase in output.

Although very high anode current can be

drawn from photomultiplier tubes, it

should be emphasized that stability cannot

be expected at such levels. These high cur-

rent levels are principally of interest in light-

pulse applications. In this case, the stability

of the tube is approximately that of the tube

operated at the integrated average current

level

Gain Variation with Temperature

It is often advisable to reduce the ambient

temperature of a photomultiplier in order to

reduce dark emission from the photocathode

and improve the signal-to-noise ratio of the

measurement. Variations of the spectral

characteristics of photocathodes with

temperature have been noted in the previous

section on

Photocathode Stability.

There are

numerous reports on the over-all variation

of the photomultiplier output current with

temperature including the effect of tempera-

ture on photocathode

sensitivity62-64.

Fig.

53 is a typical characteristic for type 8571

photomultiplier having Cs-Sb dynodes

showing the variation of gain with

temperature. These data do not include the

variation of photocathode response with

temperature which has been described in the

section on

Variations in Spectral Response

with Temperature. The gain data were ob-

tained by measuring both the anode response

variation and the photocathode response

variation with temperature and dividing out

the latter to obtain only the gain variation. It

is probable that the increase in gain at low

temperatures is the result of a decrease in

energy loss from lattice scattering. Although

the variation in over-all gain is fairly large, it

should be remembered that the variation

results from a small variation in each stage

compounded by the 9 stages. At room tem-

perature, the average gain per stage for the

8571 is 4.5 at 100 V per stage and the percen-

tage change in stage gain with temperature is

approximately

-0.06% per °C. Thus the

secondary emission temperature coefficient

is generally less that the photoresponse

temperature coefficient, both for Cs-Sb sur-

faces. See Fig. 37.

LOX

TEMPERATURE

°C

92cs-32341

Fig. 53

Typical variation of gain with tem-

perature for a

9-stage

photomultiplier tube

(type 8571) with Cs-Sb dynodes operating at

100 volts per stage.

A few words of caution regarding the am-

bient temperature of photomultipliers are

pertinent. A maximum ambient tempera-

ture, and in some instances a minimum tem-

perature, is specified for all photo-

Photomultiplier Handbook

multipliers. The specification of maximum

ambient temperatures reduces the possibility

of heat damage to the tube. Cesium, for ex-

ample, is very volatile and may be

redistributed within the tube causing loss of

secondary emission gain or loss of cathode

sensitivity.

It is recommended that photomultipliers

be operated at or below room temperature so

that the effects of dark current are mini-

mized. The variation of dark current, or

noise, is most important because of its effect

on ultimate low-light-level sensitivity.

Various cryostats and solid-state thermionic

coolers have been designed that reduce dark

current at low temperatures in low-light-level

applications. An important consideration in

the use of these devices is to prevent conden-

sation of moisture on the photomultiplier

window. A controlled low-humidity atmos-

phere or special equipment configuration

may be necessary to prevent such condensa-

tion.

Another reason for avoiding the operation

of photomultipliers at extremely low temper-

atures is the possible phase change that this

type of operation may cause in some of the

metal parts. These changes are particularly

probable when Kovar is used in

metal-to-

glass seals. Tubes utilizing Kovar in their

construction should not be operated at tem-

peratures below that of liquid nitrogen

(-196°C).

In some tubes, particularly those with

multialkali photocathodes, it is sometimes

observed that the noise actually increases as

the temperature of the photocathode is re-

duced below about -40°C. The reason for

this noise increase is not understood. How-

ever, most of the dark-current reduction has

already been achieved at temperatures above

-40°C.

In general, it is recommended that all

wires and connections to the tube be encap-

sulated for refrigerated operation. Encap-

sulation minimizes breakdown of insulation,

especially that caused by moisture condensa-

tion.

Stability and Gain

All photomultipliers have a maximum

anode current rating. The primary reason

for such a rating is to limit the anode power

dissipation to approximately one-half watt

or less. Consequently, the magnitude of the

maximum anode current is restricted to a

few milliamperes when the tube is operated

at 100 to

200

volts between the last dynode

and the anode. Many photomultipliers are

rated for only 0.1 mA or less.

Operating a photomultiplier at an ex-

cessively high anode current results in an in-

creased fatigue that occurs as the average

anode current increases. The loss in sensitiv-

ity occurs as a result of a reduction in the

secondary emission, particularly in the last

stages of the photomultiplier where the cur-

rents are the highest.

Tube fatigue or loss of anode sensitivity is

a function of output-current level, dynode

materials, and previous operating history.

The amount of average current that a given

photomultiplier can withstand varies widely,

even among tubes of the same type; conse-

quently, only typical patterns of fatigue may

be cited.

The sensitivity changes are thought to be

the result of erosion of the cesium from the

dynode surfaces during periods of heavy

electron bombardment, and the subsequent

deposition of the cesium on other areas

within the photomultiplier. Sensitivity losses

of this type, illustrated in Fig. 54 for a 1P21

TIME-MINUTES

92cs-32339

Fig. 54

Short-time fatigue and recovery

characteristics of a typical 1P21 operating at

100 volts per stage and with a light source

ad-

justed to give 100 microamperes initial anode

current. At the end of 100 minutes the light is

turned off and the tube allowed to recover

sensitivity. Tubes recover approximately as

shown, whether the voltage is on or off.

Photomultiplier Characteristics

operated at an output current of 100 micro-

amperes, may be reversed during periods of

non-operation when the cesium may again

return to the dynode surfaces. This process

of recovery may be accelerated by heating

the photomultiplier during periods of non-

operation to a temperature within the max-

imum temperature rating of the tube;

heating above the maximum rating may

cause a permanent loss of sensitivity.

Sensitivity losses for a given operating cur-

rent normally occur rather rapidly during in-

itial operation and at a much slower rate

after the tube has been in use for some time.

Fig. 55 shows this type of behavior for a

0100 200300400

TIME -HOURS

92cs-32340

Fig. 55

Typical sensitivity loss for a 1P21

operating

100 volts per stage for a period of

500 hours. Initial anode current is 100 micro-

amperes and is readjusted to this operating

value at 48, 168, and 360 hours.

1P21 having Cs-Sb dynodes, operating at an

output current of 100 microamperes. Tubes

operated at lower current levels, of the order

of 10 microamperes or less, experience less

fatigue than those operated at higher cur-

rents, and, in fact, may actually recover

from high-current operation during periods

of low-current operation.

Fatigue rates are also affected by the type

of dynode materials used in a tube. Copper

beryllium or silver magnesium dynodes are

generally more stable at high operating cur-

rents than the cesium antimony types. The

sensitivity for tubes utilizing these dynodes

very often increases during initial hours of

TIME -HOURS

Fig. 56

Typical responsivity variation on life

for a photomultiplier having silver-magne-

sium dynodes. Initial anode current was 2 mil-

liamperes and was readjusted to this operat-

ing value at 48, 168, and 360

hours.

operation, after which a very gradual de-

crease takes place, as illustrated in Fig. 56.

The operating stability of a photomulti-

plier depends on the magnitude of the

average anode current; when stability is of

prime importance, the use of average anode

currents of 1 microampere or less is recom-

mended.

In addition to the life characteristics,

which are probably the result of changes in

the dynode layer itself, other changes of a

temporary nature also occur. Not all these

changes are well understood; some are

charging of insulators in the tube.

Fig. 57 illustrates one of the peculiar in-

stabilities which are sometimes observed in

photomultiplier tubes. When the light is first

turned on, the current apparently overshoots

and then decays to a steady value. This par-

ticular phenomenon is probably the result of

the charging of the supporting insulator for

the dynodes. The effect is observed to occur

more rapidly at higher currents, presumably

Photomultiplier Handbook

because of the greater charging current.

Electrons striking the insulator probably

result

in secondary emission and a resultant

positive charge. The change in potential af-

fects the electron optics in the space between

dynodes. The effect is observed as an in-

crease in some tubes and as a decrease in

others. When the photomultiplier is designed

with metal shields or with conductive

coatings on the critical areas of the in-

sulators, this effect is eliminated.

TIME

SECONDS

92cs-32343

Fig. 57

Sudden shift in anode current prob-

ably as the result of insulator spacer charg-

ing. Observation was made using an experi-

mental photomultiplier in which the effect

was unusually large.

A related phenomenon is the variation of

pulse height with pulsecount rate in

scintillationcounting applications. Thus,

when a radio-active source is brought closer

to a scintillating crystal a greater rate of scin-

tillations should be produced, all having the

same magnitude. In a particular photomulti-

plier a few per cent change in amplitude may

result and cause problems in measurement.

Fig. 58 shows the typically minor variation

of pulse height with pulse-count rate for a

type 6342A multiplier phototube.

0.4 0.6

COUNTING

0.8

RATE

92cs-32344

Fig. 58

Typical variation of pulse

height

with pulsecount rate for a 6342A. 3

(13

source with a Nal:Tl source).

In order to investigate the phenomena of

pulse-height variation with pulsecount rate,

a purposely exaggerated experiment was

devised. Instead of a scintillating crystal, a

pulsed cathode-ray tube was used as a light

source. Two pulse rates were studied: 100

and 10,000 pulses per second. Pulse duration

was one microsecond; decay time to 0.1

maximum was 0.1 microsecond. The experi-

ment was devised to study the rate at which

the photomultiplier tube output response

changed when the pulse rate was suddenly

switched between the two rates. Tubes such

as the

6342A,

and especially the 8053 and

8054, showed practically no effect (of the

order of one per cent or less). Fig. 59 shows

Fig. 59

Variation of output pulse height as

the rate of pulsing is changed in a poorly

designed experimental tube. Light pulses

were provided from a cathode-ray tube. At the

left of the graph, which shows the pulse-

amplitude envelope with time for the output

of the photomultiplier tube, the pulses are at

100 per second. The pulse rate is increased

suddenly to 10,000 per second and again re-

duced as indicated. Changes in amplitude are

probably the result of insulator charging.

the pulses during the switching procedure for

an experimental tube. The phenomena were

completely reversible and were observed (to

a lesser extent) on many different tube types.

The time-decay period of several seconds

suggests the charging of an insulator spacer

to a new potential as the result of the in-

creased charge flow and the subsequent

modification of interdynode potential fields.

In scintillation counting it is particularly

important that the photomultiplier have very

good stability. There are two types of gain

stability tests which have been used to

evaluate photomultipliers for this

applica-

tion: (1) a test of long-term drift in pulse-

height amplitude measured at a constant

counting rate; and (2) a measure of

short-

term pulse-height amplitude shift with

change in counting rate.

In the time stability test, a pulse-height

analyzer, a

137Cs

source, and a NaI(T1)

crystal are employed to measure the pulse

height. The 137Cs source is located along the

major axis of the tube and crystal so that a

count-rate of 1000 counts per second is ob-

tained. The entire system is allowed to warm

up under operating conditions for a period

of one-half to one hour before readings are

recorded. Following this period of stabiliza-

tion, the pulse height is recorded at one-hour

intervals for a period of 16 hours. The drift

rate in per cent is then calculated as the mean

gain deviation

(MGD)

of the series of pulse-

height measurements, as follows:

where p is the mean pulse height, pi is the

pulse height at the

ith

reading, and n is the

total number of readings. Typical maximum

mean-gain-deviation values for photomulti-

pliers with high-stability Cu-Be dynodes are

usually less than 1 per cent when measured

under the conditions specified above. Gain

stability becomes particularly important

when photopeaks produced by nuclear

disintegrations of nearly equal energy are be-

ing differentiated.

In the

count-rate stability test,

the photo-

multiplier is first operated at 10,000 counts

per second. The photopeak counting rate is

then decreased to 1000 counts per second by

increasing the source-to-crystal distance.

The photopeak position is measured and

compared with the last measurement made

at a counting rate of 10,000 per second. The

count-rate stability is expressed as the

percentage gain shift for the count-rate

change. It should be noted that count-rate

stability is related to the hysteresis effect

discussed above. Photomultipliers designed

for counting stability may be expected to

have a value of no greater than 1 per cent

gain shift as measured by this count-rate

stability test.

Photomultiplier Characteristics

Life Expectancy

The life expectancy of a photomultiplier,

although related to fatigue, is very difficult

to predict. Most photomultipliers will func-

tion satisfactorily through several thousand

hours of conservative operation and propor-

tionally less as the severity of operation in-

creases.

Photomultipliers do not have

elements which “burn out” as in the case of

a filament in a vacuum tube. Furthermore,

loss of sensitivity which occurs with opera-

tion tends to recover during idle periods or

during conservative operation.

Factors which are known to affect life

adversely are high-current operation,

excessive-voltage operation, high photocath-

ode illumination, and high temperature.

Operation of photomultipliers in regions

of intense nuclear radiation or X-rays may

result in an increase in noise and dark cur-

rent as a result of fluorescence and scintilla-

tion within the glass portions of the tubes.

Continued exposure may cause darkening of

the glass and a resultant reduction in

transmission capability.

DARK CURRENT AND NOISE

The lower limit of light detection for a

photomultiplier tube is determined in many

cases by the electrical noise associated with

the anode dark current. There are several

sources of dark current in a photomultiplier.

These sources are described below,

Sources of Dark Current

Dark current in a photomultiplier tube

may be categorized by origin into three

types: ohmic leakage, dark or “thermionic”

emission of electrons from the cathode and

other elements of the tube, and regenerative

effects .

Ohmic leakage,

which results from the im-

perfect insulating properties of the glass

stem, the supporting members, or the plastic

base, is always present. This type of leakage

is usually negligible, but in some tubes it may

become excessive because of the presence of

residual metals used in the processing of the

photocathode or the dynodes. Condensation

of water vapor, dirt, or grease on the outside

of the tube may increase ohmic leakage

beyond reasonable limits. Simple precau-

tions are usually sufficient to eliminate this

sort of leakage. In unfavorable environmen-

tal conditions, however, it may be necessary

Photomultiplier Handbook

to coat the base of the tube with

moisture-

resisting materials, which may also prevent

external arc-overs resulting from high

voltage.

Ohmic leakage is the predominant source

of dark current at low-voltage operating

condition. It can be identified by its propor-

tionality with applied voltage. At higher

voltages, ohmic leakage is obscured by other

sources of dark current.

Fig. 60 shows the typical variation of dark

VOLTS PER STAGE

92CS-32346

Fig.

Typical variation of dark current

with voltage for a multiplier phototube.

current of a photomultiplier tube as a func-

tion of applied voltage. Note that in the mid-

range of voltage, the dark current follows

the gain characteristic of the tube. The

source of the gain-proportional, dark cur-

rent is the dark or thermionic emission of

electrons primarily from the photocathode.

Because each electron emitted from the

photocathode is multiplied by the secondary-

emission gain of the tube, the result is an

output pulse having a magnitude equal to the

charge of one electron multiplied by the gain

of the tube. (There are statistical amplitude

variations which will be discussed later.)

Because the emission of thermionic electrons

is random in time, the output dark current

consists of random unidirectional pulses.

The time average of these pulses, which may

be measured on a dc meter, is usually the

principal dc component of the dark current

at normal operating voltages. The limitation

to the measurement of very low light levels is

the variable character of the thermionic

dark-current component. It is not possible to

balance out this wide-band noise component

of the photomultiplier tube, as it might be to

balance out a steady ohmic-leakage current.

Nevertheless, it is usually advantageous to

operate the photomultiplier tube in the range

where the thermionic component is domi-

nant. In this range, the relationship between

sensitivity and noise is fairly constant as the

voltage is increased because both the photo-

electric emission and the thermionic emis-

sion are amplified by the same amount.

Typical dark emission current densities for

various photocathodes are given in Table I

(page 16). The resulting anode dark current

may be estimated by multiplying the dark

emission per unit area at the photocathode

by the photocathode area and by the gain of

the photomultiplier tube at the desired

operating voltage.

The thermionic component of the dark

current varies in a regular way with

temperature as illustrated in Fig. 16, and

because the thermionic component of the

dark current is a source of electronic noise in

the anode circuit, it is frequently advan-

tageous to cool the photomultiplier and take

advantage of the reduced dark current and

noise.

Various cryostats have been

de-

signed

62,66

providing low temperature

operation of photomultipliers. One practical

consideration is the prevention of condensa-

tion of moisture on the window. In a

Dewar-

type arrangement, condensation may not be

a problem; in simpler set-ups moisture con-

densation may be prevented by a controlled

low-humidity atmosphere at the external

window. On some types of photocathode,

too cool a temperature may result in the

photocathode becoming so resistive that the

photoemission is blocked by a drop in poten-

tial across the photocathode surface. See

earlier section on

Current-Voltage Charac-

teristics. Commercial cryostats or cooled

photomultiplier chambers are available

designed especially for photomultiplier

operation67.

At higher dynode voltages, a

regenerative

type of dark current develops, as shown in

Fig. 60 . The dark current becomes very er-

ratic, and may at times increase to the prac-

tical limitations of the circuit. Continued

flow of large dark currents may cause

damage to the sensitized surfaces. Some

possible causes of the regenerative behavior

will be discussed in more detail later. All

photomultiplier tubes eventually become

unstable as the gain is increased.

Dark-Current Specification

Dark-current values are often specified at

a particular value of anode sensitivity rather

than at a

fixed

operating voltage. Specifica-

tions of dark current in this manner are more

closely related to the actual application of

the photomultiplier.

The best operating range for a given

photomultiplier can usually be predicted

from the quotient of the anode dark current

and the luminous sensitivity at which the

dark current is measured. This quotient is

identified as the Equivalent Anode Dark

Current Input (EADCI) in the Technical

Data for individual photomultiplier tubes;

and is the value of radiant flux incident on

the photocathode required to produce an

anode current equal to the dark current

observed. The units used in specifying EAD-

CI are either lumens or watts at the wave-

length of maximum cathode responsivity or

watts at a specified wavelength.

The curves in Fig. 61 shows both typical

anode dark current and equivalent anode

dark current input (EADCI) as functions of

luminous sensitivity. The optimum operat-

ing range occurs in the region of the

minimum on the EADCI curve, the region in

which the signal-to-noise ratio is also near its

maximum. The increase in the EADCI curve

at higher values of sensitivity indicates the

onset of a region of unstable and erratic

operation. Many curves of this type also in-

clude a scale of anode-to-cathode supply

voltage corresponding to the sensitivity

scale.

Equivalent Noise Input-The

dark current

in a photomultiplier is the average current

value of the output pulses occurring at ran-

dom intervals plus the dc leakage current.

Fluctuations or noise associated with these

pulses limit precision of measurement,

rather than the particular dark current value.

Noise from a photomultiplier may be

evaluated in terms of a signal-to-noise-ratio

Photomultiplier

Characteristics

measurement. If the type of modulation and

bandwidth used in the measurement is

known, an equivalent noise input, ENI, can

be calculated from the signal-to-noise ratio,

Equivalent noise input is defined as the value

of incident luminous or radiant flux which,

when modulated in a stated manner, pro-

duces an rms output current equal to the rms

noise current within a specified bandwidth,

usually 1 Hz.

92c3-32347

Fig. 61

Illustrative data showing the varia-

tion of anode dark current and the equivalent

anode-dark current input (EADCI) as a func-

tion of luminous sensitivity for a type 8575.

Operation of the tube at voltages higher than

that for the minimum of the EADCI character-

istic does not provide the best signal-to-noise

ratio.

Noise Equivalent Power-Another way to

categorize the limit of detection of a

photomultiplier is by noise equivalent power

(NEP)

which is essentially the same as EN1

except the units are always in watts. NEP is

the radiant flux in watts at a specified wave-

length incident on the detector which gives a

signal-to-noise ratio of unity. The frequency

bandwidth (usually 1 Hz) and the frequency

at which the radiation is chopped must be

specified as well as the spectral content of

the radiation (most often, monochromatic

radiation at the peak of the detector

response). It should be noted that NEP is

Photomultiplier Handbook

frequently specified in units of watts

HZ-1/2. The numerical value of this for-

mulation of NEP is the same as that given in

units of watts but with a specified bandwidth

of 1 Hz.

Detectivity-Detectivity

(D)

is the recipro-

cal of NEP; it is expressed in W

Detectiv-

ity is a figure of merit providing the same in-

formation as NEP but in the reverse sense so

that the lower the radiation level to which

the photodetector responds, the higher the

detectivity.

Regenerative Effects

Dynode

Glow. Although photomultipliers

are designed to minimize regenerative ef-

fects, at some high voltage and gain almost

all photomultipliers exhibit breakdown

phenomena. One source of regeneration in

photomultipliers is the glowing of the

dynodes under electron bombardment.68

The glow has a blue spectral emission and of

course is most prominent in the latter stages

where the current is highest. The regenera-

tive effect occurs when the light from the

dynode glow is scattered and reflected back

to the photocathode. Dynode cage shields

and opaque support wafers minimize this ef-

fect .

Glass Charging Effects.

Regenerative

pho-

tomultiplier currents may also be triggered

by the electrostatic potential of the bulb

walls surrounding the dynode or photocath-

ode structure. Particularly when the poten-

tial of the bulb is near anode potential, stray

electrons may be attracted to the bulb and

cause the emission of light on impact, de-

pending upon the nature of the glass surface

and the presence of contamination. Secon-

dary electrons resulting from the impact of

stray electrons on the glass surface are col-

lected by the most positive elements in the

tube and help maintain the positive potential

of the inner surface of the glass. Under these

circumstances, it is possible to observe the

formation of glowing spots on the inside of

the glass bulb, provided the eye is dark

adapted and the applied voltage is sufficient-

ly high. Some of this fluorescent light may

be reflected back to the photocathode and

result in an increase in the photocathode

dark (or light) emission.

Shielding.

The effect just described can be

minimized by controlling the external poten-

tial of the glass envelope. Fig. 62 shows the

effect of various voltages applied to an exter-

nal shield around the tube envelope of a

1P21 photomultiplier. The graph shows the

equivalent noise input decreasing as the

shield is made negative toward the-potential

of the photocathode. It should be noted also

that an actual contact is not always required

to produce the effects noted in Fig. 62. The

proximity of a positive potential near the

glass bulb can cause a noisy operation.

Fig. 62

Effect of external-shield potential

on the noise of a 1P21 photomultiplier. Note

the desirability of maintaining a negative bulb

potential.

Operation of a photomultiplier tube with

an improper external shield may not only

cause an increase in noise or lead to an elec-

trical breakdown of the tube, but can result

in damage to the photocathode and reduced

tube life. In order to prevent these effects,

the envelope wall should be maintained near

photocathode potential by wrapping or

painting it with conductive material and con-

necting this material to cathode potential.

The connection is usually made through a

high impedance to reduce the shock hazard.

If a cathode potential shield is not provided,

the glass surface in the vicinity of the photo-

cathode must be insulated from any source

of potential difference so that leakage cur-

rents to the bulb are less than 10

-12

ampere.

In photomultiplier tubes in which the

pho-

tocathode is of a transmission type, on the

inside surface of the glass bulb, it is par-

ticularly important to avoid a positive

voltage contact on the external surface of the

photocathode window. In this case, ionic

currents can flow through the glass and pro-

duce a

fluoresence

and an accompanying

noisy photocathode current.

It should also be noted that continued

operation of a photomultiplier tube with a

positive voltage contact to the glass in the

photocathode area can cause a permanent

damage to the photocathode. The damage is

reported to be the result of ionic conduction

through the glass and poisoning of the

pho-

tocathode by sodium ions.69

Afterpulsing.

Afterpulses,

which may

be observed when photomultipliers are used

to detect very short light flashes as in scin-

tillation counting or in detecting short laser

pulses, are identified as minor secondary

pulses that follow a main anode-current

pulse. There are two general types of after-

pulses; both are characterized by their time

of occurrence in relation to the main pulse.

The first type results from light feedback

from the area of the anode, or possibly cer-

tain dynodes, to the photocathode; the in-

tensity of the light is proportional to the tube

currents. When this light feedback reaches

the cathode, the afterpulse is produced.

Afterpulses of this type, characterized by a

delay in the order of 40 to

nanoseconds,

may be a problem in many older

photomulti-

pliers having open dynode structures. The

time delay experienced with this type of

afterpulse is equal to the total transit time of

the signal through the photomultiplier plus

the transit time of the light that is fed back.

The second type of afterpulse has been

shown to be the result of ionization of gas in

the region between the cathode and first

dynode. The time of occurrence of the after-

pulse depends upon the tube dimensions, the

type of residual gas involved and the mass of

the gas ion, but usually ranges from 200

nanoseconds to well over 1 microsecond

after the main pulse. When the ion strikes

the photocathode, several secondary elec-

trons may be emitted; thus, the resulting

afterpulse has an amplitude equal to several

electron pulse-height equivalents. These

pulses appear to be identical to the larger

dark-current pulses, and it is suspected that

many of the dark-current pulses are the re-

sult of photocathode bombardment by gas

ions.

Several gases, including

+ and

+ ,

are known to produce afterpulses. Each gas

produces its own characteristic delay follow-

ing the main pulse. The most troublesome,

perhaps, is the afterpulse caused by the

Photomultiplier Characteristics

ion; this afterpulse occurs approximately 300

nanoseconds after the main pulse in a tube

of type 7850 construction. One source of

hydrogen in the tube is water vapor absorbed

by the multiplier section before it is sealed to

the exhaust system. Other gases which may

cause afterpulsing may be present as a result

of outgassing of the photomultiplier parts

during processing or operation. Present

pho-

tomultiplier processing techniques are

designed to eliminate or at least to minimize

the problem of afterpulsing.

Helium Penetration71,72

Another effect

must be considered in relation to the sources

of dark current-the penetration of helium

through the glass of photomultiplier tubes.

When the photomultiplier is operated or

stored in an environment where helium is

present, helium will gradually permeate

through the glass envelope. Because helium

is inert, it does not react with the photocath-

ode or dynode surfaces. But tubes subjected

to such an environment will exhibit a noise

increase and an increase in afterpulsing

because of the ionization of the helium by

electron impact. Depending upon the degree

of permeation, a point will be reached at

which complete ionization and electrical

breakdown occurs making the tube unus-

able.

Other Noise Sources

Excess noise or dark current can also re-

sult from field emission occurring within the

tube and from scintillations in the glass

envelope of the tube caused by radioactive

elements within glass (most glasses contain

some radioactive 40K). Fused silica is

sometimes utilized in photomultiplier face-

plates to minimize these effects.

Noise in photomultiplier tubes can

also

result from the proximity of nuclear sources

or from cosmic rays which result in glass

scintillations. Andrew T.

Young73,

* has

identified large pulses which originate from

Cerenkov light flashes produced by cosmic

rays traversing the window of end-on photo-

multipliers. The flashes correspond to

*Andrew T. Young also has written a chapter, “Photo-

multipliers: Their Cause and Cure,” in Volume 12 of

Methods of Experimental Physics: Astrophysics,

Marton, Editor, Academic Press, 1974. As well as

being

a good general reference on photomultipliers, this chap-

ter contains a further discussion of the

Cerenkov-light-

flash effect.

PhotomultIplier

Handbook

photoemission pulses of

electrons and

larger. The number of pulses is greater when

the face of the photomultiplier is upward

rather than downward because the Cerenkov

radiation is emitted in a conical pattern away

from the direction of entry. Young reports

the total number of pulses from this source

to be about 1.2 min

-2.

Another phenomenon which deserves

mention in connection with dark current is

the effect of previous exposure to light,

especially blue or near ultraviolet. Large in-

creases in photocathode dark emission may

occur as discussed in section on

Photocath-

ode Stability

with rather slow recovery, as il-

lustrated in Fig. 47. It is advisable, there-

fore, that photomultipliers be kept in the

dark at all times, or at least for many hours

before they are used for making low-level

measurements.

Noise Output of a Photomultiplier

The output current of a photomultiplier

consists of a train of unidirectional electrical

pulses whether the tube is in the dark or with

illumination on its photocathode. Each pulse

is the result of an electron emitted from the

photocathode and amplified by the secon-

dary emission of the tube. (Some pulses may

originate also from light striking the first

dynode or from thermionic emission from

the first or other dynodes.) In a system hav-

ing a wide bandwith, the individual pulses

may be measured and counted. Their spac-

ing in time will have a statistical variation as

will their height. There variations constitute

noise and limit the precision of measure-

ments.

If the bandwidth is not sufficient to

resolve the individual pulses, particularly

when the light level is relatively high, the

output will exhibit noise or rapid variation in

signal level about an average value. The

noise level relative to the signal level will

decrease with increasing signal level or with

decreasing bandwidth. The noise may be de-

scribed by giving the rms value of the current

variation in a specified bandwidth.

Dark-Current and Noise Reduction

Dark-Current Reduction.

If care is taken

to avoid damage to the photomultiplier by

operation with excessive current, the dark

current can often be reduced by a process of

operating the photomultiplier in the dark at

or near the maximum operating voltage.

This process, called

dark aging,

may require

several hours to several days. After such a

process of aging, it is recommended that a

photomultiplier be operated for several

minutes at the reduced voltage before mea-

surements are attempted.

Dark Noise Reduction with Cooling.

Because the dark emission is reduced as the

temperature of the photocathode is reduced,

the dark noise output of the photomultiplier

may also be reduced by cooling. Fig. 63

TUBE

Fig. 63

Equivalent noise input in lumens for

a 1P21 photomultiplier as a function of tem-

perature.

shows the variation of equivalent noise in-

puts, ENI, for a 1P21 (opaque

Cs3Sb

photo-

cathode) over a wide range of temperature.

The implication of these data is that by cool-

ing from room temperature to

150 °C the

low light level limit of detection can be

reduced by two orders of magnitude.

Photomultiplier Noise Characteristics

The following paragraphs describe the

noise and signal levels from both a pulse and

a dc point of view. The noise frequency spec-

trum is also described and data are presented

showing the dark noise spectrum.

At very low signal levels, the detection

limit will be shown to be determined by the

dark current of the photomultiplier and its

associated noise. At high levels, the precision

of measurement is limited by the statistical

variation in the signal pulses-or the noise

which is always present to some degree in the

dc signal level. The signal-to-noise ratio can

Photomultiplier Characteristics

be improved by decreasing the bandwidth in

dc measurements or by the equivalent of in-

creasing the time in pulse-counting applica-

tions.

Noise Spectrum. The width of an output

current pulse initiated by an electron from

the photocathode is determined by the varia-

tions in transit time through the tube. The

noise associated with these pulses of electron

current is flat with frequency out to frequen-

cies corresponding to the width of the in-

dividual pulses. (The frequency spectrum of

a delta function is flat.) A power spectrum

of the noise from type 931 has been calcu-

lated by R. D.

Sard74

and is shown in Fig. 64.

Sard’s calculation was done by considering

FREQUENCY

MHz

92cs-32349

Fig. 65

Typical Dark-Pulse Spectrum

Fig. 64

Spectral energy distribution of the

noise from type

931A

operated at 100 volts per

stage, as calculated by R.

Sard.74

the anode current from a single electron

traversing the space between the last two

dynodes and anode and the distribution

of electron arrival times due to differences in

transit time between stages; then, the Fourier

transform of the pulse shape was taken to

obtain the frequency spectrum.

The bandwidth of the noise spectrum dif-

fers in different tube designs, depending

upon transit-time variations. Usually,

high-

speed photomultipliers are characterized by

rise and fall times of the output pulses rather

than by bandwidth. This subject is discussed

in more detail in a later section.

Dark Noise Pulse Spectrum. Because of

the statistical nature of the secondary emis-

sion gain at each stage of the photomulti-

plier, the output pulses have a fairly wide

distribution of heights even though each

pulse is initiated by a single electron. Fig. 65

PULSE HEIGHT- PHOTOELECTRON EQUIVALENTS

92CM-32350

shows a differential dark-noise pulse spec-

trum-the number of pulses counted per

unit of time as a function of their height.

A differential dark-noise spectrum is ob-

tained with a multichannel pulse-height

analyzer. The calibration of the

single-

photoelectron pulse height is determined by

illuminating the photocathode with a light

level so low that there is a very low probabili-

ty of coincident photoelectron emission. The

dark-pulse distribution is then subtracted

from the subsequent combination of dark

pulses and single-photoelectron pulses, so

that the remainder represents only that

distribution resulting from single-photoelec-

tron events. By adjusting the gain of the

pulse-height analyzer, the single-electron

photopeak can be placed in the desired chan-

nel to provide a normalized distribution.

The dark-pulse spectrum of Fig. 65 is

characteristic of photomultipliers intended

for use in scintillation counting and other

Photomultiplier Handbook

low-light-level pulse applications. The curve

shown is idealized and represents an average

or typical spectrum. An actual spectrum

shows statistical variations depending upon

the length of the count.

The slope of the curve for the pulse-height

region between 1 and 4 photoelectrons is as

expected for single-electron emission, when

the statistical nature of secondary-emission

multiplication is considered. The number of

pulses in this region may be reduced by cool-

ing the photomultiplier. Below a pulse height

of one photoelectron equivalent, the curve is

determined partly by the statistical spread

due to the multiplication process and partly

from emission from some of the dynode

surfaces.

The slope of the curve for the pulse-height

region greater than 4 photoelectrons is

presumed to be caused by

multiple-electron-

emission events. These multiple pulses are

caused by processes such as ionic bombard-

ment of the photocathode. Other mecha-

nisms contributing to the noise spectrum in-

clude cosmic rays,field emission, and

radioactive contaminants that produce scin-

tillations within the glass envelope. Cooling

has little effect upon reduction of the

number of these multiple electron pulses, but

extended operation of the tube may improve

performance. Operation of the tube may

result in erosion of sharp points and reduce

the possible contribution of field effects. In

addition, improvement occurs because

residual gases are absorbed within the tube,

ion bombardment of the photocathode is

reduced, and the resulting multiple electron

emission is lessened.

For many applications it is useful to have

a summation of the total number of dark

pulses. In Fig. 65, for example, the sum of

dark-pulse counts from 1/8th electron

equivalent height to 16 electron equivalent

heights is 4 x

104

counts per minute.

Analytical Model of Noise and Signal-to-

Noise Ratio-Consider the lower

limit

light detection capability of a photomulti-

plier as determined by the fluctuation in the

thermionic dark emission. If the dark emis-

sion current from the photocathode is id, the

rms shot noise associated with this current is

given by

(16)

where e is the charge on the electron and

the bandwidth of the observation,

The photocathode dark emission and its

associated noise are both amplified by the

for the moment that the amplification pro-

cess is noise free, and that all of the current

emitted from the photocathode is collected

by the first dynode, the anode dark current is

given by

(17)

and the anode rms noise current is given by

(18)

Actually, noise is introduced by the secon-

dary emission process. (This subject is dis-

cussed at length in Appendix G.) If one as-

sumes Poisson statistics for the secondary

emission process, the anode noise current

shown in Eq. 18 should be increased by a

dary emission ratio per stage, assumed to be

the same for all stages. If a typical value of

factor of 1.15. Actual measurements indi-

cate a somewhat higher figure. On the other

hand, higher voltage on the first stage, or the

use of a GaP:Cs first dynode having a very

high secondary-emission ratio would reduce

this factor. For the purpose of the present

discussion, this increased noise from secon-

dary emission will be neglected and the

photomultiplier anode noise will be taken as

the amplified photocathode shot noise as

given by Eq. 18.

A fundamental advantage of a photomul-

tiplier as compared with a photodiode, is the

high gain and the fact that the secondary

emission process contributes very little to the

relative noise output of the tube. The high .

gain of the photomultiplier permits the use

of a relatively small load resistance

(R)

without deterioration of the photomultiplier

signal-to-noise ratio by the Johnson thermal

noise of the load resistance. The small load

resistance permits an operation of the

photo-

multiplier with the very high bandwidths in-

herent in the photomultiplier design. A large

load resistance shunted by the finite tube and

lead capacitance would otherwise seriously

limit the effective bandwidth of the system.

Photomultiplier Characteristics

In order to maintain the fundamental tion and cable used in pulse applica-

signal-to-noise capability of the photomulti- tions-the two noise sources are about

plier, the Johnson noise of the load resis- equal. Therefore, at this point in the exam-

tance must be less than the photomultiplier ple chosen there is some, but not a serious,

output noise. Johnson noise (rms) is given by

reduction of signal-to-noise ratio.

where

is Boltzmann’s constant and T is the

temperature in Kelvin units. In order to com-

pare this noise with the photomultiplier out-

put noise we may convert it to an equivalent

current noise by dividing by the load resis-

tance:

I(20)

Fig. 66 illustrates the relative magnitudes

Fig. 66

Comparison of the magnitudes of

Johnson noise in the load resistance and of

the output photomultiplier dark noise, both in

units of rms amperes per square root hertz.

For the case illustrated, both sources of

noise are equal for a load resistance of 50

ohms.

Noise in Signal. When the photocathode

current representing the signal is larger than

the photocathode dark emission, the domi-

nant noise is the noise in the signal. For a

photocathode signal current,

is,

the anode

signal current

Is,

is given by

(21)

and, paralleling equation 18, the anode rms

noise current associated with the signal cur-

rent is given by

rms

(22)

But, one must consider both the dark emis-

sion noise and the noise in the signal current.

Because there is no correlation between the

two sources, the noises may be added by

summing their squares, or for

total

rms noise

current:

(23)

An expression for the signal-to-noise ratio

may now be written:*

SNR = iS

Fig. 67 illustrates the variation of

signal-

to-noise according to equation 24 as a func-

tion of bandwidth and photocathode signal

current. The cathode dark emission is as-

sumed to be 10

15 ampere. For signal cur-

rents less than the dark emission, the dark

noise is the limit to the signal-to-noise ratio.

In this case, a narrow bandwidth is a require-

ment for detection. When dark noise is the

limit to detection, it is useful to reduce dark

emission by cooling. At wider bandwidths

and higher photocathode signal levels, the

limit to the signal-to-noise ratio is in the

*In this discussion, the signal is treated as though it were

dc. In an actual application, the signal may be

modulated and signal-to-noise figures would involve the

rms value of the modulated signal. Note also that the

noise calculation neglects the factor by which the noise

is increased by the secondary emission statistics.

Photomultiplier Handbook

noise of the signal so that cooling would not

be advantageous. In Fig. 67, it has been as-

sumed that the load resistance is sufficient

(greater than 50 ohms) so that Johnson noise

is not a factor.

In applying these signal-to-noise concepts

to an actual case, the photocathode dark

current may differ greatly from the 10- 15

ampere figure assumed. (See the data, Fig.

16.) The photocathode signal currents may

be calculated from a knowledge of the pho-

tocathode responsivity as published in a tube

data sheet. For example, a photocathode

may have a responsivity of 160

micro-

amperes per lumen for tungsten irradiation,

or 70 milliamperes per watt at the wave-

length of maximum responsivity (i.e., 420

nanometers). A photocathode current of 1

femtoampere would then correspond to

10-15/70x 10-3= 1.43x 10-14 watt flux

incident on the photocathode.

TIME EFFECTS

Terminology

Photomultiplier tubes may be character-

ized for time response in various ways. The

*These definitions follow the standards appearing in

“IEEE Standard Test Procedures for Photomultipliers

for Scintillation Counting and Glossary for Scintillation

Counting Field.”ANSI N 42.9

1972; IEEE Std 398

-1972. Published by The Institute of Electrical and Elec-

tronics Engineers, Inc., 345 East 47 St., New York,

N.Y. 10017.

terms used in these time characterizations

are defined below. In these definitions it is

assumed that the photomultiplier is activated

with a delta-function light pulse. Fig. 68 il-

lustrates the definitions of some of the more

common terms.

Fig. 68

The various time relationships in a

photomultiplier output pulse, assuming a

delta

excitation

function. Illustrated are tran-

sit time, rise time, fall time, and full width at

half maximum (FWHM).

Transit time†

“is the mean time difference

between the incidence of the light upon the

photocathode (full illumination) and the oc-

currence of the half-amplitude point on the

output-pulse leading edge.”

Rise

time “is the mean time difference be-

tween the 10- and

90-percent

amplitude

points on the output waveform for full

cathode illumination and delta-function ex-

citation.”

Fall time “is the mean time difference be-

tween the

90-

and 10-percent amplitude

points on the trailing edge of the output-

pulse waveform for full cathode illumination

and delta-function excitation.”

†This definition differs from the definition appearing in

the IEEE Standard Dictionary of Electrical and Elec-

tronics Terms (ANSI/IEEE Std. 100-1977: “The time

interval between the arrival of a delta-function light

pulse at the entrance window of the

tube

and the time at

which the output pulse at the anode terminal reaches

peak amplitude.”). This latter definition is closer to the

actual transit time which might be defined to the

average time of arrival of the electrons at the output.

However, the definition given to the half-amplitude

point may be more useful in an application where the

half-amplitude point is used for timing purposes.

Full-width-at-half-maximum

(FWHM)

the mean elapsed time between the

half-

amplitude points on the output waveform

for full cathode illumination and

delta-

function excitation.

Delta-function light pulses.

In tests related

to time characterization of photomultiplier

tubes it is useful to have light pulse sources

available which approach characterization as

a delta-function pulse. A delta-function

pulse is one whose duration is significantly

shorter than that of the output pulse to be

measured. It approaches the mathematical

concept of a function whose area is finite but

whose width approaches zero.

Various light sources have been used to

generate delta-function pulses for

time-

testing of photomultipliers.

A reverse-biased light-emitting diode

(Fer-

ranti type XP-23) has been used by

Leskovar75

to obtain light pulse widths of as

short as 200 ps. Mercury-wetted relay spark

sources have also been used and have pro-

vided pulses having rise times of 500 ps.

Pulses of radiation having a duration of 50

ps or less can be obtained with a

mode-

locked Nd:YAG laser. The laser wavelength

radiation at 532 nm by means of a nonlinear

crystal. Random pulses can be produced

with fast scintillators. A rise time of 400 ps

can be obtained for Naton 136 and a 60 Co

source.

Transit Time

is expected to increase as the

inverse half power of the applied voltage,

provided the effects of initial velocities and

secondary-emission delay are negligible. Fig.

69 shows the transit time measurements for

an 8053 photomultiplier plotted so as to

display the reciprocal square root voltage

relationship. The intercept of the line on the

time axis is probably due to the distortion of

the simple relationship by the magnitude of

the initial secondary emission velocities.

Fig. 70 shows the transit times for a

number of photomultiplier tubes plotted

over a range of operating voltages. The

larger part of the transit time is just the ac-

cumulation of times for electrons to traverse

from stage to stage. Usually, the time for the

photocathode-to-first-dynode transit is the

largest component, especially for photomul-

Photomultiplier Characteristics

tipliers with large photocathode areas. The

large photocathode necessitates a fairly long

path to the first dynode to provide good

photoelectron collection from the entire

photocathode.

SUPPLY KILOVOLTS BETWEEN

ANODE AND CATHODE

92CS-32406

Fig. 69

Transit time as a function of the

square foot of reciprocal applied voltage for a

type 8053 photomultiplier tube.

Fig, 70

Transit time as a function of supply

voltage (log scales) for a number of photomul-

tiplier tubes.

Photo- and secondary-emission times may

be as short as 10 ps, although for

negative-

electron-affinity

(NEA)

materials, the times

may be as long as 100 ps. One reason for the

PhotomultiplIer

Handbook

high performance of NEA materials is that

long diffusion paths for electrons are ob-

tained. Although this increase in diffusion

path length also increases the emission time,

emission time is not a significant part of the

transit time in most commercial photomulti-

pliers. In general, the transit time of a photo-

multiplier is not as important as its rise time

or as variations in the transit time which

would cause uncertainty in time measure-

ments made with the photomultiplier. A

fixed delay time is easily compensated for by

circuit design.

Rise Time

Fig. 71 shows the rise time-from the 10

Fig. 71

Anode-pulse rise times as a func-

tion of anode-to-cathode applied

voltage (log

scales) for a number of photomultipliers.

90%

amplitude points-for a number of

photomultipliers plotted over a range of

typical operating voltages. No correction

was made for the finite rise time of the light

pulse and measuring equipment, which is

estimated to be of the order of 0.8 ns. In us-

ing photomultipliers for

high-resolution-

time spectroscopy, the ideal point on the

output pulse to use as an indicator would be

at the half maximum of the rising character-

istic. The rising characteristic is generally

faster than the fall and thus provides the

highest precision in timing. However,

because output pulse heights vary, a fixed

discriminator level results in a loss of preci-

sion. When a fixed discriminator level is

used, the highest precision is obtained by use

of a discriminator level between 10 and 20%

of maximum. A superior method is the use

of a constant fraction of the pulse height as a

trigger

.76

Pulse Width

The width of the output pulse is deter-

mined by the variation in transit time

through the secondary emission chain of the

photomultiplier.The variations arise be-

cause of variations in emission energies and

directions of the secondary electrons as

related to the tube structure. The measure-

ment of pulse width is generally the full

width at half maximum. The output pulse

width follows the inverse half power of the

applied voltage as does the average transit

time. Fig. 72 illustrates the pulse width

(FWHM)

for an 8053. For timing experi-

ments, it is generally desirable to have a nar-

row pulse width for good timing precision

capability.

KILOVOLTS

92CS-32409

Fig. 72

Pulse width (full width at half max-

imum) for a type 8053 as a function of the in-

verse half power of the applied

voltage.

Pulse Jitter (Time Resolution)

Although pulse timing is done on the ris-

ing characteristic of the output pulse and is

more precise for a fast rise time, the ultimate

limit to time measurement is the variation in

pulse timing, or pulse jitter. Suppose single

photoelectrons initiate pulses. Variations in

transit time of photoelectrons to the first

dynode will occur because of variations in

the initial velocity and electric field resulting

from the electrode geometry. The same con-

siderations apply to the secondary electrons.

If a number of pulses initiated by single elec-

trons are observed, a histogram can be devel-

oped showing the number of pulses having a

Photomultiplier Characteristics

given transit-time difference. Such a histo-

gram, measured by Birk, Kerns, and

Tusting,77

is shown in Fig. 73. The time of

each pulse was measured by using the

leading-edge half-height point. The full

width at half maximum of this distribution is

about

360

ps and is a measure of the time

resolution capability of this particular tube.

TRANSITTIMEDIFFERENCE

92CS-32410

Fig. 73

Histogram of transit-time difference

for single-photoelectron pulses from an

RCA

developmental type photomultiplier. (From

Birk,

Kerns, and Tusting

If a number, N, of simultaneous photo-

electrons is emitted, the pulse jitter is re-

duced by the square root of N, simply by the

statistical averaging process. This relation-

ship has been demonstrated by Leskovar and

Lo75

for a microchannel-plate photomulti-

plier as shown in Fig. 74.

Data on time resolution for single photo-

electrons for a variety of photomultipliers

are given in Table IV. Values are given for

full photocathode illumination as well as for

illumination at the center point. The time

spread for full photocathode illumination is

larger than for a single point because it in-

cludes the difference in transit time from the

photocathode to the first dynode for dif-

ferent photocathode locations.

92CS-32411

Fig. 74

Time resolution of a

microchannel-

plate photomultiplier as a function of the

number of photoelectrons per pulse, mea-

sured with light pulse width of 2.6 ns, for full

photocathode illumination. Data are from

Leskovar and L

The dashed

line

showing

the inverse square root relationship has been

added.

Table IV

Time Resolution

(FWHM)*

for Single Photoelectrons for Various Photomultipliers

Photomultiplier Handbook

The improvement of the 8852 over the

8575 is the result of the use of the high-gain

first dynode. The difference between the

8850 and the 8852 may be due to the larger

photoelectron emission energies of the multi-

alkali photocathode. The poorer time resolu-

tion of the 8854 is caused by the very large

cathode area and the consequent reduced

electric field strengths and increased path

lengths.

A general treatment of single photoelec-

tron detection and timing has been provided

tion for a photomultiplier is very similar to

that for noise in a photomultiplier as given in

Appendix G. Variations in transit time in the

early stages of the photomultiplier are most

important to the over-all time resolution.

The large number of electrons in the latter

stages bring about an averaging process

which reduces the time variation.

Summary of Time-Resolution Statistics

The summary of time resolution statistics

below follows the work of Gatti and

If the secondary emission of each stage is

electron flight times including all of n stages

is given by

variance of the flight time from the

ith

dynode to the (i +

1)th

dynode. The last

term represents the variance in flight time

from the last dynode to the anode. This ex-

pression is actually a simplification because

of the induction effect of the electrons in

flight between the last stage and anode.

is negligible. If n is large and the variances of

flight times for each stage are assumed to be

follows:

(26)

When variations in secondary emission are

taken into account, assuming all stages have

a Poisson distribution, the total variance in

time of flight for the over-all tube is given by

(27)

Eq. 27 refers to the variance of the centroid

of the output pulse from a single photoelec-

tron input. If timing is done by a point on

the rising characteristic, an additional time

variance might be included, that for the

variation in pulse width. Gatti and

Svelto81

have shown that this term would add a

negligible amount to the variance as given by

Eq. 27.

PULSE COUNTING

One effective way to use a photomultiplier

for measuring very weak signals is to detect

and count pulses resulting from single

photoelectrons-sometimes referred to as

“photon counting”. (Actually, the highest

quantum efficiencies for photocathodes are

such that at best one in every three photons

would be detected, assuming a spectral

match to the peak quantumefficiency

wavelength.) Counting of single electrons

assumes that the rate of arrival of photons is

such that it would be unusual for more than

one photoelectron to be emitted in a time

equivalent to the output pulse width for a

single electron input to the multiplier.

Single-electron pulse counting is an impor-

tant technique in applications such as

Raman

spectroscopy, astronomical photom-

etry, and bio-luminescent measurements.

counting has an advantage equivalent to a

factor of 1.2 in quantum efficiency over

current-measurement techniques.

Output Pulse Height Distribution

An important consideration in photon

counting is the distribution of pulse heights

at the output of the photomultiplier. Statis-

tics of the variation in pulse heights for

single electron inputs is discussed in Appen-

dix G. (See Fig. G-6). Pulse height distribu-

tions are obtained with a multichannel

pulse-

height analyzer. Fig. 75 shows (1) a differen-

tial pulse-height distribution in which the

number of pulses in a given time interval and

in a given channel (between height, h and

an integral pulse-height distribution in which

the total number of pulses occurring in the

given time interval with a height of

greater is plotted.83 Also shown on the

graph is a rectangle whose intercept on the

abscissa is the equivalent of the output pulse

associated with one photoelectron and an

average photomultiplier gain of G. The rec-

tangle is determined by setting its height

equal to the intercept of the integral-pulse-

height distribution curve on the ordinate

axis, and by setting the area of the rectangle

equal to the area under the integral-pulse-

height curve. This area is equal to the total

charge of all the pulses counted: the total

number of pulses multiplied by the average

multiplier gain and the charge of one elec-

tron. This equality may be demonstrated by

integrating the integral-pulse-height curve

along the ordinate axis and comparing the

result with the integral of the product of the

differential pulse-height-distribution or-

dinate (number of pulses) and the abscissa

value (pulse height or charge associated).

The pulse-height resolution for single elec-

trons in this case is FWHM = 1.6 electron

equivalents. Note that the data of Fig. 75

FOR

92CS-32412

Fig. 75

Single electron (1) differential and (2)

integral pulse-height distribution curves. Type

4501, photocathode K2CsSb, counting time 10

minutes (from G. A. MortonB3). Different scales

for (1) and (2).

were taken on a tube, 4501, that has

copper-

beryllium dynodes and a modest

secondary-

emission gain. The pulse-height resolution

for single electrons in the case of

gallium-

phosphide dynodes is much better, as will be

discussed below.

When a very low light flux is measured by

the pulse-counting technique, it is also

Photomultiplier Characteristics

necessary to measure the count in the dark

and to subtract the rates to obtain that due

to the light alone. The optimum time which

should be spent on each measurement is

discussed in Appendix G-see for example,

Eq. G-103. If the signal count is much less

than the dark count, however, about equal

times should be spent on both measure-

ments. At higher signal levels, more time

should be spent on the signal count.

Copper-Beryllium Dynodes

Differential pulse-height distributions are

shown in Fig. 76 on a developmental photo-

-LIGHT

PULSE HEIGHT -ARBITRARY UNITS

92cs-32413

Fig. 76

Differential pulse-height distributions

obtained with type

Dev.

No.

C-70101B.

(Simi-

lar to type 8575, but having an S-20 response.

From R. M. Matheson84

multiplier having copper-beryllium dynodes.

Note that the dark pulse-height distribution

does not show a peak representing single

electrons. The probable explanation is that

dark emission originates not only from the

photocathode, but from the dynodes as well.

Electrons originating from the first dynode

would show a distribution similar to that of

the photoelectrons, but with a pulse height

lower by the secondary-emission ratio of the

first dynode. Pulse-height distributions also

usually show an extended foot, as in Fig. 76,

for heights greater than 7 on the figure’s ar-

bitrary scale. These counts represent pulses

Photomultiplier Handbook

of more than one electron per pulse and are

probably caused by secondary mechanisms

such as ion feedback, scintillations in the

glass envelope initiated by cosmic rays or by

radioactive traces in the glass or in the tube

environment. Note that the numbers of these

large pulses are usually several orders of

magnitude less than the numbers in the

single-electron distribution. When a

single-

electron integral count is made, the upper

discriminator level could be set to exclude

these larger pulses.

In setting the lower level of the

discriminator, an optimum level is deter-

mined by the different character of the light

shown that the minimum error in deter-

mining the signal pulse count is obtained by

adjusting the lower-level discriminator set-

ting so that for an integral count distribution

are much improved. Consequently, such

tubes are particularly advantageous for

single-electron pulse counting. The improve-

ment in multiplication is demonstrated by

the differential pulse-height distribution for

single electrons shown in Fig. 78. Note the

(28)

where

represents the number of signal

pulses counted and

represents the

number of dark pulses counted. In Fig. 77,

the value of the pulse height that satisfies the

relation shown in Eq. 28, is given by

hl.

PULSE HEIGHT-PHOTOELECTRON EQUIVALENTS

92CS-32415

Fig. 78

Resolution of a single electron peak

having a measured FWHM of 63%. The first

dynode is GaP:Cs. (Data from Morton, Smith,

Fig. 77

Single electron response (I) and dark

pulse distribution (2) of tube type 4501 for

counting time of 10 minutes; integral distribu-

tion curves. h1 and h2

are

discriminator set-

tings (From G. A.

MortonB3).

Gallium-Phosphide Dynodes

For tubes having the high-gain GaP:Cs

first dynodes, the statistics of multiplication

improvement in resolution as compared with

that of Figs. 75 or 76. When the intensity of

the light flashes is increased, a pulse-height

spectrum such as illustrated in Fig. 79

results, again for a GaP:Cs first dynode. In

this case the light level has been adjusted so

that the peak counts are nearly equal for

one, two, and three electron pulses. The

small pulse distribution to the left of the

single-electron peak distribution is probably

an artifact caused by equipment noise at the

lower channel settings.

Peak-to-Valley Ratio

Another important consideration for a

photomultiplier used in pulse counting is the

peak-to-valley ratio. Referring to Fig. 79,

for single electrons, the peak-to-valley ratio

is the value of the peak of the single-electron

distribution divided by the minimum value

of the count distribution between the first

and second peaks-in this case a ratio of

about 2.3.

Photomultiplier Characteristics

Fig. 79 - Pulse-height spectrum showing

peaks corresponding to one, two, and up to

five electrons. The first dynode is GaP:Cs.

SCINTILLATION COUNTING

Pulse Magnitude

In scintillation counting, the problems are

similar to those of counting single photoelec-

trons but the numbers of equivalent photo-

electrons per pulse can vary from a few to a

fairly large number. For example, in the case

of a soft-beta emitter, using a coincidence

liquid scintillation counter (for an un-

quenched standard, PPO and POPOP in

toluene), the total number of photoelectrons

developed in the pair of photomultipliers

imately 2.5 per keV of beta-ray energy.

Because dark noise from single electrons can

be effectively discriminated against by using

coincidence techniques, a relatively efficient

count of low-energy beta emission can be

made. On the other hand, in the case of a

NaI:Tl crystal coupled to a photomultiplier

having a K2CsSb photocathode, the number

of photoelectrons developed is approximate-

ly 8 per keV of gamma-ray energy. Thus, for

the isotope

energy is 662 keV, the pulse height would be

the equivalent of 5300 photoelectrons.

Isotope 137Cs Sources

Fig. 80 shows the pulse-height distribution

92CS-32417

taken with a NaI:Tl crystal and a two-inch

“teacup” photomultiplier (type 4902).

Pulse-height resolution is illustrated for full

width at half maximum. The peak at the ex-

treme left is the barium K X-ray peak at 32

keV. The region between the two peaks is the

result of Compton scattering.

From Appendix G, Eq. G-111, the pulse-

height resolution is given by

The following values may be assumed:

mc = number of photons corresponding to

= variance in the number of photons

per photopeak pulse

For the case illustrated in Fig. 80, the

FWHM = 7.17%. Substituting the above

325,000. The FWHM related to the crystal

statistics alone is 6.33%. The FWHM related

to the photomultiplier alone is 3.37%.

Fig. 81 shows a pulse-height distribution

energy of 5.9 keV. Note that for this spec-

trum, the FWHM is approximately 40%.

Following the same type of analysis as used

above to evaluate the relative contributions

for the photomultiplier alone is 36% and for

the scintillator, 18%.

Photomultiplier Handbook

Narayan and

Prescott86

have presented

data illustrating a trend in which the photo-

multiplier statistics dominate the pulse-

height resolution at low gamma-ray energies

and the scintillator statistics dominate at

high gamma-ray energies. This trend is il-

lustrated in the above analysis of the data in

Figs. 80 and 81.

Fig. 81

Pulse-height distribution for

5sFe

taken with type 8850 photomultiplier and

Nal:TI

crystal.

As the gamma ray energy is increased, of

course, the relative pulse-height resolution

attributable to the photomultiplier decreases

as the square root of the number of photo-

electrons per pulse or as the square root of

the gamma ray energy. The relative pulse-

height resolution attributable to the crystal

also decreases with gamma ray energy, but

not as rapidly. The light-pulse statistics of

the crystal also varies from crystal to crystal.

Key Photomultiplier Characteristics for

Good Pulse-Height Resolution

Several characteristics of a photomulti-

plier tube determine its suitability for obtain-

ing good pulse-height resolution. Of prime

importance is the

quantum efficiency

of the

photocathode, especially as it matches the

generally blue spectrum of scintillators.

Secondly, of course, the photomultiplier

must have a suitable electron-optical con-

figuration so that all or nearly all of the

emitted photoelectrons effectively impact

the first dynode. The statistics of secondary

emission, especially in the first stage, also af-

fect the ultimate resolution. It is desirable,

therefore, to provide a fairly

high

secondary-

emission yield either by the nature of the

dynode surface material or by applying a

rather high voltage between the photocath-

ode and the first dynode. Finally, it is

generally advantageous that the

photoemis-

sion be uniform across the photocathode

and that a uniform collection of all the

photoelectrons be provided. This latter

characteristic is particularly important in the

case of thin scintillators where the light from

a scintillation is more localized on the

photocathode than it would be for thicker

scintillators. Light pipes may be used to im-

prove the uniformity of light on the

photocathode and therefore improve the

pulse-height resolution in the case of thin

scintillators. For a crystal whose length is

comparable to its diameter, the output

light-

flux distribution is generally uniform so that

photocathode uniformity becomes statisti-

cally less significant.

Peak-to-Valley Ratio

As in pulse-counting applications, another

way of classifying the performance of photo-

multipliers in scintillation counters is by the

peak-to-valley ratio of the distribution. This

way is illustrated in Fig. 81. The peak will be

higher, of course, if the resolution of the

tube is better. Good resolution also results in

a lower valley point before the pulse-height

distribution increases on the low energy side

as a result of photomultiplier dark current.

The peak-to-valley ratio is a particularly

valuable parameter for measurements with

sources characterized by low emission

energy.

Plateau Concept

A plateau curve for a scintillation counter

is illustrated in Fig. 82. The curve is devel-

oped by using a source such as

137Cs

and

counting all pulses higher than a fixed

discriminator level. The total counts are then

plotted as a function of the photomultiplier

voltage. The development of the curve in

Fig. 82 may be understood by referring to

the pulse-height distribution curve of Fig.

80. At the lowest value of photomultiplier

Photomultiplier Characteristics

voltage, the pulse heights are less than the

equivalent of the discriminator setting. As

the voltage is increased, the discriminator

setting moves in effect from right to left on

Fig. 80. The rising portion of Fig. 82 then

corresponds to the discriminator moving

through the photopeak, through the

Comp-

ton scattering region of the distribution, and

through the barium X-ray peak. The plateau

corresponds to the dark background87 of the

photomultiplier which would appear at the

extreme left of Fig. 80 except for the

scale-the barium X-ray peak at 32

keV

cor-

responds to about 250 photoelectrons. The

dark background pulse distribution of a

photomultiplier is shown in Fig. 76. As the

voltage on the photomultiplier is increased,

the dark current of the tube increases be-

cause various regenerative effects increase,

and the plateau is terminated at the upper

end of Fig. 82.

PHOTOMULTlPLlER VOLTAGE-VOLTS

92CS-32419

Fig. 82

Typical plateau is defined as por-

tion of integral-bias characteristic in which

change of counting rate per 100-volt interval

is less than a selected value (Engstrom and

Weaver

The concept of utilizing a plateau charac-

teristic probably stemmed from the plateau

of Geiger-Muller tubes. In the case of photo-

multipliers, the length of the plateau is deter-

mined by the stability of the tube at high

gain (a minimum of regenerative effects) and

by the characteristic variation of gain with

voltage. That is, a tube having Cs-Sb

dynodes would generally have a shorter

plateau than one having Cu-Be dynodes

simply

because of the more rapid increase in

gain with voltage for the Cs-Sb dynodes. The

plateau would also be shorter as the number

of stages is increased.

Photomultiplier plateau is of particular in-

terest in the application to oil-well logging.

A rather intense gamma-ray source such as

137Cs

is mounted in the sonde near the

photomultiplier and crystal assembly, but

shielded from it. The gamma-rays result in

Compton scattering in the materials near the

probe. The scattered radiation is detected

and measured with an integral count with the

discriminator set to correspond to a point on

the plateau so that essentially all of the in-

tercepted radiation is counted. As the depth

of the measurement in the drill hole in-

creases, the temperature does also. It is,

therefore, important that the counting char-

acteristic of the scintillation detector be

essentially independent of temperature. But,

as the temperature increases, both the dark

background count and the instability of the

photomultiplier increases and, as a result,

the length of the plateau decreases. Fig. 83

il-

92cs-32420

Fig. 83

Plateau characteristics at room

temperature and at 150 °C for a

photomulti-

plier having a Na2KSb photocathode. The

photomultiplier’s principal application is in

oil-well-logging.

lustrates plateau characteristics taken at

22°C and 150°C for a photomultiplier

having a Na2KSb photocathode. The curves

indicate a range of photomultiplier voltage

that would be satisfactory for this range of

temperature.

In scintillation counting applications

where gamma-ray energy resolution is im-

portant, the plateau concept is not par-

ticularly applicable. Direct measurement of

pulse-height resolution would be more

use-

Photomultiplier Handbook

ful. But in special applications such as the

oil-well logging, the plateau characterizes the

useful range of operating voltage.

LIQUID SCINTILLATION COUNTING

Liquid scintillators are most commonly

used for the evaluation of low-energy beta

emitters because of the generally short range

of beta particles and the need to provide an

intimate contact between the source and the

scintillator

.88,89

There are numerous fluors

or solutes which may be used, such as PPO

(2,5 diphenyloxazole). A common solvent is

toluene which must be miscible with the li-

quid radioactive sample. The emission of the

typical liquid scintillator is in the near ultra-

violet and blue spectrum so that a photomul-

tiplier having a“bialkali” photocathode

(K2CsSb)

provides a good spectral match.

Counting Techniques

When liquid-scintillation counting was

first introduced, the technique involved the

use of a single photomultiplier to observe the

liquid scintillator .Because of the low

energies involved and the need to measure

samples of relatively low activity, the

background count of the photomultiplier

was a severe limitation. An improved ap-

proach is the use of a pair of photomultiplier

tubes facing a common scintillator chamber.

Because of the number of photons involved

when a scintillation occurs in the sample, it is

quite likely that both photomultipliers will

sense the flash, thus permitting the use of

coincidence circuitry. Events that initiate

background counts in one photomultiplier

are only infrequently coincident with those

in the second tube and so a high degree of

discrimination against background counts is

possible.

The three most commonly used radioiso-

topes in liquid-scintillation counting are

tritium,

3H;

carbon, 14C; and phosphorus,

32P.

The beta-ray energies from these radio-

isotopes may vary over a fairly broad range;

for example,

emits betas having energies

varying from zero to a maximum of 18.6

keV.

Maximum beta-ray energies for 14C

and

32P

are 156

keV

and 1.71

MeV

respec-

tively. Fig. 84 illustrates the distribution of

pulse heights (which are proportional to

energy) for tritium. (The average energy is

about 6

keV.)

A block diagram of a liquid-scintillation

spectrometer is shown in Fig. 85. If a photo-

multiplier having a fast time characteristic is

used in the spectrometer, the coincidence

resolving time may be as small as 10 nano-

seconds.

Fig. 84

Pulse-height spectrum representing

the beta energy for tritium (18.6

keV

max-

imum) from a liquid-scintillation counter.

Maximum and minimum discriminator levels

are indicated (From

E.D.

Bransome

88)

92cs-32422

Fig. 85

Block diagram of a liquid-scintilla-

tion spectrometer.

Photomultiplier

Characteristics

Counter Efficiency

A conventional figure of merit for a liquid

scintillation coincidence counter is the ratio,

E2/B, where E is the counting efficiency ex-

pressed in per cent, and B is the number of

background coincident counts per minute.

The efficiency, E, is measured utilizing a

standard sample whose disintegration rate is

known. The recorded count rate is compared

with that of the standard, taking into ac-

count the exponential decay in the disinte-

gration rate. The background count value,

B, is measured by utilizing a blank sample of

the scintillator .

Note that E2/B is proportional to the

square of a signal-to-noise figure for the

system. Thus, the efficiency E is propor-

tional to the signal; and the square root of B

is the expected standard deviation in the

statistical count.

Efficiencies of the scintillators and the

photomultipliers in

liquid-scintillation-

counting equipment (using the most efficient

liquid scintillator and photocathodes) are

such that about 2.5 photoelectrons are emit-

ted per

keV

of energy of the beta ray. The

best counting efficiency for a given radioiso-

tope is obtained when a highly efficient

scin-

tillator and a photomultiplier having a high

quantum efficiency in the spectral emissivity

range of the scintillator are used. The optical

system containing the two photomultipliers

and the counting vial is also of major impor-

tance. The system should be designed so

that, as far as possible, the photons pro-

duced in a scintillation are equally divided

between the photomultipliers. This division

assures that a coincidence pulse results from

as many scintillations as possible. In some

cases two or more different isotopes may be

counted simultaneously. It is desirable,

therefore, that the photomultipliers have

matched gains and good energy

(pulse-

height) resolution capability to provide best

isotope separation. A typical value for E is

about 60%. Quantum efficiency of the

bialkali photocathode for the scintillation

radiation is approximately 25 %.

Sources of Background Counts

There are various sources of the coin-

cident background counts. One source is the

random dark-noise pulses in each of the two

photomultipliers which are occasionally

coincident. The coincident rate may be

predicted by the following equation:

(29)

where C is the chance coincidence rate per

minute,

is the dark-noise count rate in

counts per minute from tube No. 1,

is the

dark-noise count rate in counts per minute

of the coincidence circuit in seconds. In a

liquid-scintillation spectrometer employing

two tubes each having dark-noise rates of

30,000 counts per minute each and a coin-

cidence circuit having a resolving time of 10

nanoseconds, the number of accidental coin-

cidences is approximately 0.3 count per

minute.

Another source of background count is

cross talk between the two photomultipliers

as a result of light flashes in one tube which

are sensed by the other.

Cosmic rays and other natural radiation

can result in flashes in the scintillator or

possibly in the photomultiplier envelope.

Shielding the equipment with lead can

reduce the background from these sources.

However, the photomultipliers themselves

also contain radioactive isotopes. A com-

mon contaminant is 40K, a naturally oc-

curring isotope of potassium (0.1 %), that is

present in many glasses. Photomultipliers

designed for use in liquid-scintillation

counters may utilize quartz face plates or

thin face plates of a glass having a minimum

potassium content and with a low yield of

scintillations from gamma rays.

When a vial filled only with a scintillator is

placed between the photomultipliers, and the

output from the coincidence circuit is ex-

amined by use of a multichannel analyzer, a

pulse-height distribution such as that shown

in Fig. 86 is obtained. Clearly, not many of

the background pulses shown are caused by

the accidental coincidences of dark-noise

pulses from the photomultipliers, but are

caused by cosmic rays of scintillations in the

material of the vial and photomultiplier

envelope resulting from the presence of

radioisotopes in the materials of which they

are constructed.

Typical values for B, the number of

background counts,in coincidence

liquid-

Photomultlpller

Handbook

scintillation counters are in the range 15 to

20 counts per minute, and

E2/B

values are

typically in the range 150 to 250 for tritium.

92CS

32423

Fig. 86

Background distribution obtained

from a liquid scintillator using a coincidence

system.

Photomultiplier Selection for

Liquid Scintillators

The

E2/B

figure of merit is, however, not

necessarily the best way of evaluating all

systems. It is valid at very low counting rates

where the background count is the dominant

factor, but is not of great help at

high-

counting rates where the background count

of the system becomes less important than

the efficiency in determining the merit of the

system.

In selecting a photomultiplier tube for a

liquid-scintillation application, the following

items are of major importance: high photo-

cathode quantum efficiency, low dark-noise

count rate, minimum internal-light

genera-

tion, low scintillation-efficiency envelope,

fast time response, and good energy resolu-

tion. The 4501V3 photomultiplier has been

specifically designed to meet these require-

ments.

ENVIRONMENTAL EFFECTS

Although photomultiplier tubes are not

overly sensitive to their environment and can

be handled without exercising undue cau-

tion, there are various considerations the

user should be aware of in order to maximize

tube life, avoid permanent damage, and pro-

vide the optimum performance. This section

provides guidance relating to specific

envi-

ronmental limitations

timizing performance

tions.

Temperature

or to means for

op-

under adverse

condi-

The maximum temperature to which a

photomultiplier should be exposed during

either operation or storage is determined

primarily by the characteristics

the photo-

cathode. Temperature ratings vary with tube

type depending upon the particular photo-

cathode. For a specific type, its published

data should be consulted. In general, how-

ever, a temperature of 75 °C should not be

exceeded. Excessive exposure to higher

temperatures will alter the delicate balance

of chemicals of the photocathode and result

in a loss of responsivity and/or a shift in

spectral response.Another reason for

avoiding operation of photomultipliers at

elevated temperatures is that it causes an in-

crease in the dark current. (See Fig. 16.)

There are also hazards in cooling photo-

multiplier tubes to reduce the dark emission

from the photocathode. Rapid cooling

should be avoided to minimize thermal

shock which could result in cracking of the

bulb or stem. Temperatures below

50°C

should be avoided because the difference in

expansion between the glass and the base

material could result in cracking of the glass.

This hazard is also present for photomulti-

plier tubes having metal envelopes or for

tubes having stiff lead glass stems when

socketed in a material of different expansion

coefficient.

In the cooling of photomultiplier tubes

care must also be taken to avoid moisture

condensation. One method is to provide a

dry atmosphere for the tube and, especially,

its base and socket. Another method is to

cover the critical areas of the tube with a

silicone rubber such as General Electric

RTV-11. The tube should first be cleaned

and then primed (4004 primer for RTV-11)

before the silicone rubber coating is applied.

Differential cooling in which only the pho-

tocathode end of the tube is cooled is not

recommended. This practice may result in

loss of photocathode sensitivity because of

the vapor transport of alkali metals from the

dynodes to the photocathode.

Voltage

The published data for photomultiplier

tubes contain information on the maximum

supply voltage as well as information on the

specific maxima for voltages between adja-

cent electrodes such as between dynode No.

1 and the photocathode. Too high a voltage

between closely spaced electrodes can result

in electrical breakdown, particularly across

insulator surfaces. The over-all tube voltage

maximum is determined by considerations of

the maximum gain which a tube can tolerate.

Depending upon the construction of the

photomultiplier, at some gain in excess of

107 or 10 8, there may be sufficient feed-

back-perhaps by light generated in the out-

put section of the tube-to result in a sus-

tained electrical current. This current

breakdown, if sufficiently large, can per-

manently damage the tube by causing an in-

crease in the dark current and a decrease in

responsivity .

Ground potential and shielding of the

photomultiplier can also be a problem. In

order to minimize regenerative effects it is

generally recommended that the wall of the

photomultiplier envelope be maintained at a

potential at or near cathode potential. This

recommendation offers no problem in a cir-

cuit in which the cathode is at ground poten-

tial. But when the anode is at ground poten-

tial, it may be advisable for safety reasons to

provide a double shield: the inner shield at

cathode potential, surrounded by an in-

sulator, and an outer shield at ground poten-

tial. The inner shield should be maintained

at cathode potential through a high resis-

tance (usually 5 to 10 megohms) to avoid

hazard through accidental contact or

breakdown of the insulation.

It is strongly recommended that no dif-

ference of potential be maintained between

the semitransparent photocathode layer of a

photomultiplier and its outer glass faceplate.

A potential on the outside of the glass that is

positive with respect to that of the photo-

cathode can result in damage to the photo-

cathode by ionic transport through the glass

faceplate.69 (See section on “Shielding”

earlier in this Chapter.)

In supplying the tube voltage by means of

a resistive voltage divider, care must be

taken to avoid physical contact between any

of the resistors. Such contact can cause

miniature electrostatic discharges resulting

in noise spikes in the photomultiplier output.

Photomultlpller

Characteristics

Light Level

It is generally advisable to store photomul-

tiplier tubes in the dark and to avoid ex-

cessive exposure of the tubes to any light rich

in blue or ultraviolet such as that from fluor-

escent lamps or sunlight. Exposure of the

photocathode to such radiation will general-

ly cause a very substantial increase in the

dark current originating from the photocath-

ode. Complete recovery from this effect may

take as long as several days. See Fig. 47. Ex-

posure of the photocathode to sunlight

even

though no voltage is being applied may also

result in photocathode response changes

such as loss of infrared response in a tube

having a multialkali photocathode.

Before a photomultiplier tube is used for

critical low-light-level measurements, it is

recommended that it be aged for a period of

24 hours in the dark with voltage applied.

This precaution is particularly recommended

after a long period of idleness even if the

tube has not been exposed to ambient

lighting.

When the photomultiplier has the ap-

propriate voltage applied, it is of course pru-

dent to avoid any excessive light exposure.

Particularly, if the voltage divider has a

substantial current flow, an excessive current

may flow in the photomultiplier which could

result in loss of gain and an increase in dark

current. Photomultiplier data sheets gener-

ally contain ratings of the maximum anode

current which may be drawn.

Some semitransparent photocathodes are

very resistive. See Fig. 36. Even though the

light flux on the photocathode may be

relatively small, it is possible that the drop in

voltage across the photocathode between the

electrical contact and the light spot may in-

hibit the photoelectron current flow and

cause a non-linear behavior that could upset

careful measurements.

When measurements are made at very low

light levels, it is important that the tube be

totally shielded from unwanted stray light to

avoid an increase in background noise out-

put. A person’s photopic vision is not a good

judge of the presence of light leakage that

the photomultiplier might sense readily. For

example, light may leak through an

open-

ended coaxial cable connector such as BNC

or through certain bases or sockets. It may

be noted that light-tight caps are generally

Photomultlpller

Handbook

commercially available to terminate unusual

coaxial connectors.

Magnetic Fields

Care should be exercised to keep the

magnetic field environment of a photomulti-

plier to a minimum. The electron optical

operation of a photomultiplier can be con-

siderably altered by magnetic fields, as

shown in Fig. 51. It is also possible to induce

a permanent magnetization of some photo-

multiplier parts such as dynodes or dynode

side rods constructed of nickel. If

magnetization occurs, degaussing may readi-

ly be accomplished by placing the tube at the

center of a coil operated at an alternating

current of 60 Hz with a maximum field

strength of 8000 ampere turns per meter and

then gradually withdrawing the tube from

the coil.

Magnetic shields are generally available

commercially for photomultiplier tubes of

various constructions. In providing such

protection, it may be important to use a

shield which extends beyond the semitrans-

parent photocathode a distance of at least

half the diameter of the photocathode.

Atmosphere

In some applications it may be necessary

to operate the photomultiplier in other than

normal atmospheric pressure. Most photo-

multipliers will tolerate pressures to three at-

mospheres. For the larger tubes, however,

this pressure may present an implosion

hazard. In these special cases, it would be

well to check with the tube manufacturer.

Pressures less than one atmosphere also pre-

sent a problem in that electrical breakdown

can more readily occur. In such cases it may

be necessary to coat all exposed connections

and wiring with an insulator such as silicone

rubber.

Corrosive atmospheres must be avoided,

especially on photomultipliers having metal

envelopes. Corrosion could destroy the

glass-to-metal seal and result in loss of

vacuum.

High-humidity conditions should be

avoided if possible because condensation on

the base and socket can result in additional

electrical leakage or even breakdown. If the

moisture situation is unavoidable, it may

again be advisable to coat exposed connec-

tions with an insulator such as silicone

rub-

ber as described earlier under “Tempera-

ture.”

Again, a word of caution about the prob-

lem of helium penetration of glass. Even at

room temperature, if there is a significant

helium content in the ambient photomulti-

plier atmosphere, some helium will penetrate

the glass envelope. The result is an increase

in dark current and after pulsing because of

ionization of the helium gas. Eventually,

with sufficient helium pressure a complete

electrical breakdown can occur with voltage

applied to the photomultiplier tube.

If helium cannot be avoided, it may be

desirable to use a separate enclosure for the

photomultiplier through which a small flow

of purging gas such as dry nitrogen is pro-

vided. It is also

reported90

that helium

penetration can be blocked by a thin layer of

an epoxy-Epon 828 resin (registered trade-

mark of Shell Chemical Company) and

Belsamid 125 hardening agent (registered

trademark of General Mills Incorporated).

Shock and Vibration

Most photomultipliers will survive only a

reasonable amount of shock or vibration

(less than 10-g shock, depending on shock

duration and direction). Although special

photomultipliers have been designed to sur-

vive in extreme environments (shock values

from 30 to 1500 g), the user should make

every effort to avoid excessive shock or vi-

bration, possibly by the use of special

vibration-isolation fixtures. The photomulti-

plier tube should be handled as the delicate

instrument that it is. Excessive shock or

vibration can actually cause physical damage

to the tube to the point of shorting out some

of the elements or even resulting in breakage

of the envelope and loss of vacuum. A lesser

degree of shock may cause deformation of

the tube elements and can result in loss of

gain or deterioration of other performance

parameters. If measurements are being made

while a tube is vibrated, it is likely that the

output will be modulated by the vibration

not only because the light spot may be

deflected to different positions on the

photocathode but also because some of the

dynodes may actually vibrate and cause

modulation of the secondary-emission gain.

Many photomultipliers have been de-

signed for use under severe conditions of

shock and vibration and, in many cases,

spe-

cifically for use in missile and rocket applica-

tions. Such tubes, however, find uses in

many other applications including oil-well

logging or other industrial control applica-

tions where the tube may be subjected to

rough usage. These tubes are available with

most of the electrical and spectral character-

istics typical of the more general-purpose

types. These types differ primarily in

mechanical construction in that additional

supporting members may be employed and

an improved cathode connection may be

used to assure positive contact when the tube

is subjected to these environments.

Rug-

gedization of tubes using the glass envelope

has also been accomplished by moving the

dynode cage close to the stem (thereby

drastically shortening the lead lengths and

raising their mechanical resonant frequen-

cy), by using heavier leads and extra spacers

to hold the dynode cage in place, and by a

special heavy-duty welding process on the

metal-to-metal joints.

Tubes recommended for use under severe

environmental conditions are usually de-

signed to withstand environmental tests

equivalent to those specified in the ap-

plicable portions of MIL-E-5272C or

MIL-

STD-810B

in which the specified accelera-

tions are applied directly to the tubes.

Sinusoidal vibration tests are performed on

apparatus that applies a variable sinusoidal

vibration to the tube. The sinusoidal fre-

quency is varied logarithmically with time

from a minimum to a maximum to a mini-

mum value. Each tube is vibrated in each of

the three orthogonal axes.

Random vibration tests are performed by

subjecting the tube to a specified spectral

density (g2/Hz) in a specified frequency

band.

Shock tests are performed on an apparat-

us that applies a half-wave sinusoidal shock

pulse to the photomultiplier tube. The tube

is subjected to the shock in each of the three

orthogonal axes. The shock pulse is ex-

pressed in terms of the peak acceleration of

the pulse and the time duration of the pulse.

The tubes may receive more than one shock

pulse in each of the orthogonal axes.

Nuclear Radiation

In specialized applications such as the use

of photomultiplier tubes in satellites, the

Photomultiplier Characteristics

tubes may be subjected to high levels of

radiation such as occur in the Van Allen

belts. A summary of the effects of radiation

on photomultiplier tubes may be found in a

paper by S.M. Johnson, Jr.90a. Temporary

effects of intense radiation are principally an

increase in background current and noise.

Continued exposure, however, will also

cause permanent damage to the face-plate

glass, and to a much lesser extent the photo-

cathode, and the dynodes.

The origin of the increased background

current in photomultipliers exposed to

nuclear radiation is a fluorescence or scin-

tillation in the glass faceplate that causes

electron emission from the photocathode.

For example, irradiation with 60Co gamma

rays produces the equivalent of 10-12

W/cm2 of 420-nm flux for an input of 10

rad/s.

The major damage to a photomultiplier is

the browning of the faceplate glass. Signifi-

cant changes in transmission are observed

for 9741 and 7056 glass with an exposure of

105

rads of 60Co gamma radiation. Fused

silica shows much less change for the same

irradiation. Lime glass is reported to be very

susceptible to browning. Sapphire is the least

degraded by gamma radiation of windows

used in photomultipliers. Of the far-ultra-

violet windows,

LiF

is reported to be very

susceptible to radiation damage.

MgF2

recommended down to the Lyman-alpha

wavelength level in the presence of high

radiation exposures.

Although most of the damage to photo-

multipliers from nuclear radiation is in the

faceplate, some change may be expected in

the photocathode. The damage is not great

compared with that to the glass, probably

because of the very small absorption of the

very thin photocathodes. The same may be

said for dynode material where, again, the

surface layer is most important.

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Spicer

and F. Wooten, “Photo-

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H. Martin, Cooling photomultipliers

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Ramberg,

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1091-2 (June 1966).

51. H. Hora,“Experimental evidence of

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52. W.D. Gunter, Jr., E.F. Erickson, and

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512 (April

1965).

53. J.R.

Sizelove

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443-446,

(March 1967).

54.

J.B. Oke and R.E.

Schild,

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multiple reflection technique for improving

the quantum efficiency of photomultiplier

tubes,”Appl. Opt.,Vol. 7, No. 4, p.

617-622 (Apil 1968).

55. W. D. Gunter, Jr., G.R. Grant, and

S.A. Shaw,“Optical devices to increase

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Opt.,

Vol. 9, No. 2, p. 25 l-257 (Feb. 1970).

56. D.P. Jones,“Photomultiplier sensitiv-

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photocathode,”

Appl. Opt.,

Vol. 15, No. 4,

p. 910-914 (April 1976).

57. F.A. Helvy and R.M. Matheson,

“Photosensitive cathode with closely adja-

cent light-diffusing layer,” U.S. Patent

3 242,626, (Mar. 29, 1966).

58. R.W. Engstrom, “Improvement in

photomultiplier and TV camera tubes for

nuclear medicine,”

IEEE Trans.

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Sci.,

Vol. NS-24, No. 2, p. 900-903 (Apil 1977).

59. W. Widmaier and R.W. Engstrom,

“Variation of the conductivity of the semi-

transparent cesium-antimony photo-

cathode,”RCA Rev., Vol. 16, No. 1, p.

109-115 (March 1955).

60.

H.A. Zagorites and D.Y. Less, “Gam-

ma and X-ray effects in multiplier photo-

tubes,” Naval Radiological Defense

Laboratory, USNRDL-TR-763, (7 July

1964).

61. W. Viehmann, A.G. Eubanks, G.F.

Pieper,

and J.H. Bredekamp, “Photomulti-

plier window materials under electron ir-

radiation: fluorescence and phosphores-

cence”Appl. Opt., Vol. 14, No. 9, p.

2104-2115 (Sept. 1975).

62. R.B. Murray and J.J. Manning,

“Response of end-window photomultiplier

tubes as a function of temperature,” IRE

Trans.

Nucl.

Sci., Vol. NS-7, No. 2-3, p.

80-86 (June-Sept . 1960).

63. A.T. Young,“Temperature effects in

photomultipliers and astronomical photo-

metry,”Appl. Opt., Vol. 2, No. 1, p.

5 l-60-(Jan. 1963).

64.

R.E. Rohde, “Gain vs. temperature ef-

fects in

NaI(T1)

photomultiplier scintillation

detectors using 10 and 14 stage tubes,” Nucl.

Intr. and Methods, Vol. 34, p. 109-115

(1965).

65. D.F. Cove11 and B.A. Euler, “Gain

shift versus counting rate in certain

multiplier phototubes,” USNRDL-TR-521,

U.S. Naval Radiological Defense

Laboratory, San Francisco (1961).

66. C.S. Wiggins and K. Earley,

“Photomultiplier refrigerator,” Rev. Sci.

Instr., Vol. 33, p.

1057-8

(1962).

67. For example, among others: Products

for Research, Inc., 78 Holten Street,

Danvers, Massachusetts 01923

(617-774-3250).

Pacific Precision Instruments, 1040

Shary Court, Concord, California 945 18

(415-827-9010).

68.

H.R.

Krall,

“Extraneous light emission

from photomultipliers,” IEEE Trans.

Nucl.

Sci. Vol. NS-14, No. 1, p. 455-9 (Feb.

1967).

69.

Louis Lavoie, “Photomultiplier cath-

ode poisoning,”Rev. Sci. Instr., Vol. 38,

No. 6, pp 833-4, (1967).

70. G.A. Morton, H.M. Smith and R.

Wasserman,

“Afterpulses in photomulti-

Photomultlpller

Characteristics

pliers,” IEEE

Trans.

Nucl. Sci.,

Vol. NS-14,

No. 1, pp 443-448,

(1967).

71. V.O. Altemose, “Helium diffusion

through glass,

”

J. Appl. Phys., Vol. 32, No.

7, pp 1309-1316,

(1961).

72. W.C. Paske,“He + afterpulses in

photomultipliers: Their effect on atomic and

molecular lifetime determinations,” Rev.

Sci.

Instr., Vol. 45, No. 8, p.

1001-3

(1974).

73. A.T. Young,“Cosmic ray induced

dark current in photomultipliers,” Rev. Sci.

Instr., Vol. 37, No. 11, pp 1472-1481 (1966).

74. D. Sard, “Calculated frequency spec-

trum of the shot noise from a

photo-

multiplier tube,”J. Appl. Phys., Vol. 17, p.

768-777 (1946).

75. B. Leskovar and C.C. Lo, “Transit time

spread measurements of microchannel-plate

photomultipliers,”

European Conference on

Precise Electrical Measurements,

Brighton,

Sussex, England, 5-9 Sept. 1977 (London,

England: IEEE

(1977),

pp 41-3.)

76. D.A. Gedcke and W.J. McDonald,

“Design of the constant fraction of pulse

height trigger for optimum time resolution, ”

Nucl.

Instr. and Meth., Vol. 58, No. 2, pp

253-60

(1968).

77. M. Birk, Q.A. Kerns, and R.F. Tusting,

“Evaluation of the

C-70045A

high-speed

photomultiplier,”IEEE Trans.

Nucl.

Sci.

Vol. NS-11, No. 3, pp 129-138

(1964)

78. F. de la Barre,

“Influence of transit time

differences on photomultiplier time resolu-

tion,” IEEE Trans.

Nucl

Sci., Vol. NS-19,

No. 3, pp 119-121 (1972).

79. B. Leskovar, C.C. Lo, “Performance

studies of photomultipliers having dynodes

with

GaP(Cs)

secondary emitting surface,”

IEEE Trans.

Nucl.

Sci., Vol. NS-19, No. 3,

pp 50-62 (1972).

80. S.K. Poultney,“Single Photon Detec-

tion and Timing: Experiments and Techni-

ques,”

Advances in Electronics and Electron

Physics,

Vol. 31, Academic Press, Inc., New

York, (1972).

81. E. Gatti and V. Svelto, “Review of

theories and experiments of resolving time

with scintillation counters,”

Nucl.

Instr. and

Methods,

Vol. 43, pp 248-268, (1966).

82. W.A. Baum,“The detection and mea-

surement of faint astronomical sources,”

Astronomical Techniques,

Edited by W .A.

Hiltner, The University of Chicago Press,

(1962).

83. G.A. Morton,“Photon Counting,”

Appl. Opt., Vol. 7, No. 1, pp l-10, (1968).

84. R.M. Matheson, “Recent photomulti-

plier developments at RCA,” IEEE Trans.

Nucl.

Sci. Vol. NS-11, No. 3, pp 64-7 1,

(1964).

85. G.A. Morton, H.M. Smith, and H.R.

Krall, “Pulse-height resolution of high gain

first dynode photomultipliers,” Appl. Phys.

Lett.,

Vol. 13, No. 10, pp 356-7, (1968).

86. G.H. Narayan and J.R. Prescott,

“Line-widths in

NaI(Tl)

scintillation count-

ers for low energy gamma-rays,” IEEE

Trans.

Nucl.

Sci.,Vol. NS-13, No. 3, pp

132-7, (1966).

87. R.W. Engstrom and J.L. Weaver, “Are

plateaus significant in scintillation count-

ing?” Nucleonics, Vol. 17, No. 2, pp 70-74,

(1959).

88. The Current Status of Liquid-

Scintillation Counting,

Edited by Edwin D.

Bransome, Grune and Stratton, New York

and London, (1970).

89. Liquid-Scintillation Counting, Recent

Developments, Edited by Philip E. Stanley

and Bruce A. Scoggins, Academic Press

(1974).

90. Yasunori Nikaido et al, “Electron tube

for photoelectric conversion,” Patent

Publication No. SHO 42-16002 (Shimizu Pa-

tent Office, Fukiai-Ku, Kobe, Japan).

90a. S.M. Johnson, Jr., “Radiation effects

on multiplier phototubes,” IEEE Trans.

Nucl.

Sci., Vol. NS-20, No. 1, pp 113-124,

Feb. 1973.

Photomultiplier Handbook

Photomultiplier

Applications

SUMMARY OF SELECTION

CRITERIA

A comparison of the relative advantages

of photomultipliers and solid-state detectors

is given in the introductory chapter of this

manual. The following is a summary of the

application requirements that indicate when,

in general, a photomultiplier is the most

suitable detector.

The most important consideration is the

light or radiation level to be detected. The

use of photomultipliers is recommended

when the level of light flux is very low. If

levels are relatively high, it may be simpler to

use a solid-state detector. Furthermore, a

high level of light flux could overload and

damage the photomultiplier. A photomulti-

plier may be used if the light flux is 100

microlumens or less or of the order of 0.1

microwatt or less at the peak of the spectral

response.

When a fairly large detector area is re-

quired, a photomultiplier is also recom-

mended, as is the case in scintillation count-

ing where a crystal scintillator may be several

centimeters in diameter. Silicon avalanche

photodiodes have good low-light-level

capability but their area is generally limited

to a few square millimeters.

Spectral response of the photomultiplier,

of course, must be a reasonable match to

that of the source of radiation. Photomulti-

pliers are useful in the range 120 nanometers

to 1100 nanometers, depending upon the

type of photocathode and window material.

Responses further in the infrared than

1100

nanometers require an infrared photocon-

ductive

detector or some other infrared-

sensitive device.

When a fast response time is an important

requirement, the photomultiplier is usually

the most suitable detector. Photomultiplier

tubes have response-time capability down to

the nanosecond range and even better in the

case of specially designed tubes.

APPLIED VOLTAGE

CONSIDERATIONS

Proper operation of a photomultiplier

depends critically upon the applied voltage

and the voltage distribution to the dynode

stages. A small variation in voltage can

result in a much larger percentage change in

the tube gain. For example, see Fig. 49 which

shows the log of the gain as a function of the

log of the voltage. For a 9-stage tube the

slope of the log-gain vs log-voltage curve is

approximately 7; for a 14-stage tube the

slope is about 12. (If the secondary emission

ratio were linear with voltage, see Fig. 19,

one would expect the slope of the log-gain vs

log-voltage curves to be equal to the number

of stages.) For a 12-stage tube, a 1% change

in voltage results in a 10% change in gain.

Thus, there is need for more than ordinary

precaution to provide a well-regulated power

supply

Power Supply, Regulation, Polarity, and

Shielding

Commercial power supplies are generally

available that have a line-regulation of

0.005% with some available at 0.001% for a

10% change in line voltage when working in-

to a fixed divider network. Voltage variation

with temperature might be of similar magni-

tude for a 1 °C change in temperature. It

would not be too difficult, therefore, to pro-

vide voltage supplies that would result in

gain stability of the photomultiplier tube due

to line voltage and temperature changes of

better than 0.1%. For requirements not this

critical, BURLE manufactures a simplified

compact power supply that includes a socket

and voltage-divider network for 1-l/8inch

diameter, 9-stage, side-on photomultipliers.

Also manufactured by BURLE are Inte-

grated Photodetection Assemblies

(IPA’s)

that package a photomultiplier tube, optical

filter, power supply, signal-conditioning

amplifier, and electrostatic/magnetic

shielding. The IPA operates from a

12-volt

dc supply.

The recommended polarity of the photo-

multiplier power-supply voltage with respect

to ground depends largely on the application

intended. Of course, the cathode is always

negative with respect to the anode. In some

pulsed application, however, such as scin-

tillation counting, the cathode should be

grounded and the anode operated at a high

positive potential with a capacitance-coupled

output. In this case the scintillator and any

magnetic or electrostatic shields should also

be connected to ground potential.

In applications in which the signal cannot

be passed through a coupling capacitor, the

positive side of the power supply should be

grounded. The cathode is then at a high

negative potential with respect to ground.

When this arrangement is used, extra pre-

cautions must be taken in the mounting and

shielding of the photomultiplier. If there is a

potential gradient across the tube wall, scin-

tillations occurring in the glass will increase

dark noise. If this condition continues for a

sufficiently long period of time, the photo-

cathode will be permanently damaged by the

ionic conduction through the glass. To pre-

vent this situation when a shield is used, the

shield is connected to photocathode poten-

tial. Light-shielding or supporting materials

used in photomultipliers must limit leakage

currents to 10-

ampere or less. Fig. 62

shows a curve of the effect of external-shield

potential on photomultiplier noise.

To reduce the shock hazard to personnel,

a very high resistance should be connected

between the shield and the negative high

voltage.

Voltage Divider Design

The interstage voltage gradients for the

photomultiplier elements may be supplied by

individual voltage sources. The usual source,

however, is a resistive voltage divider placed

across a high-voltage source, as shown in

Fig. 87.

A resistive voltage divider must be de-

signed to divide the applied voltage equally

or unequally among the various stages as re-

quired by the electrostatic system of the

tube. The most common voltage between

stages is usually referred to as the stage vol-

tage and the voltage between other stages as

Photomultiplier Applications

92CS

32424

Fig. 87

Schematic diagram of a resistive

voltage divider.

The following formula may be used to

calculate the voltage between stages:

The voltage-divider resistor values re-

quired for each stage can be determined

from the value of the total resistance re-

quired of the voltage divider and the voltage-

divider ratios of the particular tube type.

Photomultiplier Handbook

The interstage resistance values are in pro-

portion to the voltage-divider ratios as

follows:

where

is the resistance between elements

Dyj

and

Dyj.

The recommended resis-

tance values for a photomultiplier voltage

divider range from 20,000 ohms per stage to

megohms per stage; the exact values are

usually the result of a compromise. If low

values of resistance per stage are utilized, the

power drawn from the regulated power sup-

ply may be excessively large. The resistor

power rating should be at least twice the

calculated power dissipation to provide a

safety margin and to prevent a shift in

resistance values as a result of overheating.

The highest suitable value of stage resistance

(after consideration has been given to

average anode current as described below) is

dictated by leakage currents in the photo-

multiplier and socket wiring.

One criterion for the selection of a suitable

range of voltage-divider resistance values is

the expected maximum anode current that

may be drawn from the photomultiplier.

When the anode current is of the same order

of magnitude as the divider current, non-

linear response results. This non-linearity is

illustrated in Fig.88 which shows the

response of a 931A photomultiplier as a

function of light level using a conventional

voltage divider such as shown in Fig. 87 with

equal voltage per stage.91 (The value of

was essentially zero for this measurement.)

The anode current is shown relative to the

divider current at zero light level. The

superlinearity region is explained by a

change which takes place in the voltage

distribution between stages from uniform to

non-uniform as the light level is increased.

Thus, the electron current flow from the last

dynode to the anode causes less current to

flow through the voltagedivider resistor,

voltage between dynodes and a decrease in

the collection voltage between the last

dynode and the anode. The reduced collec-

tion voltage tends to reduce the output cur-

rent slightly, but the increased dynode

voltages more than compensate for this

reduction by an increased gain.

Fig. 88

The relative response of a 931A

pho-

tomultiplier as a function of the light flux us-

ing the circuit of Fig. 87 with equal stage

voltage (at zero light level). (From Engstrom

and

Fischer91)

The decrease in sensitivity that occurs

beyond region A of Fig. 88 results from the

extension of voltage losses to the last two or

three dynode resistors causing defocusing

and skipping in the associated dynode

stages. In order to prevent this loss and

assure a high degree of linearity, the current

through the voltage-divider network should

be at least ten times the maximum average

anode current required. In calculating the

voltage-divider current, the average anode

current must first be estimated; this estimate

requires knowledge of the value of the input

(light) signal and the required output (elec-

trical) signal.

Photo-multiplier noise or a shift in gain

may result from heat emanating from the

voltage-divider resistors. The divider net-

work and other heat-producing components,

therefore, should be located so that they will

not increase the tube temperature. Resis-

tance values in excess of five megohms

should be avoided because current leakage

between the photomultiplier terminals could

cause a variation of the interstage voltage.

The type of resistor used in a divider

depends on the dynode structure with which

the divider will be used. Close-tolerance

resistors, such as the laser-trimmed

metal-

film types, are normally required with the

focused structures. On the other hand, inter-

dynode voltages in the Venetian-blind struc-

tures can vary widely with but little effect on

the photomultiplier. For this reason, the

resistors used with a Venetian-blind structure

can be of a less stable variety, such as com-

position.

The over-all performance of a tube that

has a cathode-to-first-dynode region essen-

tially electrostatically isolated from the re-

maining dynode region can be improved by

maintaining a high electric field, in the

cathode-to-first-dynode region to reduce the

transit-time spread of photoelectrons arriv-

ing at the first dynode and minimize the ef-

fect of magnetic fields. A high first-dynode

gain, which implies a high

have the disadvantage of reducing the gain in

Intermediate Stages

In applications in which it is desirable to

control the anode sensitivity without

changing the over-all voltage, the voltage of

a single dynode may be varied. Fig. 89 shows

the variation of anode current for a 931A

photomultiplier when one of the dynode

voltages is varied while the total supply

voltage is held constant. The dynode should

be selected from the middle of the dynode

string because a variation of dynode poten-

tials near the cathode or anode would have a

detrimental effect on photomultiplier opera-

tion.

Operation with Fewer Stages

A photomultiplier need not be operated

with all dynode stages, although the design

of the tube has been optimized for this con-

dition. In some situations where the light

level being detected is so high that it would

Photomultiplier Applications

overload the output stages of the photomul-

tiplier, it is possible to eliminate these stages

from the circuit. For example, the last

several stages of the tube and the anode may

be tied together electrically to form an

anode. The tube is then operated with the

reduced gain of the remaining stages. In an

extreme case, the photomultiplier may be

operated just as a photodiode using only the

photocathode and several or all of the re-

maining elements together as an anode. It

must be realized in operating the photomul-

tiplier in such a manner that many photo-

cathode types are very resistive (see Figs. 35

and 36) and, therefore, the photocurrent will

not be linear with light at the high level

which may be available. Photocathodes of

the opaque type with a conductive substrate,

or the semitransparent type with a conduc-

tive undercoating will tolerate a much higher

light level without loss of linearity.

700800

92CS-32426

Fig. 89

The output-current variation of a

931A

when the voltage on one dynode (No. 6)

is varied while the total supply voltage re-

mains fixed.

Voltage Dividers for Pulsed Operation

In applications in which the input signal is

in the form of pulses, the average anode cur-

rent can be determined from the peak pulse

current and the duty factor. The total

resis-

Photomultiplier Handbook

tance of the voltage-divider network is

calculated for the average anode current.

In cases in which the average anode cur-

rent is much less than a peak pulse current,

dynode potentials can be maintained at a

nearly constant value during the pulse dura-

tion by use of charge-storage capacitors at

the tube socket. The voltage-divider current

need only be sufficient to provide the

average anode current for the photomulti-

plier. The high peak currents required during

the large-amplitude light pulses are supplied

by the capacitors.

The capacitor values depend upon the

value of the output charge associated with

the pulse or train of pulses. The value of the

final-dynode-to-anode capacitor C is given

where C is in farads, q is the total anode

charge per pulse in coulombs, and V is the

voltage across the capacitor. The factor 100

is used to limit the voltage change across the

capacitor to a l-per-cent maximum during a

pulse. Capacitor values for preceding stages

should take into account the smaller values

of dynode currents in these stages. Conser-

vatively, a factor of approximately 2 per

stage is used. Capacitors are not required

across those dynode stages at which the peak

dynode current is less than 1/10 of the

average current through the voltage-divider

network. For pulse durations in the 1 to 100

ns range, consideration should be given to

the inherent stage-to-stage capacitances

which are in the order of 1 to 3

pF.

Medium-Speed Pulse Applications

In applications in which the output cur-

rent consists of pulses of short duration, the

capacitance

of the anode circuit to

ground becomes very important. The capaci-

tance

is the sum of all capacitances from

the anode to ground: photomultiplier-anode

capacitance, cable capacitance, and the in-

put capacitance of the measuring device. For

pulses having a duration much shorter than

the anode time constant

RLCL,

the output

voltage is equal to the product of the charge

and 1/CL because the anode current is simp-

ly charging a capacitor. The capacitor charge

then decays exponentially through the anode

load resistor with a time constant of

RLCL.

To prevent pulses from piling up on each

other, the maximum value of

RLCL

should

be much less than the reciprocal of the

repetition rate.

An important example of this type of

operation is scintillation counting, for exam-

ple with NaI:Tl. The time constant of the

scintillations is 0.25 microsecond and, be-

cause the integral of the output current pulse

is a measure of the energy of the incident

radiation, the current pulse is integrated on

the anode-circuit capacitance for a period of

about 10 microseconds. Because the scin-

tillations occur at random, the maximum

average counting rate is limited to about 10

kHz.

If circumstances require a higher

counting rate, the integration time must be

reduced accordingly.

Fast-Pulse Applications

In fast-pulse light applications, it is

recommended that the photomultiplier be

operated at negative high voltage with the

anode at ground potential. A typical

voltage-

divider circuit with series-connected capaci-

tors is shown in Fig. 90. The parallel con-

figuration of capacitors may also be used, as

shown in Fig. 91. The parallel arrangement

requires capacitors of higher voltage ratings.

Regardless of the configuration, the

capacitors must be located at the socket. The

capacitor arrangements just described may

also be applied to negative-ground applica-

tions.

Fig. 90

Series-connected

voltage-divider circuit using

for pulse-light applications.

capacitors in

positive ground

The wiring of the anode or dynode

“pick-

off” circuit is very critical in pulse applica-

tions if pulse shape is to be preserved. Most

pulse circuitry uses

50-ohm

characteristic im-

pedance cables and connectors because of

Photomultiplier Applications

92cs-32428

Fig. 91

Parallel-connected capacitors in voltage-divider circuit for pulsed-light

ap-

plication.

their ready availability, although 75- and

92-ohm

components are also used. Careful

wiring is required. Figs 92 and 95 illustrate

schematically and pictorially the best loca-

tion of pulse bypass capacitors that return

the anode pulse current to

Dyn

and Dyn

by paths of minimum residual inductance. It

should be noted that these bypass capacitors

also serve the function of charge-storage

capacitors, as shown in Fig. 91.

92cs-32429

Fig. 92

Bypass capacitors used to make

the last two dynodes appear as a ground

plane to a fast-pulsed signal.

Wiring Techniques

Good high-frequency wiring techniques

must be employed in wiring photomultiplier

sockets and associated voltage dividers if

pulse-shape distortion is to be minimized.

Fig. 93 illustrates the pulse shape obtained

from an 8575 photomultiplier excited by a

0.5-nanosecond

rise-time light pulse. The

pulse shape is most easily seen with the aid of

a repetitive light pulser and a high-speed

real-time oscilloscope. Fig. 94 indicates the

general type of distortion encountered with

the use of improperly wired or excessively in-

ductive capacitors.The output pulse il-

lustrates the ringing that may occur in an im-

properly wired socket.

Fig. 93

Pulse shape obtained from

photo-

multiplier excited by a light pulse having an

0.5-nanosecond rise time.

92cs-32430

Fig. 94

Pulse shape distortion (ringing) en-

countered with improperly wired or excessive-

ly inductive capacitors.

Photomultiplier Handbook

Fig. 95 shows a socket wired for a negative

high-voltage application. The disk-type

bypass capacitors are mounted in series with

minimum lead length because the

self-

inductance of the lead wires becomes critical

in nanosecond-pulse work. Care should also

be taken in dressing the bypass capacitors

and coaxial cable.

The resistors for the

voltage divider are shown mounted on the

socket. In applications requiring minimum

dark current, the resistors should be remote

from the photomultiplier to minimize

heating effects.

92cs-32432

Fig. 95

Anode detail of socket wired for a

negative high-voltage application showing

location of charge-storage capacitors.

Checking Socket and Tube Performance

Some photomultipliers typically display a

reflected-pulse rise time of the order of 1.5

nanoseconds for an initiating pulse of the

order of 50

picoseconds

when the tube and

socket are tested with a time-domain

reflec-

tometer, the instrument used to test the

anode-pin region of the socket.

During tube operation, the output pulse

can be viewed with a real-time high-speed

oscilloscope. The pulse shape can be in-

spected for signs of clipping or ringing.

The effectiveness of the charge-storage

capacitors can be verified with a simple

linearity test. The output of a photomulti-

plot of pulse amplitude as a function of the

logarithm of the voltage value should yield a

straight line.

A two-pulse technique may be employed

to test linearity at constant operating

voltage. Consider two sequential pulses hav-

ing amplitude ratios in the range from 2:1 to

10:1. The ratio of the amplitude of the two

photomultiplier current pulses is established

at low pulse amplitudes where linear opera-

tion is assured. Next, the light flux is in-

creased in a series of increments. A constant-

current pulse amplitude ratio indicates linear

operation. A pair of light-emitting diodes

may be used to provide the two light pulses.

The over-all control of the light levels may

be done with a series of neutral-density

filters. Exact filter calibration is not required

because the system is self-calibrating in the

pulse current measurements.

Tapered Dividers

Some applications require that photomul-

tipliers sustain high signal currents for short

time intervals, tens of nanoseconds or less.

In general, photomultipliers are capable of

supplying 0.2 ampere or more into a

50-ohm

load for short durations. However, the vol-

tage divider must be tailored to the applica-

tion to allow a photomultiplier to deliver

these high currents.

The principal limitation on current output

(into a

50-ohm

system) is space charge at the

last few stages. This space charge can be

overcome if the potential difference across

the last few stages is increased by use of a

tapered divider rather than an

equal-volts-

per-stage divider. The tapered divider places

3 to 4 times the normal interstage potential

difference across the last stage. The pro-

gression leading to the 4-times potential dif-

ference should be gradual to maintain prop-

er electrostatic focus between stages; a pro-

gression of 1, 1 . . .1, 1, 1.5, 2.0, 3, 4.2 is

recommended.

Another application of the tapered divider

is to provide a large signal voltage across a

high-impedance load. Voltage excursions of

400 to 500 volts can be obtained from photo-

multipliers. A possible application is in driv-

ing an electro-optical modulator. Because

the photomultiplier is nearly an ideal con-

stant current source, its output voltage signal

is limited only by the potential difference

between the last dynode and the anode. If

the potential difference is 100 volts, the

anode cannot swing through more than a

100-volt excursion. By impressing a much

higher potential difference between the

anode and last dynode by means of a tapered

divider, greater voltage swings can be ob-

tained. Tube data sheets should be consulted

for maximum voltage ratings.

Dynamic Compression of Output Signal

Most photomultipliers operate linearly

over a dynamic range of six or seven

orders

of magnitude, a range few monitor devices

can accommodate without requiring range

changes. When compression of the dynamic

range is desired, a logarithmic amplifier is

sometimes used. The photomultiplier may

also be operated in a compressed-output

mode, however, without the need for addi-

tional compression circuitry.

For example,in liquidscintillation

counting where several different isotopes

may be present, the output pulse height

might normally vary by as much as 100: 1. By

limiting the potential difference between the

last dynode and anode to, for example,

volts, space charge will limit the maximum

current that can be drawn. The anode pulse

can then be used for the coincidence timing

with a more convenient range of pulse

heights. Energy measurements can then be

made using the current at an earlier dynode

where space-charge saturation is not present.

Current Protection of Photomultiplier

If a photomultiplier is accidentally ex-

posed to an excessive amount of light, it may

be permanently damaged by the resultant

high currents. To reduce this possibility, the

resistive voltage-divider network may be

designed to limit the anode current. The

average anode current of a photomultiplier

cannot much exceed the voltage-divider cur-

rent; therefore, the zero-light level

voltage-

divider network serves as an overload

pro-

ANODE

Photomultiplier

Applications

tection for the tube. If overexposure is ex-

pected frequently,interdynode currents,

which can be quite excessive, may cause loss

of gain. In some applications it may be

worthwhile to protect against dynode

damage by using resistors in series with each

dynode lead

.91

Active

Divider Network

Although normally, the divider network

current should exceed the maximum photo-

multiplier output current by a factor of 10 or

more, it is possible by using the

emitter-

follower characteristics of transistors to pro-

vide a power supply requiring much less

divider current. Fig. 96 is an example of such

an active divider network devised by C. R.

Kerns92 As the dynode current increases,

the added current is diverted from the

high-

beta transistors rather than from the

divider-

resistor string, thus improving the voltage

regulation by a large factor. The capacitors

shown are the usual ones for high-frequency

bypassing. The circuit also retains a current-

limiting action that prevents damage to the

photomultiplier.

MECHANICAL CONSIDERATIONS

Handling

Because most photomultipliers have glass

envelopes, they should be handled with care

to avoid damage to the tube seals and other

parts. This caution is especially important

for tube types having graded-seal envelope

construction. The pins or leads of the tube

should also be treated with care.

Fig. 96

An active divider network for an 8575 photomultiplier designed to minimize

voltage changes at the dynodes. (From C.R.

Kerns92)

Photomultiplier Handbook

Basing

Photomultiplier tubes may have either a

temporary or a permanently attached base.

Dimensional outline diagrams such as those

shown in Fig. 97 are provided in the pub-

lished technical data for individual tubes. In-

dicated on the diagrams are the type of base

employed, maximum mechanical dimen-

sions, radii of curvature where applicable,

pin/lead details, location and dimensions of

magnetic parts used (in tubes utilizing mini-

mum number of magnetic materials), and

notes regarding restricted mounting areas,

again where applicable.

Photomultiplier tubes intended to be

soldered directly to circuit boards or hous-

ings are supplied with semiflexible or “fly-

ing” leads and a temporary base, intended

for testing purposes only, that should be re-

moved prior to permanent installation.

A lead-terminal diagram that shows

photomultiplier-tube lead orientation with

the temporary base removed, shown in Fig.

98, provides a lead indexing reference. A

lead-connection diagram, such as the one

shown in Fig.

99,

relates terminal to elec-

trode. Care must be exercised in interpreting

basing and lead-terminal diagrams to insure

against possible damage to the photomulti-

plier resulting from incorrect connections.

Terminal Connections

BURLE photomultipliers are supplied with

either semiflexible leads, semiflexible leads

attached to temporary bases, permanently

PHOTOCATHODE

attached bases, or stiff leads. Semiflexible

leads may be soldered, resistance (spot)

welded, or crimp connected into the

associated circuitry. When soldering or

welding is employed for such connections,

care should be taken to prevent tube destruc-

tion due to thermal stress of the seals at the

stem. A heat sink, such as locking forceps,

should be placed in contact with the

semi-

flexible leads between the point being

soldered or welded and the tube seals. If

soldering is employed, only a soft solder

(e.g., 60% Sn, 40% Pb) should be used.

Heat should be applied only long enough to

permit the solder to flow freely. By the term

semiflexible, it is implied that excessive

bending may break the leads, most common-

ly at the stem surface. Some photomulti-

pliers are supplied with insulating wafers at-

tached to the stem to prevent such an occur-

rence. The semiflexible leads are normally

made of dumet or Kovar and are usually

plated to facilitate soldering.

Photomultipliers supplied with perma-

nently attached bases or stiff leads should

use only high-grade, low-leakage sockets to

minimize leakage currents between adjacent

electrode terminals. Teflon and mica-filled

sockets should be used.

Mounting and Support

Photomultipliers having permanently at-

tached bases normally require no special

mounting arrangements. When special

mounting arrangements are used, however,

92CM - 32434

Fig. 97

Typical dimensional-outline drawings showing the type of base supplied

with each tube and pertinent notes.

INDEX

Fig. 98

Lead-orientation diagram.

BOTTOM VIEWS

PIN I

DYNODE NO. I

PIN 2

DYNODE NO. 3

PIN 3

DYNODE NO.5

PIN 4

DYNODE NO.7

PIN 5

DYNODE NO. 9

PIN 6

ANODE

PIN 7

DYNODE NO.10

PIN 8

DYNODE NO. B

PIN 9

DYNODE NO. 6

PIN IO

DYNODE NO 4

PIN II

DYNODE NO. 2

PIN 12

PHOTOCATHODE

LEAD I

DYNODE NO. I

LEAD 2

DYNODE NO. 3

LEAD3

DYNODE

NO.

LEAD4

DYNODENO.

LEAD

DYNODE

NO.

LEAD6

ANODE

LEAD7

DYNODE NO.10

LEAD 8

DYNODE NO. 8

LEAD9

DYNODE NO.6

LEAD 10

DYNODE NO.4

LEAD II

DYNODE NO.2

LEAD 12

PHOTOCATHODE

92CS-32436

Fig. 99

Lead-connection diagram: (a) with

base connected, (b) with temporary base

removed.

the envelope, especially that region near the

photocathode, must be maintained at

cathode potential. Care should also be taken

so that tube performance is not affected by

extraneous electrostatic or magnetic fields.

Side-on photomultipliers should be mounted

to allow rotation of the tube about its major

axis to obtain maximum anode current for a

given direction of incident radiation. An

angular tolerance with respect to incident

light direction is normally specified in tube

data sheets.

Direct clamping with non-resilient

materials to the envelope of tubes not having

permanently attached bases is not recom-

mended nor should clamping be made to any

Photomultiplier Applications

metal flanges employed in the construction

of a tube. Such flanges, when present, are

part of the tube’s vacuum enclosure and any

undue force or stress applied to them can

damage the seals and destroy the tube.

The use of resilient potting compounds or

rubber washers is recommended when

pho-

tomultipliers are clamp-mounted. If a pot-

ting compound is used, its characteris-

tics-over the temperature range in which

the tube is to be operated-must be such that

its resilience is maintained at low

temperature and its expansion, in confined

space, is not excessive at high temperature.

The electrical insulation properties of any

materials supporting or shielding the photo-

multiplier should be considered. If such

materials come into contact with high

voltage with respect to photocathode,

minute leakage currents can flow through

the material and the tube envelope to the

photocathode. Not only does this condition

introduce excessive noise at the tube output

but it can also permanently damage the

photocathode sensitivity of the tube through

electrolysis of the glass envelope. This cau-

tion is only true when the tube is operated at

high negative potential with respect to

ground. Under this operating condition, a

decrease in sensitivity can occur if the

faceplate of the tube comes into contact with

ground. Cathode sensitivity does not recover

after such an occurrence. Photocathode

“poisoning” due to envelope electrolysis can

destroy the usefulness of a photomultiplier

in a very short time. Therefore, the in-

sulating property of materials supporting the

tube should be such that leakage current to

the tube envelope is limited to 1 x 10 -

ampere, or less.

Shielding

Electrostatic and/or magnetic shielding of

most photomultipliers is usually required.

When such shields are used and are in con-

tact with the tube envelope, they must

always be connected to photocathode poten-

tial.

In applications where the dc component of

the signal output is of importance, the

cathode is normally operated at high nega-

tive voltage with respect to ground. As a

result, the shield is at high negative voltage

and precautions must be taken to avoid

shorts to ground and to prevent shock

Photomultiplier Handbook

hazard to personnel. A 10-megohm resistor

should be placed between the negative high

voltage and shields to avoid such hazards.

In scintillation counting applications, it is

recommended that the photocathode be

operated at ground potential. In this case,

the shields should be operated at ground

potential.

Magnetic shielding of most photomulti-

pliers is highly desirable. Characteristic

curves showing the effects of magnetic fields

on anode current are provided in many data

sheets.

Storage

Photomultipliers should be stored in the

dark. Storage of tubes in areas where light is

incident on the tube results temporarily in a

higher than normal dark-current level when

the tubes are placed in operation. This in-

crease in dark current is primarily due to

phosphorescence of the glass and can persist

for about 24 hours. Additionally, storage of

tubes designed for operation in the near IR

region of the spectrum (above 700 nano-

meters) in illuminated areas may decrease

the “red” sensitivity of the tube.

The phototube should never be stored or

operated in areas where there are concentra-

tions of helium because helium readily

permeates glass. The composition of the

envelope material is a major factor govern-

ing the rate of helium permeation. As the

silica content in the glass is reduced, the rate

of permeation decreases. Accordingly, the

rate of permeation is greatest for fused silica

and decreases to a minimum in lime glass. It

is also to be noted that the rate of permea-

tion is proportional to temperature and

varies directly with

pressure.691

92a 92b

Moisture Condensation

A very small anode current is observed

when voltage is applied to the electrodes of a

photomultiplier in darkness. Among the

components contributing to this dark cur-

rent are pulses produced by thermionic emis-

sion, ohmic leakage between the anode and

adjacent elements,secondary electrons

released by ionic bombardment of the

pho-

tocathode, cold emission from the elec-

trodes, and light feedback to the photocath-

ode. Other conditions contributing to anode

*Distributed by Arthur H. Thomas Company,

Vine Street and

3rd,

Philadelphia, PA 19105.

dark current include external leakage caused

by condensation on the tube base and/or

socket when conditions of high humidity ex-

ist and contamination of the tube base

and/or socket by handling.

Moisture condensation can be minimized

by potting the tube socket assembly in

silicone rubber compounds such as RTV-11,

or equivalent. If a tube is suspected of

having high ohmic leakage as a result of

handling, it is recommended that it and its

socket be washed with a solution of alkaline

cleaner such as Alconox*, or equivalent, and

de-ionized or distilled water having a tem-

perature not exceeding 60 °C. The base or the

socket should then be thoroughly rinsed in

de-ionized or distilled water (60 °C) and then

air-blown dry.

OPTICAL CONSIDERATIONS

Incident Light Flux

Photomultiplier tubes are capable of

operating usefully over a very wide range of

incident light flux. At the very lowest levels,

the limit is determined by the

useable

signal-

to-noise ratio. Upper range of usefulness is

determined by the saturation of the photo-

cathode or by the magnitude of the output

current levels which either result in

space-

charge limitations or cause damage to the

secondary emission dynodes.

The lower limit of detection is discussed in

detail in Chapter

4, Photomultiplier Charac-

teristics, in the section “Dark Current and

Noise.”See in particular Fig. 67. The lower

limit in a current-measurement mode is de-

termined by dark emission and bandwidth.

Photocurrent levels as low as 10

ampere

(625 photoelectrons per second) can be

measured. This photocurrent level cor-

responds to

x 10

lumens for a

200-microampere/lumen photocathode. In a

photoelectron-counting mode (see Appendix

G), the limit is determined by the statistics of

discriminating between photoelectrons and

thermally emitted electrons. For a

Na2KSb

photocathode, the dark emission from the

photocathode at room temperature may be

of the order of 10

ampere or 62 electrons

per second. Counting for one minute in the

dark and one minute in the light, one should

be able to detect a photocurrent of only a

few photoelectrons per second.

For resistive photocathodes (See Table II

Photomultiplier Applications

in Chapter IV,

Photomultiplier Characteris-

tics, in the section “Photocathode-Related

Characteristics.“) such as K

CsSb, the max-

imum recommended photocurrent is only of

the order of 10

-9

ampere which cor-

responds to a flux of the order of 10

-5

lumen.

When the photocathode resistivity does

not limit the input light flux, the maximum

light flux is determined by output currents in

the photomultiplier. For example, the 931A

maximum average anode current is 1.0

milliampere which corresponds to an input

light flux of about 10

-5

lumen with a total

applied voltage of 1000 volts. If the over-all

voltage were reduced to 500 volts, the tube

could tolerate an input flux of 10

lumen.

At times it may be useful to reduce the

light level on the photomultiplier by a

calibrated amount. Neutral-density filters

can be useful for this purpose. It should be

appreciated, however, that such filters are

frequently not neutral and their stated

density may be only an approximation. A

comparison of the spectral transmittance of

a metalized filter and a gelatin type is shown

in Fig. 100. Wratten filters are reasonably

Fig.

100

Comparison of spectral transmit-

tance

of metalized- and organic-type

neutral-

density filters (ND 1.0).

neutral in the visible range but become

transparent in the infrared, which could

cause a problem when they are used with a

photomultiplier having a near-infrared

response. This effect is much more pro-

nounced for the more dense neutral-density

filters.

Another method for reducing light level is

by the use of a mesh or screen. This method

*670 E. Arques St., Sunnyvale, Calif. 94086

has the advantage of providing a reduction

factor that is essentially independent of

wavelength. Such neutral density screens can

be obtained from the Varian Instrument Di-

vision. **

Large reduction factors can be achieved

by the use of opal glass, which scatters trans-

mitted light in an approximate Lambertian

(cosine) distribution (although a bit more

concentrated near the normal angles than the

cosine prediction). Unfortunately, the scat-

tered flux is not neutral, providing more red

than blue. See Fig. 101.

WWAVELENGTH

NANOMETERS

92cs-32430

Fig. 101

The effective spectral transmit-

tance (for scattered light) of an opal-glass

filter,

3-mm

thickness, into a solid angle of

approximately 0.01 steradian.

Convex front-surface spherical aluminum

mirrors can also be used to provide a

calibrated light-reduction factor. Aluminum

mirrors have the advantage of having a

reasonably flat spectral reflectance-from

92.3% at 300 nanometers to 86.7% at 800

nanometers.93 If a lamp of CP

candelas

located at a distance a from the surface of

the mirror having a reflectivity

and a

radius

the flux (L) in lumens through a test

aperture of area A located at a distance b

from the mirror surface is given

by94

Photomultlpller

Handbook

nent rays on the mirror.

Calibration

For some purposes it may be useful to pro-

vide a calibration of the photomultiplier

anode or photocathode responsivity. A con-

venient test source is the tungsten lamp (see

Appendix F). Lamps with candle power and

color temperature (2856 K) traceable to the

National Bureau of Standards can be ob-

tained from BURLE, Lancaster, PA (Type

AJ2239). Some method of reducing the light

flux such as discussed above would normally

be required.

Spectral response measurements generally

require a sophisticated monochromator set-

up. Reasonably good results, however, can

be obtained by use of calibrated narrow-

band-pass color filters and a calibrated

tungsten lamp. The power through a par-

ticular filter may be obtained by integrating

the product of the filter transmission and the

tungsten irradiance over the wavelength

band of the filter. Spectral irradiance for a

tungsten lamp calibrated to a color tempera-

ture of 2856 K may be obtained from Appen-

dix F, Table F-I, with a multiplying factor

appropriate to the particular total luminous

flux. Details are given in the footnote ac-

companying Table F-I.

Spot Size

It is generally advisable to utilize a large

part of the photocathode area rather than to

focus a small spot of the light flux on the

photocathode. If the light flux is fairly high,

concentrating it on a small area could cause

damage to the photocathode or result in

non-linearity effects because of the

resistiv-

ity of the photocathode layer. Furthermore,

variations in photocathode sensitivity across

the surface area could cause some uncer-

tainty in the measurement if the light spot is

too small. In scintillation-counting applica-

tions, spot size is not generally a problem

because of the diffuse nature of the flux and

the size of the crystal. But, in

flying-spot-

scanner applications it is particularly impor-

tant that the image being scanned should not

be in focus on the surface of the photomulti-

plier because any non-uniformities of

photocathode sensitivity would cause fixed

pattern modulation.

Angle of Incidence

Photocathode response varies somewhat

with the angle of incidence (see Fig. 41). It is

also possible to increase the effective quan-

tum efficiency of a photocathode by the use

of a specially constructed optical coupling to

the photocathode window that utilizes at the

glass-air interface angles of incidence greater

than the critical angle. (See Fig. 42).

Light Modulation

For some purposes, it may be advan-

tageous to modulate the light signal with a

light chopper. A synchronous motor may be

used to rotate a chopper disk which could

then provide an approximate square-wave

modulation. The output signal could then be

analyzed with a narrow-band-pass amplifier

tuned to the chopper frequency. When the

modulated signal is in the presence of an un-

modulated background whether from the

dark current of the tube itself or from an ex-

ternal source of light, a significant improve-

ment in signal-to-noise may be achieved.

Regarding the testing of photomultiplier

tubes with delta-function light pulses, see the

section on “Time Effects” in Chapter 4,

Photomultiplier Characteristics.

Cerenkov

radiation also provides very short pulses of

light.

SPECIFIC PHOTOMULTIPLIER

APPLICATIONS

This section discusses various applications

of photomultipliers and some of the special

considerations for each application. This

catalogue of applications is not complete

even for presently known applications, and

new ones are being continually devised. The

applications discussed, however, are some of

the major ones and the information given

can be readily adapted to other applications

or to new ones.

Scintillation Counting

A scintillation counter is a device used to

detect and register individual light flashes

caused by ionizing radiation, usually in the

form of an alpha particle, beta particle,

gamma ray, or neutron, whose energy may

be in the range from just a few thousand

electron-volts to many million

electron-

volts. The most common use of scintillation

counters is in gamma-ray detection and spec-

troscopy.

The gas, liquid, or solid in which a scin-

tillation or light flash occurs is called the

scintillator. A photomultiplier mounted in

contact with the scintillator provides the

means for detecting and measuring the scin-

tillation. Fig. 102 is a diagram of a basic

Fig. 102

Diagram of a scintillation counter.

scintillation counter. The three most prob-

able ways in which incident gamma radiation

can cause a scintillation are by the photoelec-

tric effect, Compton scattering, or pair pro-

duction. The reaction probabilities associ-

ated with each of these-types of interaction

are a function of the energy of the incident

radiation as well as the physical size and

atomic number of the scintillator material.

In general, for a given scintillator, the

photoelectric effect predominates at small

quantum energies, the Compton effect at

medium energies, and pair production at

energies above 1.02

MeV.

Scintillation Processes. In the photoelectric

effect, a gamma-ray photon collides with a

bound electron in the scintillator and im-

parts virtually all its energy to the electron.

In the Compton effect a gamma-ray photon

tron in the scintillator and transmits only

part of its energy to the electron, as shown in

Fig. 103. A scattered photon of lower energy

92CS-32440

Fig. 103

Compton-effect mechanism.

Photomultiplier Applications

also results. To satisfy the conditions of con-

servation of energy and momentum, there is

a maximum energy that can be transferred to

the electron. This maximum energy, known

as the Compton edge, occurs when

in Fig.

given by

the speed of light. The resultant energy im-

parted to the electron can then range from

zero to a maximum of

TCM.

In pair production, the energy of a gamma

ray is converted to an electron-positron pair

in the field of a nucleus. The gamma ray

must have energy at least equal to two times

the rest-mass-energy equivalent of an elec-

energy is transferred as kinetic energy. When

the positron is annihilated, two photons are

produced 180 degrees apart, each with an

energy of 0.51

MeV.

The photons are then

subject to the normal probabilities of in-

teraction with the scintillator .

In neutron detection, unlike alpha- or

beta-particle or gamma-ray detection, the

primary interaction is with the nuclei of the

scintillator atoms rather than its atomic elec-

trons. The interaction may consist of scatter-

ing or absorption; in either case, some or all

of the energy of the neutron is transferred to

the recoil nucleus which then behaves

similarly to an alpha particle.

In each interaction between a form of

ionizing radiation and a scintillator, an elec-

tron having some kinetic energy is produced.

A secondary process follows which is in-

dependent of both the kind of ionizing radia-

tion incident on the scintillator and the type

of interaction which occurred. In this secon-

dary process, the kinetic energy of the ex-

cited electron is dissipated by exciting other

electrons from the valence band in the

scin-

tillator material into the conduction band.

When these excited electrons return to the

valence band, some of them generate light or

scintillation photons. The number of pho-

tons produced is essentially proportional to

the energy of the incident radiation. In the

photoelectric interaction described above, all

Photomultiplier Handbook

of the incident photon energy is transferred

to the excited electrons; therefore, the

number of photons produced in this secon-

dary process, and hence the brightness of the

scintillation, is proportional to the energy of

the incident photon.

Scintillation Mechanism.

The exact mecha-

nism of the scintillation or light-producing

process is not completely understood in all

types of materials; however, in an inorganic

scintillator, the phenomenon is known to be

caused by the absorption of energy by a

valence electron in the crystal lattice and its

subsequent return to the valence band. Fig.

104 shows a simplified energy-band diagram

IMPURITY

LEVELS

92CS-32441

Fig. 104

A simplified energy diagram of a

scintillation crystal: ionized electron-hole

pairs; an exciton; an activator center through

which an electron may return to the ground

state causing fluorescence; or through which

an electron may return to the ground state by

a thermal, non-radiative process; impurity

levels that can trap an electron for a time

causing phosphorescence.

of a scintillation crystal. The presence of

energy levels or centers between the valence

and conduction bands is the result of im-

perfections or impurities in the crystal lat-

tice. Three types are important: (1) fluor-

escence centers in which an electron, after

excitation, quickly returns to the valence

band with the emission of a photon; (2)

quenching centers in which the excited elec-

tron returns to the valence band with the

dissipation of heat without emission of light;

and (3)

phosphorescence

centers in which the

excited electron can be trapped in a

metastable state until it can absorb some ad-

ditional energy and return to the valence

band with the emission of a photon. An im-

portant process in the transfer of energy to

the fluorescence, or activator, centers in the

generation of

excitons

or bound

hole-

electron pairs. These pairs behave much like

hydrogen atoms and, being electrically

neutral, can wander freely through the

crystal until captured by fluorescence

centers. The emission of a photon from a

phosphorescence center is a relatively slow

or delayed process. Of the three types of

centers through which an excited electron

can return to the valence band, the first,

fluorescence, is that sought for in the

preparation of scintillators. The second

type, quenching, tends to lessen the effi-

ciency of the scintillator because it does not

cause the emission of photons, and the third,

phosphorescence, produces an undesirable

background glow.

Scintillator

Materials. The most popular

scintillator material for gamma-ray energy

spectrometry is thallium-activated sodium

iodide, NaI:Tl. This material is particularly

good because its response spectrum contains

a well-defined photoelectric peak; i.e., the

material has a high efficiency or probability

of photoelectric interaction. In addition, the

light emitted by the material covers a spec-

tral range from approximately 350 to 500

nanometers with a maximum at about 410

nanometers, a range particularly well

matched to the spectral response of conven-

tional photomultipliers, NaI:Tl does not,

however, have a fast decay time in com-

parison to other scintillators, and, therefore,

is generally not used for fast-time resolution.

As a comparison, the decay time constant

for NaI:Tl is approximately 250 nano-

seconds, while for a fast plastic scintillator

the dominant decay time constant is in the 1

to 4 nanosecond range.

The decay time of a scintillator involves

the time required for all the light-emitting

luminescence centers to return excited elec-

trons to the valence band. In some of the

better scintillators the decay is essentially ex-

ponential, with one dominant decay time

constant. Unfortunately, most scintillators

have a number of components each with dif-

ferent decay time.

Collection Considerations. Because scin-

tillations can occur anywhere in the bulk of

the scintillator material and emit photons in

all directions, there exists the problem of col-

lecting as many of these photons as possible

on the faceplate of the photomultiplier. If it

is assumed that the fluctuation in the

number of photons resulting from a single

ionizing event follows simple Poisson statis-

tics*, the relative standard deviation in the

number arriving at the face plate is given by

(32)

where

is the average number of photons

arriving at the faceplate of the photo-

multiplier per incident ionizing event.

ber of photons per unit energy for the scin-

tillation, E is the energy of the incident

radiation, and

is the fraction of the total

number of photons produced which arrive at

the faceplate of the photomultiplier.

Eq. (32) implies that it is highly desirable

that all emitted photons be collected at the

photomultiplier faceplate. This collection

problem can be simplified by careful selec-

tion of the shape and dimensions of the

scin-

tillator to match the photomultiplier photo-

cathode dimensions. The coating of all sides

of the scintillator except that which is to be

exposed to the photomultiplier faceplate

with a material that is highly reflective for

the wavelengths of the photons emitted by

the scintillator also proves helpful. Because

NaI:Tl is damaged by exposure to moist air,

it is usually packaged in an aluminum case

lined with highly reflective

MgO

or Al2O3

powder; the NaI:Tl scintillator is provided

with an exit window of glass or quartz. To

avoid total internal reflection, it is important

that the indices of refraction of the

scin-

tillator material, its window, any light guide,

and the photomultiplier faceplate match as

closely as possible.

If it is not convenient for the photomulti-

plier to be directly coupled to the scintillator,

as when the photomultiplier entrance win-

dow is not flat, light guides can be used.

Again, care should be taken in the design of

the light guide to assure maximum light

transmission. The outer side or surface of

the light guide should be polished and coated

with a highly reflective material. In some

*Actually, the variance in the number of photons ex-

iting from the crystal per event is much higher than ex-

pected from the simple expression of Eq. (32).

See

the

discussion in Chapter 4 on “Scintillation Counting”

related to

Eq.

G-l 11.

Photomultiplier Applications

cases, a flexible fiber-optics bundle can be

used. An optically transparent silicone-oil

coupling fluid should be applied at the

scin-

tillator (-light guide, if used)-photomultiplier

interface regardless of the light-conduction

method used.

The next important consideration in scin-

tillation counting is the conversion of the

photons to photoelectrons from the photo-

cathode. The photocathode should have the

greatest quantum efficiency possible over the

spectral range defined by the spectral

emissivity curve of the scintillator. The

method of determining the quantum effi-

ciency of a photocathode as a function of

wavelength is explained in Appendix E. Of

the various photocathodes available, bialkali

types,

K2CsSb

and Rb2CsSb, having peak

quantum efficiencies in excess of 25%, pro-

vide the best spectral match to the emission

from most

scintillators.

The uniformity of the photocathode, i.e.,

the variation in quantum efficiency at a

given wavelength as a function of position

on the photocathode, is also important.

Because the number of photoelectrons

emitted for a constant number of photons

incident on the photocathode is proportional

to the quantum efficiency any variation in

quantum efficiency as a function of position

results in an undesirable variation in the

number of photoelectrons emitted as a func-

tion of position.

When the scintillation is fairly bright, i.e.,

when a large number of photons are pro-

duced per scintillation, and when the

scin-

tillator is thick in comparison to its

diameter, as shown in Fig. 105(a), the

photocathode is approximately uniformly il-

luminated during each scintillation. How-

ever, if the scintillator is thin in comparison

to its diameter, as shown in Fig.

105(b),

or if

the scintillation is very weak, the illumina-

tion of the photocathode as a function of

position is closely related to the position of

origin of the scintillation; therefore, photo-

cathode uniformity is much more important

when a thin scintillator is used. In this case,

the use of a light guide may be advan-

tageous. Photocathode uniformity also be-

comes more important as the energy of the

incident radiation becomes less and the

number of photons per disintegration is

reduced.

Photomultiplier Handbook

REFLECTIVE

,-COATING

REFLECTIVE

/-COATING

COUPLING

FLUID

(b)

92CS -32442

Fig. 105

Scintillator geometries: (a) thick in

comparison to diameter;

(6)

thin in com-

parison to diameter.

In some photomultipliers, the collection

efficiency for photoelectrons decreases near

the outer edges of the photocathode;

therefore, best results are obtained when the

scintillator is slightly smaller in diameter

than the photocathode.

Significant Photomultiplier Characteristics.

In scintillation-counting applications (See

also the section on “Pulse Counting” in

Chapter

4, Photomultiplier Characteristics),

a photomultiplier should be selected to pro-

vide a photocathode diameter matching that

of the crystal to be used. The most important

characteristic of the tube to be used in scin-

tillation counting is then the effective photo-

cathode sensitivity to blue and near-ultra-

violet wavelengths. The effective photocath-

ode sensitivity includes the basic quantum

efficiency and the collection efficiency of the

electron-optical system. Tubes having

Venetian-blind dynodes provide a fairly large

opening to the first dynode area and thus

frequently have better collection efficiency

than tubes having a focused-dynode struc-

ture, although the focused structure is much

better for time resolution. Photomultipliers

having the recent “tea-cup” type of first

dynode generally have very good collection

efficiency and, in addition, are designed to

provide increased photocathode emission

either by reflected light or by excitation of

photoelectrons from photocathode

deposited on the side walls of the envelope.

At high count rates, tubes having

copper-

beryllium dynodes generally provide greater

stability than tubes having cesium-antimony

dynodes, although for low count rates the

latter prove to be satisfactory. A tube having

good stability may be expected to shift in

gain by no more than 7% in several months

of continuous operation at a count rate of

10,000 per second. Variation of pulse height

or gain with count rate is also of importance.

Well-designed tubes should show a variation

in pulse height of less than 1% between

count rates of 1000 per second and of 10,000

per second, related to the photopeak, using

137Cs

and a NaI:Tl crystal.

Photomultiplier dark noise is of particular

importance in scintillation-counting applica-

tions when the energy of the ionizing radia-

tion is small, or when very little energy is

transferred to the scintillation medium; in

short, when the flash per event represents

only a few photons. If a photomultiplier is

coupled to a scintillator and voltage is ap-

plied, the composite of all noise pulses

coming from the photomultiplier is referred

to as the background of the system. The plot

of a frequency distribution of these pulses as

a function of energy is shown in Fig. 106.

PULSE HEIGHT

92CS-32443

Fig. 106

Distribution of radiation back-

ground pulses as a function of energy.

cali-

bration spectrum showing the

137CS

photo-

peak is included for reference.

The well-defined peaks seen in the figure are

caused by external radiations and can be

reduced by placing the photomultiplier and

scintillator in a lead or iron vault to reduce

background radiation.

Photomultiplier Applications

It is desirable that the background count

of the scintillation counter be as low as

possible. Photomultipliers have been devel-

oped in which the background count is kept

low by the use of radioactively clean metal

cans instead of glass-bulb enclosures and

faceplates of either

Lucalox*

or sapphire in-

stead of glass. As an additional aid in reduc-

ing background, the scintillation counter can

be surrounded by another scintillator, such

as a plastic sheet equipped with its own

photomultipliers. The output from this scin-

tillation shield is then fed into an anticoin-

cidence

gate with the output from the scin-

tillation counter. The gating helps to reduce

the background contributions from both in-

ternal and external radioactive con-

taminants.

Liquid Scintillation Counting

In liquid scintillation counting, the scin-

tillation medium is a liquid and the ionizing

radiations to be detected are usually

low-

energy beta rays. Because low-energy beta

rays cannot penetrate a scintillation con-

tainer, the radioactive material is dissolved

in a solution containing the scintillator. The

scintillator is generally one or more fluores-

cent solutes in an organic solvent. The

beta-

ray energy is transferred by ionization and

by excitation that in turn results in the emis-

sion of photons in the near ultraviolet. In

some cases, a wavelength shifter is also used

to provide radiation at somewhat longer

wavelengths more compatible with the spec-

tral responsivity of the photomultiplier.

Important characteristics of the photo-

multiplier in this application are again high

responsivity in the spectral region of the

scintillation and low background count rate.

A useful figure of merit is the square of the

counting efficiency divided by the back-

ground coincident count rate in a paired

photomultiplier arrangement. Further

details of liquid scintillation counting ap-

plications are discussed in the section “Pulse

Counting” in Chapter 4, Photomultiplier

Characteristics.

Cerenkov Radiation Detection

Cerenkov radiation is generated when a

charged particle passes through a dielectric

*Registered Trade Name for General Electric Co.

material.

with a velocity greater than the velocity of

light in the dielectric (i.e., v greater than c/n,

where n is the index of refraction of the

dielectric). Polarization of the dielectric by

the particle results in the development of an

electromagnetic wave as the dielectric

relaxes. If the velocity of the particle is

greater than the velocity of light, construc-

tive interference occurs and a conical

wave-

front develops, as illustrated in Fig. 107.

with respect to the direction of the particle is

given by the expression

(33)

where

is the index of refraction of the

velocity to the velocity of light. The spectral

energy distribution of the radiation increases

limited by the absorption of the medium.

Cerenkov radiation is the electromagnetic

counterpart of the shock wave produced in a

gas by an object traveling faster than sound.

It is highly directional and occurs mostly in

the near-ultraviolet part of the elec-

tromagnetic spectrum. Because the radiation

is propagated in the forward direction of

motion of the charged particle, as shown in

Fig. 107, Cerenkov detectors can be made to

detect only those particles that enter the

system from a restricted solid angle.

Cerenkov radiation produced in an

aqueous solution by beta emitters can be

useful in radioassay techniques because it is

unaffected by chemical quenching and

because it offers the advantages, over

liquid-

scintillation counting techniques, of simpli-

fied sample preparation and the ability to ac-

commodate large-volume samples. Because

a fast particle is required to produce

Cerenkov radiation, rather high-energy beta

rays are required; e.g., the threshold for

Cerenkov radiation is 261

keV

for electrons

in water. Because the photon yield for

Cerenkov light is usually very low, the same

considerations concerning photomultiplier

selection apply for the Cerenkov detection as

for liquid-scintillation counting. The tube

selected should also be equipped with a

faceplate capable of good ultraviolet trans-

mission. In some experiments it may be

im-

Photomultiplier Handbook

portant to select photomultipliers for their

speed of response. Depending upon the

dispersion of the medium, the duration of

the Cerenkov flash can be very short.

Time Spectroscopy

In addition to the energy spectroscopy

described above, there are occasions when it

is of advantage to measure time differences

such as between a pair of gamma rays or a

combination of gamma rays and particles in

cascade de-exciting some level in a nucleus.

In time spectroscopy, some special con-

siderations must be made in selecting the

scintillator, photomultipliers, and technique

of analyzing the signals from the photomul-

tipliers.

In a photomultiplier, time resolution is

proportional to (n)

1/2, where n is the

average number of photoelectrons per event.

It is therefore important to choose a

scin-

tillator material that provides a high light

yield for a given energy of detected radia-

tion. It is also important that the variation of

the time of interaction of the radiation with

the scintillator be as small as possible. This

minimum variation is assured by attention to

scintillator thickness and source-to-detector

geometry. The decay time constant of the

light-emitting states in the scintillator should

be as short as possible, and the geometry and

reflective coatings of the scintillator should

be selected so that variations in path lengths

of photons from the scintillator to the

photocathode of the photomultiplier are

minimized. The photocathode of the photo-

multiplier selected should have a high quan-

tum efficiency. In addition, the transit-time

dispersion or jitter (variations in the time re-

quired for electrons leaving the photocath-

ode to arrive at the anode of the tube) should

be small over the entire photocathode area.

The major contribution to transit-time

spread occurs in the photocathode-to-first-

dynode region and may be a result of the in-

itial kinetic energies of the emitted photo-

electrons and their angle of emission. Focus-

ing aberrations,and the single-electron

response or rise time, i.e., the output-pulse

shape at the anode for a single photoelectron

impinging on the first dynode, may also be

of some importance. Although the

single-

electron response theoretically does not have

much effect on time resolution, it does

change the triggering threshold at which the

best time resolution can be obtained.

The most commonly used time-spectro-

scopy techniques include leading-edge tim-

ing, zero-crossover timing, and

constant-

fraction-of-pulse-height-trigger timing.

The

technique used depends on the time resolu-

tion and counting efficiency required and the

range of the pulse heights encountered. A

block diagram of a basic time spectrometer

is shown in Fig. 108.

Leading-edge timing

makes use of a fixed

threshold on the anode-current pulse and

provides good time resolution over a narrow

range of pulse heights. The fractional pulse

height F at which the triggering threshold is

set is defined as follows:

(34)

where

is the discriminator threshold, and

is the peak amplitude of the anode-

current pulse. The fractional pulse height

has a considerable effect on the time resolu-

tion obtained; best results are usually ob-

tained with F equal to 0.2.

In fast zero-crossover timing, the anode

pulse is differentiated. This differentiation

produces a bipolar output pulse that triggers

Photomultiplier Applications

DETECTOR I

TIMING

PULSE

NUMBER

TIME

OF EVENTS

SPECTRUM

SOURCE OF

COINCIDENT

RADIATION

DETECTOR 2 MULTICHANNEL

(PULSE HEIGHT)

ANALYZER

Fig. 108

Generalized block diagram of a time spectrometer.

the timing discriminator at the zero

crossover, the time required to collect ap-

proximately 50 per cent of the total charge in

the photomultiplier pulse. Zero-crossover

timing is second to leadingedge timing for

time-resolution work with narrow

pulse-

height ranges, but is better than the

leading-

edge method for large pulse-height ranges.

constant-fraction triggering,

the point

on the leading edge of the anode-current

pulse at which leading-edge timing data in-

dicate that the best time resolution can be

obtained is used regardless of the pulse

height. For this reason, constant-fraction-

of-pulse-height timing is the best method for

obtaining optimum time resolution no

matter what the pulse-height range.

Oil-Well Logging

Logging is the term given to the method of

determination of the mineral composition

and structure a few miles under the earth’s

surface. Oil-well logging companies gather

data by means of probes, or sondes, that ex-

amine the geological media along very deep

bore holes. The probes determine various

physical and chemical characteristics of the

material in their vicinity. Measurements

made by the probes comprise the log.

A variety of sondes are used in selected

combinations to determine various aspects

of the lithology (the character of a rock for-

mation), including density, of the media

along the bore hole. The combinations of

sondes used depend very much on the

bore-

hole media. For example, when a formation-

density sonde is used in combination with a

neutron sonde in liquid-filled bore holes,

both lithology and porosity can be deter-

mined. The same pair of sondes allows the

measurement of gas and liquid saturation in

bore holes drilled through reservoirs of

low-

pressure gas. The use of the formation-

density sonde, along with either an induction

or a sonic sonde, permits a similar type of

determination. The final result of the

logging activity is information concerning

the existence of hydrocarbons and other

geological media of interest in establishing

an oil field.

The formation-density sonde is one of the

more sophisticated logging devices. Its

operational elements are encased in a rugged

cylindrical housing, as shown in Fig. 109. It

is designed to withstand the high tempera-

ture and shock encountered in probing bore

holes miles deep under the earth’s surface.

The sonde contains a gamma-ray source,

such as radioactive cesium 137, a detector

consisting of a sodium iodide crystal and a

photomultiplier tube, a gamma-ray shield

Photomultiplier Handbook

ELECTRONICS

PRESSURE FOOT

GAMMA SHIELD

Fig. 109

Formation-density sonde sends gamma rays into rock formation and then detects

returns with photomultiplier.

for the detector, a pressure foot that presses

the sonde against the bore-hole wall, and

operating electronics. The objective of this

sonde is to determine the bulk density of the

material in the region of the probe.

Gamma rays from the source interact with

the atoms in the geological medium.

Compton-scattered gamma rays are detected

by the crystal and photomultiplier in the

sonde. A knowledge of gamma-ray penetra-

tion of bulk media, mass absorption data,

and the special effects resulting from the

chemical nature of various geological media

allows the information imparted by the

pulses to be deciphered by the electronics

into bulk-density data, which becomes the

substance of the geological

formation-

density log.

lustrated in Fig. 110. Most of the decrease is

the result of photocathode sensitivity loss,

but some is associated with the crystal. Be-

cause of the loss in photocathode response

and the increase in thermionic emission with

temperature, the desired signal is finally lost

in the background noise at a temperature

near 200°C. Permanent damage to the

pho-

tocathode may also be expected after many

cycles of operation at 200°C.

Because of the increase in temperature

with the depth of the bore hole-to perhaps

150°C at 15,000 feet-a most important

characteristic of the photomultiplier is its

resistance to high temperature. Of the

numerous photocathodes which have been

developed, the

Na2KSb

“bialkali” photo-

cathode is the most stable at elevated

tem-

peratures.95 (See Fig. 83 and related text.)

92cs

32447

Fig. 110

Pulse-height resolution and pulse

height as a function of temperature for a

pho-

tomultiplier having a

Na2KSb

photocathode

and used with a

Nal:TI

crystal and a 137Cs

source.

As the temperature is increased on a

pho-

tomultiplier with a

Na2KSb

photocathode

coupled to a NaI:Tl crystal, the pulse height

(137Cs source) gradually decreases, as

il-

Gamma-Ray Camera

The gamma-ray camera originally de-

scribed by Anger96is a more sophisticated

100

version of the scintillation counter, and is

used for locating tumors or other biological

abnormalities. The general principle of the

gamma camera is illustrated in Fig. 111. A

92CS-32448

Fig.

111

Structure of gamma-ray camera.

radioactive isotope combined in a suitable

compound is injected into the blood stream

of the patient or is ingested orally. Certain

compounds or elements are taken up prefer-

entially by tumors or by specific organs of

the body, such as iodine in the thyroid gland.

As the radioactive isotope disintegrates,

gamma rays are ejected from the location of

the concentration.

A lead collimator permits gamma rays to

pass through it only when they are parallel to

the holes in the lead; gamma rays at other

angles are absorbed in the lead. In this way,

the location of the gamma-ray source may be

determined because gamma rays originating

Photomultiplier Applications

on the left side of the organ are caused to im-

pact the left side of the scintillation crystal,

etc. The crystal covers an area about 10

inches or more in diameter.

Behind the crystal are, perhaps, 19 photo-

multiplier tubes in a hexagonal array. The

light of the individual scintillation is not col-

limated but spreads out to all of the 19 tubes.

The location of the point of scintillation

origin is obtained by an algorithm depending

upon the individual signals from each of the

photomultipliers. Resolution is obtained in

this manner to about 1/4 inch. Each scintilla-

tion is then correspondingly located by a

single spot on a cathode-ray tube. Counting

is continued until several hundred thousand

counts are obtained and the organ in ques-

tion is satisfactorily delineated. Fig. 112 is a

reproduction of such a scintigram. The par-

ticular advantage of the gamma camera over

other techniques such as the CT scanner

(described below) is that the gamma camera

provides functional information. For exam-

ple: Tc-99m polyphosphate is used to reveal

bone diseases;

123

I is used in thyroid studies;

127Xe is inhaled to provide information on

lung ventilation.

Of great concern to the designer of gamma

cameras and, of course, to the ultimate

customer is the inherent resolution capability

of the device. Generally, with more photo-

multiplier tubes sampling the scintillation

distribution, the delineation becomes more

precise. Most of the original cameras utilized

Fig. 172

Scintigrams obtained by Lancaster General HospitaI with a gamma camera. The

scin-

tiphoto on the left shows multiple emboli in the right lung; photo on right shows lungs after

clearing. The isotope

technitium-99m

was used to tag albumin microspheres-a colloidal form

of the albumin protein, with particles ranging in size from 2 to 50 microns. These particles are

injected into the bloodstream and are filtered and trapped in the lung capillary bed; the scan

can then determine those areas where the capillary bed is intact. Areas of diminished blood

flow show as “cold spots.”

101

Photomultiplier Handbook

a hexagonal array of 19 tubes. By adding

another circumferential row, an array of 37

provided better resolution. In the same man-

ner cameras are now available with 61 tubes

and even 91. (Note the numerical progres-

sion of these hexagonal arrays: 1 + 6 + 12 +

18 + 24 + 30.) Different photomultiplier

dimensions also are used to provide in-

struments with appropriate portability or

coverage. Most common has been the 3-inch

photomultiplier,but a large number of

2-inch

tubes are also used and there is con-

sideration of the use of 1 1/2-inch tubes. Hex-

agonal

and 3-inch tubes have been

developed to provide better space utilization,

and a photomultiplier with a square

faceplate is now available.

Resolution of the gamma-ray camera de-

pends fundamentally upon the pulse-height

resolution of the photomultiplier and,

hence, the quantum efficiency of the photo-

cathode. The most common photocathode

selected for gamma-ray-camera application

is the bialkali (K2CsSb). Also important in

the determination of good pulse-height

resolution is collection efficiency of the

photoelectrons. Large first dynodes are ap-

propriate and the “tea-cup” configuration is

used to advantage.

Because each scintillation is sensed by a

number of the photomultipliers in the array,

the spatial uniformity of the photomultiplier

response and its angular response become

important especially as related to the

algorithm of the gamma-ray-camera design.

The camera design may also incorporate

modifications in the light pipe such as

grooves or etched patterns to provide im-

proved spatial location of the scintillations.

Finally, the stability of the photomulti-

plier is important in maintaining the resolu-

tion and spatial integrity of the display.

Although the photomultiplier currents are

generally small in this application, changes

can occur in the photomultiplier gain which

would then result in positional errors in the

CRT display.Some gamma-ray-camera

designs include a means of recalibrating the

array of photomultipliers so that the

pulse-

height “window” is the same for each tube.

"For a more detailed discussion, see the article in

Scientific American, October, 1975, p 56, “Image

Reconstruction from Projections,” R. Gordon, G.T.

Herman, and S.A. Johnson.

102

This recalibration could be done periodically

(once a day) and would contribute signifi-

cantly to consistent, distortion-free opera-

tion.

Computerized Tomographic

X-Ray Scanners

The computerized tomographic (CT) scan-

ner produces an X-ray image in an entirely

different manner from that of conventional

radiography.* The standard X-ray picture is

a shadowgraph. Thus, a chest X-ray pro-

duces an overlay of shadows from the rib

cage and internal body structure. Interpreta-

tion is frequently difficult because of the in-

terfering images. The CT scanner, on the

other hand, provides a density image that

represents a cross section of the patient-a

tomograph. Thus, a CT scan of the head

would show the outer bone structure, the

folds of the brain, and possibly a tumor in-

side the skull as though a complete thin slice

had been taken through the middle of the

head. This tomographic image is produced

by exposing the head to X-rays at many dif-

ferent angles of entrance. The multiplicity of

shadow-type images thus formed are ana-

lyzed by a computer, which produces a

reconstructed cross-section density image.

A diagram of a typical CT scanner is

shown in Fig. 113. An X-ray source pro-

ducing a fan beam rotates around the patient

in a few seconds. (Short periods are desirable

to minimize artifacts in the final recon-

structed image of the patient’s cross section

caused by unavoidable motions.) An array

of several hundred detectors in an outer cir-

cle surrounding the patient provides the data

from which a computer derives a tomo-

graphic view of the patient’s body or head.

A typical tomograph is shown in Fig. 114.

Three different X-ray detector systems

have been developed for use in CT scanners.

All are in use at present by the various

CT-

scanner manufacturers. Each detector

system has its problems, but all seem com-

parable in ultimate performance.

The original CT-scanner development

utilized a crystal and photomultiplier. A

subsequent development used a tube con-

taining xenon gas at several atmospheres.

The density of the gas and the high atomic

weight of xenon provide sufficient absorp-

tion so that a large fraction of the X-rays are

detected by the resulting ion current. More

Photomultiplier

Applications

ARRAY OF STATIONARY

DETECTORS ENCIRCLING

PATIENT

92cS-32662

Fig-

113

Diagram of a typical CT scanner. X-ray source producing

fan beam

rotates around the patient. Array of several hundred detectors in outer circle pro

vides

the data from which a computer derives a tomographic

(cross-section)

view

the patient’s body or head.

recently, detector packages have been

developed using silicon photocells and a

CdWO

crystal. This latter detector package

appears to have cost advantages. Detection

with the silicon photocell was not feasible in

the early development of CT scanners, but a

combination of higher-speed machines and

the improved light output of the CdWO4

crystal, relative to the Bi3Ge4O12 (BGO)

crystal, has made this combination an

economic possibility.

Fig. 114

Tomograph showing a “slice” of

the thoracic cavity. Lung tissue, blood ves-

sels, air passages, ribs, and spine may be ob-

served. (Courtesy Pfizer Medical Systems,

Inc.).

The photomultiplier system uses a BGO

(bismuth germanate) crystal usually coupled

to the photomultiplier by means of a light

pipe. Spectral emission from the crystal is il-

lustrated in Fig. 115. It is rich in the

blue-

I I

300 400 500 600

WAVELENGTH-NANOMETERS

92CS-32452

Fig. 115

Fluorescence spectrum of

Bi4Ge3O12 (Data from Weber and

Mon-

champ

)

green region and thus is a good match for

bialkali photocathodes. Recently, it has been

found that the Rb2CsSb photocathode is

more suitable for use with the BGO crystal

than the

K2CsSb

photocathode because of

103

Photomultiplier Handbook

the somewhat higher response and better

spectral match. See Fig. 10. The Rb2CsSb

photocathode is also more suitable because it

has a lower surface resistance than the

K2CsSb

photocathode. See Fig. 36.

In the CT-scanner systems, pulse-height

discrimination is not used nor are individual

scintillations counted. The photomultiplier

is used as an integrator of the light flux in

each density measurement. It is important

that the photomultiplier provide a linear

translation of the light flux into output cur-

rent and also that there be no overshoot or

undershoot in photocurrents as the X-ray

beam path changes from outside to through

the patient resulting in a signal variation of

1000:1 or more. Typical maximum photo-

cathode current (X-ray path through air

only) is of the order of 2 nanoamperes de-

pending on operation conditions. The pho-

tomultiplier is usually operated at a modest

gain figure of the order of 60,000. High

photocathode sensitivity is important in pro-

viding minimum noise in the signal so that

the inherent signal-to-noise ratio in the

transmitted X-ray beam is retained and the

X-ray dose to the patient in minimized. The

photomultiplier tubes used are either

inch

or 3/4 inch in diameter, depending upon the

configuration of the particular CT scanner.

Positron Camera

vides tomographic presentations based on

coincident gamma-ray emission accompany-

ing annihilation of a positron and an elec-

tron. Tracer radionuclides such as

or 15O emit a positron upon disintegration.

In the presence of matter such as the brain,

the positron interacts almost instantly with

an electron resulting in the simultaneous

emission of two gamma rays each having an

energy of 511 keV, but moving in nearly op-

posite directions. When a pair of detectors,

one on either side of the patient, observes

coincident events, the point of radionuclide

disintegration lies on a line joining the two

detectors. See Fig. 116.

A number of positron cameras have been

designed and built, some commercially.

Various geometries and reconstruction tech-

niques have been utilized. Complications in

design arise when each detector can be in

coincidence with any of several detectors on

the opposite side of the patient. More than

104

92CS -32453

fig. 116

Principle of positron coincidence

detection. A disintegration at “a

”

results in

oppositely directed gamma rays which are

detected in coincidence

the pair of scintil-

lator-photomultiplier

detectors. An event at

"b” results in a count in only one of the detec-

tors. (From G.L.

Brownell

and C.A. Burn-

ham

)

one plane may be utilized to provide

three-

dimensional data.

Reponse time of the photomultiplier is of

particular importance in positron

cameras

because the discrimination against

spurious

coincidences improves as the resolution time

of the system decreases. Photomultiplier

transit time spreads of the order of 2

nanoseconds or less are advantageous in

these systems

although other time- factors,

related to thecrystal scintillator

(CsF

has a

time constant of 5 nanoseconds) and cir-

cuitry may limit the system discrimination

time to perhaps 10-20 nanoseconds. Photo-

multiplier tubes of

3/4-inch

and 1 1/2-inch

diameter have been the sizes preferred. Of

course, high quantum efficiency matching

the spectral emission of the scintillators

(NaI:Tl,

CsF,

) is

also

important.

Photometric and Spectrometric Applications

The side-on photomultiplier has in gener-

al, been the most widely used tube in photo-

metric and spectrometric applications. The

side-on tube is relatively small and has a

rectangularly shaped photocathode that

matches the shape of the light beam from an

exit slit. Spectrometric applications require

tubes with good stability, high anode sen-

sitivity, low dark current, and broad spectral

sensitivity. A high signal-to-noise ratio is im-

portant because of the generally small signal

levels.

Before the development of the new

negative-electron-affinity type photoemit-

ters,

such as gallium arsenide, it was

necessary to use more than one detector in

the measurement of radiant energy from the

near-ultraviolet to the near-infrared part of

the spectrum. A photomultiplier having a

gallium arsenide photoemitter can now be

used to detect radiant energy from the cutoff

point of the photomultiplier window in the

near-ultraviolet to 910 nanometers.

Spectrophotometry.

Spectrophotometers

measure the optical density of materials as a

function of wavelength and require photo-

multipliers having a broad spectral range.

The results of measurements of the absorp-

tion characteristics of substances are fre-

quently expressed in terms of optical density.

This logarithmic method correlates with the

way the human eye discriminates differences

in brightness.

The transmission density D is defined by

where

is the radiant flux incident upon a

sample,

is the radiant flux transmitted by

a sample, and T is the transmission figure

equal to

Pt/Po.

Density measurements are

useful in various applications to films and

other transparencies in addition to chemical

analysis where concentration of a solution is

studied as a function of wavelength.

Color-Balancing Photometry. A color-

balancing photometer is used to determine

color balance and exposure times necessary

to produce photographic color prints from

color negatives. Such a device is shown in

block diagram form in Fig. 117. It allows the

matching of the relative proportions of red,

blue, and green light transmitted by a pro-

duction negative to those of a master color

negative. As the first step in the matching

process, the master negative is used to make

an acceptable print. This first print is pro-

duced through trial and error by measure-

ment of the relative proportions of red, blue,

and green light transmitted through a key

area of the master negative; the key area

usually consists of a flesh tone or a gray

area. The amount of light transmitted is a

Photomultiplier Applications

measure of the density. The exposure time

using white light and the lens opening used

to obtain the satisfactory print are noted.

Next, an area of the production negative

similar to that on the master negative is

chosen and the relative proportions of the

red, blue, and green light transmitted are

measured again. By use of magenta and

yellow correcting filters, the color trans-

mitted by the production negative can be

balanced with that of the master negative.

When the production negative is used, the

lens opening is adjusted so that the exposure

time with white light is the same as it was for

exposure of the master negative. When the

values of color-correcting filters and ex-

posure times thus determined are used,

prints from the production negative can be

obtained which are very nearly as good as

those obtained with the master color

negative.

Most color-balancing photometers employ

steady light sources and handle

small-

amplitude signals. Consequently, the photo-

multiplier used must have low values of dark

current and good stability. The life expectan-

cy of tubes used in this application is long

because the small signal levels reduce the ef-

fects of fatigue which might otherwise

adversely affect measurement accuracy and

repeatability. Low-current operation also

provides for linear operation where the

anode current is proportional to the input

flux over the range of transmission values

measured. The intensity range of a color-

balancing photometer, given in terms of the

ratio of the radiant flux incident upon a

sample to the radiant flux transmitted by a

sample, is usually of the order of 1000 to 1 or

more. It must be remembered that as in most

photomultiplier applications, the power sup-

plies used must be capable of providing

voltages sufficiently regulated and free from

ripple to assure minimum variation in sen-

sitivity with possible line-voltage variation.

Densitometry. Although techniques such

as those employed in the color-balancing

photometer may be used successfully to

measure density over an intensity range of 10

or 100 to 1 (density 1 to density

2),

their use

becomes increasingly difficult as the range is

increased to 1000 to 10,000 to 1 (density 3 to

density 4) or more. The large dynamic ranges

encountered in color-film processing place

105

Photomultiplier Handbook

92CS-32454

Fig. 117

Block diagram of a color-balancing photometer.

severe requirements upon the photomulti-

plier because it must be operated in a

constant-voltage mode. Problems develop in

this mode at high-density values at which the

dark current may become a significant pro-

portion of the signal current. The random

nature of this dark current, which precludes

its being “zeroed out”, may lead to output-

signal instability. As a result of the type of

operation needed to produce the dynamic

range required in density measurements, the

photomultiplier anode current is high at low

density values. These high currents may

result in excessive fatigue, and, depending

upon the operating point, perhaps non-

linear operation. As with the color-balancing

photometer,a well regulated low-ripple

power supply is needed to assure accurate

measurements.

Logarithmic Photometry.

In the measure-

ment of absorption characteristics, the

changes in brightness levels vary over such a

large range that it is advantageous to use a

photometer whose response is approximately

logarithmic. This response enables the

pho-

106

tometer to be equipped with a meter or read-

out scale that is linear and provides precise

readings even at high optical densities.

A simplified circuit of a logarithmic

photometer capable of measuring film

density with high sensitivity and stability and

of providing an appropriate logarithmic

electrical response and linear meter indica-

tion of density over three or four density

ranges is shown in Fig. 118.

The circuit of Fig. 118, in which the

photomultiplier operates at a constant cur-

rent, minimizes photomultiplier fatigue and

eliminates the need for a regulated

high-

voltage supply. The feedback circuit il-

lustrated develops a signal across

propor-

tional to the anode current. This signal con-

trols the bias applied to the control device

and automatically adjusts the current in the

voltage-divider network. By this means, the

dynode voltage is maintained at a level such

that the anode current is held constant at a

value

selected to minimize the effects of

fatigue. At optical densities of 3, the dynode

supply voltage may be 1000 volts; at optical

densities of less than 1, the dynode voltage

Photomultiplier Applications

COLOR FILTERS

92CM-32455

Fig.

118

Block diagram of a logarithmic photometer.

may be as low as

300

volts. Dynode voltage

is translated into density by means of the

scale on voltmeter V1 .

Because there is an approximately ex-

ponential variation of sensitivity of a

photomultiplier with applied voltage (see

Fig,

48),

in a constant-current mode, the

voltage varies almost linearly with the

logarithm of the input light flux. The voltage

is thus close to a linear measure of the

density. A compensating circuit for the lack

of linearity is indicated in Fig. 118 in the

form of a variable and automatic shunt

across the voltmeter V1 . As the density

values increase, the effective value of the

shunt resistance is reduced. This circuit not

only compensates for photomultiplier non-

linearity, but also for the non-linearity of the

optical system employed.

Spectrometry

Spectrometry, the science of spectrum

analysis, applies the methods of physics and

physical chemistry to chemical analysis.

Spectrometric applications include absorp-

tion, emission, Raman, solar, and vacuum

spectrometry, and fluorometry.

Absorption Spectrometry. Absorption

spectrometry, used to detect radiant energy

in the visible, ultraviolet, and infrared

ranges, is one of the most important of the

instrumental methods of chemical analysis.

It has gained this importance largely as a

result of the development of equipment

employing photomultipliers as detectors.

The principle underlying absorption

spec-

trometry is the spectrally selective absorp-

tion of radiant energy by a substance. The

measurement of the amount of absorption

aids the scientist in determining the amount

of various substances contained in a sample.

The essential components of an absorp-

tion spectrometer are a source of radiant

energy, a monochromator for isolating the

desired spectral band, a sample chamber, a

detector for converting the radiant energy to

electrical energy, and a meter to measure the

electrical energy. The spectral ranges of the

107

Photomultiplier Handbook

source and detector must be appropriate to

the range in which measurements are to be

made. In some cases this range may include

one wavelength. In others, it may scan all

wavelengths between the near-ultraviolet

and the near-infrared.

Raman

Spectroscopy.

There are two types

of molecular scattering of light,

Rayleigh

and Raman. Rayleigh scattering is the elastic

collision of photons with the molecules of a

homogeneous medium. Because the scat-

tered photons do not gain energy from or

lose energy to the molecule, they have the

classic example of

Rayleigh

scattering of

light from gas molecules is the scattering of

the sun’s light rays as they pass through the

earth’s atmosphere. This scattering accounts

for the brightness and blueness of the sky.

Raman

scattering is the inelastic collision

of photons with molecules that produces

scattered photons of higher or lower energy

than the initial photons. During the collision

there is a quantized exchange of energy that,

depending on the state of the molecules,

determines whether the initial photon gains

or loses energy. The differences in energy

levels are characteristic of the molecule. If

emitted after the interaction has a frequency

from the molecule that was in the excited

state. In the reverse case, the initial photon

has given up energy to the molecule in the

unexcited state.

Early

Raman

instruments had a number

of disadvantages and were difficult to use. It

was difficult to find a stable high-intensity

light source and to discriminate against

Rayleigh scattering of the exciting line. With

the recent development of high-quality

monochromators and the advent of the laser

light source, a renewed interest in the Raman

effect as an analytical method of chemical

analysis has taken place. Raman

spec-

trophotometers are generally used to in-

vestigate the structure of molecules and to

supplement other methods of chemical

analysis, particularly infrared-absorption

spectrometry.

The scattered photons from a Raman

in-

108

teraction are so few in number that only the

highest-quality photomultipliers can be used

as detectors. The tube should have high col-

lection efficiency, high gain, good multipli-

cation statistics, low noise, and high quan-

tum efficiency over the spectral range of in-

terest. Fig. 119 shows a Raman spectrum. To

reduce the effects of

Rayleigh

scattering, a

source of noise in Raman spectroscopy, the

photomultiplier is placed at right angles to

the light beam, whose wavelength has been

chosen as long as possible within the range

of interest.

92CS-32456

Fig. 119

Typical

Raman

spectrum.

Fluorometry. The fluorometer is another

instrument which utilizes the photomulti-

plier’s capability for low-light-level detec-

tion, its high gain, and its good

signal-to-

noise ratio. There are numerous applications

of this instrument in the fields of

biochemistry,

medical research, and in-

dustrial toxicology. Typical applications in-

clude the detection and measurement of

minute quantities of air pollutants and of

components of the blood or urine. Some

materials are detected by their own

fluorescence; others by their quenching ef-

fects on the fluorescence of other materials.

Illustrative of fluorometer operation is the

optical design of a model designed by G. K.

Turner

associates103

shown in Fig. 120. The

sample is irradiated by an ultraviolet source

filtered to eliminate longer wavelength com-

ponents of the lamp irradiation. Fluorescent

spectra at a longer wavelength pass through

a second filter which eliminates scattered

ultraviolet radiation.

A second

beam fromthe ultraviolet source

provides acalibratedreference flux. Both

the fluorescent beam and the reference beam

Photomultiplier Applications

are directed to the photomultiplier and alter-

nately sampled by means of the light inter-

rupter. The output ac signal from the

photomultiplier is processed to provide a

null signal by means of the light cam and the

attached dial records the fluorescence level.

The null feature of the device eliminates er-

rors from changes in the photomultiplier or

from the ultraviolet source. A third flux in-

dicated by the FORWARD LIGHT PATH is

provided so that even for an absence of

fluorescence a balance is always possible.

The BLANK KNOB provides an adjustment

so that the calibration can be adjusted to the

zero fluorescence point.

Low-Light-Level Detection

Systems for the detection of low light

levels make use of two basic techniques:

charge integration, in which the output

photocurrent is considered as an integration

of the anode pulses which originate from the

individual photoelectrons, and the digital

technique in which individual pulses are

counted.

Charge-Integration Method. In the

charge-integration method, either the

transit-time spread of the photomultiplier or

the time characteristics of the anode circuit

cause the anode pulses to overlap and pro-

duce a continuous, though perhaps noisy,

anode current. The current is modulated by

turning the light off and on by means of

some mechanical device such as a shutter or

light “chopper”.

The signal becomes the dif-

ference between the current in the light-on

and light-off conditions.

Detection is limited by noise in the anode

current. At low light levels the noise is

caused primarily by the fluctuations in dark

current of the photomultiplier (as discussed

in the section, “Dark Current and Noise” in

Chapter

4 Photomultiplier Characteristics

and in Appendix G, “Statistical Theory of

Noise in Photomultiplier Tubes. ") The noise

may be minimized by reducing the band-

width of the measuring system. For example,

a dc system may be used with a bandwidth of

only a few hertz if an appropriate low-level

current meter is selected. Bandwidth can also

be reduced by some technique of averaging

the current fluctuations over a period of

time.

Another technique of charge integration is

to chop the light signal with a motor-driven

chopper disk having uniformly spaced holes

or slots. The output current is then fed

through an amplifier having a narrow band-

width tuned to the frequency of chop. Band-

widths of the order of 1 Hz are typical.

Digital Method. In the digital method for

the detection of low light levels a series of

output pulses, each corresponding to a pho-

toelectron leaving the photocathode of a

photomultiplier, appears at the anode. All of

the output pulses from the tube are shaped

by a preamplifier before they enter a

pulse-

amplitude discrimination circuit. Only those

pulses having amplitudes greater than some

predetermined value and having the proper

rise-time characteristics pass through to the

signal-processing circuits. The digital tech-

nique is superior to charge integration at

109

Photomultlpller

Handbook

very low light levels because it eliminates the

dc leakage component of the dark current as

well as dark-current components originating

at places other than the photocathode. Fig.

121 shows a digital system in block form.

In the special case in which the digital

technique is used to count single photons in-

cident upon the photocathode of a photo-

multiplier, signal pulses appear at the anode

with an average pulse amplitude PH equal to

em, where e is the electron charge and

the photomultiplier gain. The number of

signal pulses

arriving at the anode is

given by

(36)

where N

is the number of photons incident

on the ph

quantum efficiency of the photocathode at

the photon wavelength including a factor for

the loss of light by reflection and absorption,

and a factor for the loss resulting from im-

perfect electron-collection efficiency of the

front end of the photomultiplier.

The dark-noise pulses present in addition

to the signal pulses originate mainly from

single electrons and have a pulse-height

distribution as shown in the simplified dark-

noise pulse-height-distribution spectrum of

Fig. 122. Region A of Fig. 122 includes

circuit-originated noise, some single-electron

pulses, and pulses caused by electrons

originating at the dynodes in the multiplier

section. Pulses originating at the dynodes ex-

hibit less gain than the single-electron pulses

from the cathode. Region B represents the

single-electron pulse-height distribution and

is the region in which the single-photon

signal pulses appear. Region C is caused by

cosmic-ray muons, after-pulsing, and radio-

active contaminants in the tube materials

and in the vicinity of the tube. To maximize

the ratio of signal pulses to noise pulses in

single-photon counting, lower- and

upper-

level discriminators should be located as

shown in the figure. For a discussion of

signal-to-noise ratio and the statistics related

to pulse counting, see “Pulse Counting Sta-

tistics” in Appendix G.

The signal-to-noise ratio may be improved

by the square root of the total count time.

Increasing the time of count is analogous to

decreasing the bandwidth in the

charge-

integration method. The signal-to-noise

110

ratio can also be improved by decreasing the

number of dark-noise pulses. Because these

pulses originate at the photocathode surface,

the number can be reduced by reducing the

area of the photocathode or by reducing the

effective photocathode area by using elec-

tron optics to image only a small part of the

photocathode on the first dynode. It may

also be desirable to cool the photomultiplier

and thereby reduce the thermionic emission

from the photocathode.

In some applications not requiring the use

of the full area of the semitransparent

photocathode, it may be useful to restrict the

active area of the photocathode by specially

arranged magnetic

fields104

and thus reduce

the collection of thermally emitted electrons

from non-utilized areas of the photocath-

ode. Fig. 123 illustrates the design of such

a magnetic defocusing system type

PF-

1011 which includes an 8852

photomulti-

plier (2-inch diameter, ERMA photocath-

ode, and

GaP

first dynode). The basic ele-

ment is a permanent magnet in the form of

an annular ring (1) polarized so that the

S-pole or N-pole is toward the photocath-

ode. Pole pieces, spacers, and a magnetic-

shield cylinder complete the arrangement.

The resulting magnetic field is such that only

electrons emitted from the central portion of

the photocathode arrive at the first dynode.

Typical variation of photocathode response

as a function of the position of the incident

light is shown in Fig. 124. Note that the ef-

fective diameter of the photocathode has

been reduced to about one eighth of an inch.

Total dark emission at room temperature

from the magnetically controlled photo-

cathode is reduced by approximately 80:1. If

the tube is then cooled to

20°C,

the dark

count is still further reduced to about two

electrons per second.

Very-Low-Light-Level Photon-Counting

Technique.

Before beginning very-low-light-

level photon counting, the following special

precautions must be taken:

1. The power supply and interconnecting

circuits must have low-noise characteristics.

2. The optical system must be carefully

designed to minimize photon loss and to pre-

vent movement of the image of the object on

the photocathode, a possible cause of error

if the cathode is non-uniform. It is generally

good practice to defocus the image on the

Photomultiplier Applications

PHOTOMULTIPLIER

ADJUSTABLE

PULSE AMPLITUDE

DISCRIMINATOR

92cs-32458

Fig.

121-

Block diagram illustrating a digital system for detection of low light levels.

photocathode, especially in the case of a

point source, to minimize problems which

may result from a non-uniform photocath-

ode.

3. The photomultiplier must be allowed

to stabilize before photon counting is begun.

The tube should not be exposed to

ultraviolet radiation before use and should,

if possible, be operated for 24 hours at the

desired voltage before the data are taken.

4. The photomultiplier should be

operated with the cathode at ground poten-

tial if possible. If the tube is operated with

the photocathode at negative high voltage,

care must be taken to prevent the glass

envelope of the tube from coming into con-

tact with conductors at ground potential or

noisy insulators such as bakelite or felt.

Without this precaution, a very high dark

noise may result as well as permanent

damage to the photocathode.

5. Large thermal gradients must not be

permitted across the tube as it is cooling. In

addition, care must be taken to avoid ex-

cessive condensation across the leads of the

tube or on the faceplate.

PULSE HEIGHT 92cs-32459

Fig. 122

Dark-noise pulse-height-distribu-

tion spectrum.

92CS-32460

Fig. 123

Magnetic defocusing system

(PF1011) which includes type 8852 photomul-

tiplier.

This system permits collection of elec-

trons only from the central part of the pho-

tocathode and thus substantially reduces dark

emission. Shown are (7) ceramic magnet, (2)

spacer-insulator, (3) magnet pole piece of

black iron, and (4) magnetic shield cylinder.

The assembly is designed to fit over the 8852

envelope.

Photomultiplier Selection for Photon

Counting. In the selection of a photomulti-

plier for use in photon counting, several

im-

portant parameters must be considered.

First, and most important, the quantum effi-

ciency of the photocathode should be as high

as possible at the desired wavelength. To

minimize the thermionic dark-noise emis-

sion, the photocathode area should be no

larger than necessary for signal collection;

the multiplier structure should utilize as

large a fraction as possible of the electrons

from the photocathode. The over-all tube

should have as low a dark noise as possible.

In some applications, the rise time and

time-

resolution capabilities of the tube may also

be important.

111

PhotomultIplier

Handbook

DISTANCE FROM CENTER OF PHOTOCATHODE

INCHES

92CS-32461

Fig. 124

Typical variation of photocathode sensitivity as a function of incident-light

spot position for an 8852 photomultiplier with the magnetic defocusing system of

Fig. 123 affixed to the tube.

A number of more recent developments in

photomultiplier design are of considerable

significance in photon counting. These

developments fall into two major categories,

secondary-emission materials and photo-

cathodes. Because of the superior statistics

of gallium phosphide (one of the more

recently developed secondary-emission

materials discussed in the section “Secon-

dary Emission”in Chapter 2

Photomulti-

plier

Design),

a tight single-electron distribu-

tion curve can be obtained, as shown in Fig.

79. The tight distribution permits easy loca-

tion of the pulse-height discriminator, as is

particularly evident when some of the signal

pulses are originated by two or more photo-

electrons leaving the cathode simultane-

ously. If the average number of photoelec-

trons leaving the photocathode per pulse

were three,a pulse-height distribution

similar to that shown in Fig. 125 would be

obtained. Gallium phosphide provides a

higher signal-to-noise ratio than would con-

ventional secondary-emitting materials such

as Be0 or

Cs3Sb

when used in the same tube.

Photocathode developments include

ERMA (Extended Red Multi-Alkali), a

semitransparent photocathode having a

response to 940 nanometers, and several

negative-electron-affinity materials. Perhaps

the most significant of these materials is

indium gallium arsenide, whose threshold

wavelength increases with increasing indium

content. Spectral-response curves for some

of the more recent photocathodes are shown

in the section “Photoemission” in Chapter 2

Photomultiplier Design.

92CS-32462

Fig. 125

Pulse-height distribution obtained

with a photomultiplier having a gallium

phosphide first dynode. Light level is such

that three photoelectrons per pulse time is

the most probable number.

Astronomy

105

Astronomers were among

the first users of photomultiplier tubes. The

high quantum efficiency, low background

noise, and high gain recommend the use of

photomultipliers in various astronomical ap-

plications. Requirements are often similar to

112

those for low-level photometry or

spec-

trometry as discussed above. Applications

include the guidance of telescopes, intensity

measurements,

stellar spectrophotometry,

Doppler measurement of radial velocities,

and the measurement of stellar magnetic

fields by measuring Zeeman displacements.

Side-on photomultipliers such as the 1P21

are frequently used in these applications.

Refrigeration may be utilized to reduce

background. Photon counting may be

adopted for very low signal measurement.

For applications requiring a wide spectral

range, tubes with the GaAs:Cs photocathode

are very useful.

Pulsed Photomultipliers

Some years ago,

Post106

demonstrated

improved characteristics of photomultiplier

tubes by operating them with a pulsed

voltage supply. He used side-on photomulti-

pliers types 931A and 1P21 with an applied

voltage of between 4 and 5 kilovolts pulsed

for 2.5 microseconds or less. At these

voltages, 400 to 500 volts per stage, the

secondary emission for Cs-Sb dynodes is at a

maximum (see Fig. 19) so that the gain

becomes relatively insensitive to the varia-

tion of the maximum voltage applied. Nor-

mally, these tubes could not be operated at

such high voltages without incurring destruc-

Photomultiplier Applications

tive run-away dark currents. But, by pulsing

the applied voltage, regenerative effects that

depend upon transit time of ions are

eliminated. Post’s circuit is shown in Fig.

126. Not all tubes he tried could tolerate this

high voltage without field emission and

break-down effects, but on those tubes

which were satisfactory initially or which he

was able to stabilize by application of

repeated high-voltage pulses, he found some

very interesting advantages. Peak pulse

amplitudes of 0.7 ampere were possible and

provided a 70-volt pulse to drive an

oscilloscope directly. Thus, a single

photoelectron resulted in a 10-to-15-volt out-

put pulse; the tubes were operated at gains in

the neighborhood of 109

The measured rise

time he found to be slightly more than one

nanosecond. Refer to Fig. 71. Note in Fig.

126 the termination of the coaxial output

line at the photomultiplier end which re-

duced the output RC time constant. It is

pointed out by Post “the loss of a factor of

two in voltage by terminating at the multi-

plier is compensated by voltage doubling at

the unterminated end.”

Laser Range Finding

A simplified block diagram of a laser

range finder is shown in Fig. 127. Such a

device may be utilized by the military for

-4

PULSE

Fig. 126

Post’s voltage divider and bypass capacitor connections of photomulti-

plier to output coaxial line for pulsed high-voltage operation.

105

113

Photomultiplier Handbook

92CS

32464

Fig. 127

Simplified block diagram of a laser range finder.

tank fire control or in various industrial

surveying type applications. Typically, a

laser is pulsed in a time range of 25

nanoseconds. Measurements are made of

beam travel time with a photomultiplier pick

may be used with a 3/4-inch diameter photo-

multiplier having an S-20 spectral response.

and solid-state junction lasers such as

of the longer wavelengths, the detectors are

more often silicon avalanche diodes. An in-

terference filter is used to pass the

wavelength of radiant energy of the laser

with a minimum of background radiation.

Photomultipliers used in laser range finding

may have a relatively small photocathode,

but they must exhibit high quantum efficien-

cy, low dark noise, and fast rise time or an

equivalent large bandwidth. Most

photomul-

tipliers can provide bandwidths exceeding

100 MHz and at the same time maintain

relatively large output signals. The band-

width of a photomultiplier can be limited by

the RC time constant of the anode circuit.

The range of a laser-range-finding system

depends on system parameters and opera-

tional environment. The maximum range of

a given system may be signal-photon limited

or background limited. The photon-limited

case exists when the background and de-

tector noise can be considered negligible.

The maximum range in this case is deter-

mined by the signal-to-noise ratio in the

photoelectron pulse corresponding to the

114

scattered laser return beam. The

signal-to-

noise ratio of such a pulse is proportional to

the square root of the product of the number

of incident photons on the photomultiplier

and the quantum efficiency of the photoelec-

tric conversion. If the atmospheric attenua-

tion is neglected and it is assumed that the

laser spot falls entirely within the target, the

maximum range in this case would be in-

creased as the square root of the quantum ef-

ficiency of the photocathode. This increase

follows because the number of photons col-

lected by the aperture of the receiving system

varies inversely as the square of the distance

from the target.

In cases where the laser pulse must be

detected against the background of a

daylight scene, it is said to be background

limited. If atmospheric attenuation again is

neglected, the signal is proportional to the

number of incident photons times the quan-

tum efficiency of the photocathode. The

number of incident photons from the return

beam is inversely proportional to the square

of the range, again assuming that the target

is larger than the laser spot. The noise,

however, is independent of the range and is

determined by the square root of the product

of the incident background radiation and the

quantum efficiency. Thus, to a first approx-

imation the range is increased in the

background-limited case only by the fourth

root of the quantum efficiency. Because the

photomultiplier current caused by the

background radiation is proportional to the

solid angle of the scene from which the

photomultiplier collects radiation, the

background current in the photomultiplier

may be minimized by the use of a

photocath-

ode having a small area or by the use of a

limiting aperture on the faceplate of the

photomultiplier. No loss in collected laser

light need result because the return beam

may generally be considered as originating

from a point source. The system aperture, of

course, must be large enough to avoid op-

tical alignment problems.

Scanning Applications

A number of photoelectrically sensed

scanning systems have been devised such as

facsimile scanners in which the material to

be transmitted-a photograph or printed

message-is mounted on a drum that is

rotated to provide scan in one dimension. A

scanning head moves parallel to the drum

axis to provide the other dimension. The pic-

ture element is illuminated with a focused

light spot and the scattered light is picked up

by a photomultiplier, providing high-speed

transmission utilizing the modulated output

electrical signal.

Two more recently developed scanning

systems are the flying-spot scanner for the

development of television signals and the

supermarket checkout system which recog-

nizes a coded symbol on each product.

Flying-Spot Scanning. The elements of a

flying-spot scanning system are shown in

Fig. 128. A cathode-ray tube, in conjunction

with its power supplies and deflection cir-

cuits, provides a small rapidly moving light

source which forms a raster on its face. This

raster is focused by the objective lens in the

optical system onto the object being

scanned, a slide transparency or a motion-

picture

film.

The amount of light passing

through the film varies with the film density.

This modulated light signal is focused upon

the photomultiplier by means of the

condensing-lens system. The photomultiplier

converts the radiant-energy signal into an

electrical video signal. The amplifier and its

associated equalization circuits increase the

amplitude of the video signal as required.

The flying-spot scanning system is capable

of providing high-resolution monochromatic

performance. With the addition of (a) ap-

propriate dichroic mirrors which selectively

reflect and transmit the red, blue, and green

wavelengths,

(b)

two additional photomulti-

Photomultiplier Applications

pliers with video amplifiers, and (c) ap-

propriate filters, color operation is possible.

In color operation, the primary wavelengths

are filtered after separation by the

light-

absorbing filters before being focused upon

each of three photomultipliers, one for each

color channel. The output of each photo-

multiplier is then fed to a separate video

amplifier.

Flying-spot video-signal generators are

used in the television industry primarily for

viewing slides, test patterns, motion-picture

film, and other

fixed

images. Systems have

been developed for the home-entertainment

industry that allow slides and motion-picture

film to be shown on the picture tube of any

type of commercial television receiver.

A similar system is utilized by the

photographic industry to provide accurate

exposure control for film copying. By means

of the three color controls, the operator may

produce a color television display represent-

ing the film and can adjust and measure each

color component to produce a visually satis-

fying balance.

Several important considerations must be

taken into account if the cathode-ray tube in

the system is to produce a light spot capable

of providing good resolution. The cathode-

ray tube should be operated with as small a

light spot as possible. The cathode-ray-tube

faceplate should be as blemish-free as

possible and the tube should employ a

fine-

grain phosphor. Blemishes adversely affect

signal-to-noise performance and contribute

to a loss of resolution.

The spectral output of the cathode-ray-

tube phosphor should match the spectral

characteristic of the photomultiplier. This

match can be rather loose in a monochro-

matic flying-spot generator. The spectral

output of the phosphor of the cathode-ray

tube used in a three-color version, however,

must include most of the visible spectrum.

Phosphors used in monochromatic systems

may provide outputs in the ultraviolet region

of the spectrum and still perform satisfac-

torily. Phosphors such as P16 and P15,

when used with an appropriate ultraviolet

filter, display the necessary short persistence

required in a monochromatic system.

The visible portion of the P15 or P24

phosphor is used in color systems. The P15

and P24 phosphors are, however, much

115

VERTICAL

SCANNING

HORIZONTAL

SCANNING

92CM

-32465

Fig.

128

Elements of a flying-spot scanning system.

slower than the P16 phosphor and cause a

lag in buildup and decay of output from the

screen.

The phosphor lag results in trailing, a con-

dition in which the persistence of energy

output from the cathode-ray tube causes a

continued and spurious input to the photo-

multiplier as the flying spot moves across the

picture being scanned. The result is that a

light area may trail into the dark area in the

reproduced picture.

Similarly, the lag in buildup of screen out-

put causes a dark area to trail over into the

light area. The result of these effects on the

reproduced picture is an appearance similar

to that produced by a video signal deficient

in high frequencies. Consequently,

high-

frequency equalization is necessary in the

video amplifier.

The objective lens used in a flying-spot

generator should be of a high-quality

enlarger type designed for low magnification

and, depending upon the cathode-ray-tube

light output,should be corrected for

ultraviolet radiation. The diameter of the

objective lens should be adequate to cover

the slide to be scanned. An enlarging f/4.5

lens with a focal length of 100 millimeters is

suitable for use with

35-millimeter

slides.

The optics should not image the film on the

photomultiplier because shading effects

would result from non-uniformities of the

photomultiplier response. In some cases it

may be useful to employ a beam splitter

116

between the cathode-ray tube and the lens

and to use a second photomultiplier to sense

any non-uniformities in the cathode-ray tube

raster display. This sensed signal may then

be used to eliminate this source of signal

distortion in the primary photomultiplier

pickup.

The spectral characteristics of the

photomultiplier (or photomultipliers in the

case of the three-color system) and the

cathode-ray-tube phosphor should match.

Usually, a photomultiplier having an S-4 or

S-l1

(Cs3Sb)

spectral response is suitable for

use in a monochromatic system or as the

detector for the blue and green channels. An

S-20 (Na2KSb:Cs) response is very often

utilized for the red channel. The bialkali

cathode (K-CS-Sb) is also well suited for the

blue channel. The speed of the detector must

be sufficient to provide the desired video

bandwidth. Most requirements do not ex-

ceed 6 to 8 MHz, a figure well within photo-

multiplier capabilities.

The anode dark current of the photomulti-

plier should be small compared to the useful

signal current. The signal-to-noise ratio will

be maximized by operation of the

photomul-

tiplier at the highest light levels possible. If

necessary, the over-all photomultiplier gain

should be reduced to prevent excessive anode

current and fatigue.

The amplitude of the light input is usually

a compromise between an optimum

signal-

to-noise ratio and maximum

cathode-ray-

Photomultiplier Applications

tube life. Tube life may be reduced because

of loss of phosphor efficiency at high

beam-

current levels. The signal-to-noise ratio can

also be improved by the selection of photo-

multipliers having the highest photocathode

sensitivities possible. However, because the

spread of photocathode sensitivities is

seldom greater than two or three to one, the

improvement afforded by such selection is

limited to two or three

dB,

an improvement

difficult to detect during observations of a

television display but desirable and necessary

in some critical applications.

Photomultiplier gain need only be suffi-

cient to provide a signal of the required level

to the succeeding video-amplifier stages.

These stages, in addition to providing the

necessary amplification and bandwidth to

assure good picture quality, incorporate

equalization circuits composed of networks

having different time constants. The

relatively long decay time of these circuits

generally results in appreciable reduction of

the useful signal-to-noise ratio. Therefore,

the use of short-persistence phosphors is

recommended to reduce the required amount

of equalization.

In addition to the video amplifier, a

gamma-correction amplifier is required in

each channel. The gammacorrection ampli-

fier assures maximum color fidelity by mak-

ing the linearity or gamma of the system uni-

ty.

Supermarket Checkout Systems.

Fig. 129

shows the elements of a supermarket check-

out system. The Universal Product Code

(UPC), which is to be marked on all prod-

ucts, is scanned by laser beam by means of

an oscillating mirror and a rotating mirror

system. The pattern of the scan is a network

of overlapping sine waves. The reflected pat-

tern contains the modulation of the UPC

symbol and is converted by the photomulti-

plier to an electronic signal that is then

analyzed for the content of the code.

The laser used in the supermarket

checkout system is usually a low-power

He-

Ne type with the principal emission line at

633 nanometers. A suitable photomultiplier

for this application is a two-inch end-on type

having an S-20 (Na2KSb:Cs) spectral

response or an extended-red multialkali

type. Stability and good signal-to-noise ratio

are important characteristics of the photo-

multiplier tube.

ROTATING MIRROR

POLYGON

92CM

32466

Fig. 129

Point-of-sale supermarket checkout system. At left is Universal Product

Code (UPC) symbol that provides 10-digit product identification. At right is sche

matic

of optical system for scanning the product symbol and detecting the modula-

tion with a photomultiplier.

117

Photomultiplier Handbook

REFERENCES

91. R.W. Engstrom and E. Fischer, “Ef-

fects of voltage-divider characteristics on

multiplier phototube response,” Rev. Sci.

Instr.,

Vol. 28, pp 525-527, July, 1957.

92. C.R. Kerns, “A high-rate phototube

base,”

IEEE Trans. Nucl. Sci.,

Vol. NS-24,

No. 1, pp 353-355, Feb. 1977.

92a. V.O. Altemose, “Helium diffusion

through glass,”

J. Appl. Phys.,

Vol. 32, No.

7, pp 1309-1316, 1961.

92b. F.J. Norton, “Permeation of gases

through solids,” J. Appl. Phys., Vol. 28,

No. 1, pp 34-39, 1957.

93. American Institute of Physics Hand-

book, Third Edition, McGraw-Hill, 1972.

94. R.W. Engstrom, “Luminous

Micro-

flux standard,” Rev. Sci.

Instrum.

Vol. 26,

No. 6, pp 622-623, June, 1955.

95. D.G. Fisher, A.F.

McDonie,

and A.H.

Sommer, “Bandbending effects in

Na2K

and

K2Cs

Sb photocathodes,” J.

Appl.

Phys., Vol. 45, No. 1, pp 487-8, Jan. 1974.

96. H.O. Anger, “Scintillation camera,”

Rev. Sci. Instrum.,

Vol. 29, pp 27-33, 1958.

97. M.J. Weber and R.R. Monchamp,

“Luminescence of Bi4Ge3O12: spectral and

decay properties,” J.

Appl.

Phys., Vol. 44,

No. 12, pp 5495-99, 1973.

98. G.L.

Brownell

and C.A.

Burnham,

“Recent developments in positron

scin-

tigraphy,” Instrumentation in Nuclear

Medicine, Vol. 2, Ed: G.J. Hine and J.A.

Sorenson, Chapter 4, pp 135-159, Academic

Press, 1974.

99. M.E. Phelps, E. J. Hoffman, N.A.

Mullani and M.M. Ter-Pogossian, “Ap-

plication

of annihilation coincidence detec-

tion to transaxial reconstruction tomogra-

phy,” J.

Nucl.

Medicine, Vol. 16, No. 3, pp

210-224, 1975.

100. S.E. Derenzo, H. Zaklad, and T.F.

Budinger ,

“Analytical study of a

high-

resolution positron ring detector system for

transaxial reconstruction tomography,” J.

Nucl.

Med., Vol. 16, No. 12, pp

1166-73,

Dec. 1975.

101. M.E. Phelps, E.J. Hoffman,

Sung-

Cheng Huang, and D.E. Kuhl, “ECAT: a

computerized tomographic imaging system

for positron-emitting radiopharmaceuti-

cals,” J.

Nucl.

Med., Vol. 19, No. 6, pp

635-647, June 1978.

102. G.L. Brownell, C. Burnham, B.

Ahluwalia, N.

Alpert,

D. Chester,

Cahavi, J. Correia, L. Deveau, “Positron

imaging instrumentation,” IEEE Trans.

Nucl. Sci.,

Vol. 24, No. 2, pp 914-916, Apr.

1977.

103.

Operating and Service Manual, Model

111 Fluorometer,”

G.K. Turner Associates,

2524 Pulgas Ave., Palo Alto,

Calif.

94303.

104. G.Y. Farkas and P. Varga, “Reduc-

tion of dark current in transparent cathode

photomultipliers for use in optical measure-

ments,” J. Sci. Instrum., Vol. 41, pp.

704-705,

1964.

105. Astronomical Techniques, Edited by

W.A. Hiltner, The University of Chicago

Press, 1962.

106. R.F. Post, “Performance of Pulsed

Photomultipliers,”

Nucleonics,

Vol. 10, No.

pp 46-50, May 1952.

118

Typical Photomultiplier Applications and Selection

Guide

Appendix

A-

Typical Photomultiplier Applications and Selection Guide

The many and varied requirements of

equipment designers and experimenters

preclude the recommendation of a single

photomultiplier as the optimum device for

any given application category. In most ap-

plications, some trade-offs must be made in

electrical characteristics; tube size must be

considered; the environment in which the

device is to be operated can be an influential

factor; and of course, over-all cost is impor-

tant. Each of these constraints can be best

evaluated by the individual designer for the

specific application.

This Appendix defines a number of the

more common applications of photomulti-

pliers and lists tube types which are suitable

or are frequently used for the particular ap-

plication. The listing is not all-inclusive and

is intended to serve only as a general guide

for initial type selection. Other photomulti-

pliers may be satisfactory for the specified

applications when all system requirements

are considered.

CATALOGUE OF PHOTOMULTIPLIER

APPLICATION CATEGORIES

Astronomy:

The guidance of telescopes, in-

tensity measurements, stellar

spectropho-

tometry, and the like.

Colorimetry: The quantitative color com-

parison of surfaces (reflectance) and solu-

tions (transmission).

C-T Scanners: A medical X-ray equipment

that provides a cross section density map

(tomograph) of a patient. A photomultiplier

is used to detect and measure the light flux

from a scintillating crystal.

Densitometry: The measurement of optical

density of photographic negatives, neutral

density filters, and similar materials.

Gamma-Ray Cameras: A scintillation

counter having a single large crystal and a

number of photomultiplier tubes used in

medical applications to map the location of

isotope disintegrations.

High-Temperature Environments:

Applica-

tions such as the logging of deep oil wells, or

geological exploration, and steel-mill process

controls.

Imaging Devices: Acathode-ray tube or

moving mirrors can be used as a light source

to sequentially illuminate a film positive or

negative or a printed page. This system is

used in (1) optical character recognition, (2)

scanning or printed or written material for

transmission by telephone, (3) parts inspec-

tion, and (4) reproduction of motion pic-

tures, slides, and educational material on a

television receiver (color or black and white).

Inspection, High-Speed:

Small objects such

as fruits, vegetables, seeds, candy, toys,

paper products and even glass, metal, and

other industrial parts can be examined for

color and defects as they move at high speed

past one or more photomultiplier tubes.

Laser Detection: Lasers provide unique

light sources; they are spectrally pure and

produce very narrow collimated beams.

They can be very intense and can be made to

produce light pulses of extremely short dura-

tion. The photomultiplier provides time

resolution in the nanosecond and

subnano-

second ranges and is capable of detecting

very low light levels such as those received

from weak reflected laser light pulses.

Photometry: The measurement of illumina-

tion or luminance. Levels of light flux vary

over a wide range in photography,

astronomy, television, and other applica-

tions.

Photon Counting: A method of detecting

photons by counting single photoelectrons

released from the photocathode.

Pollution Monitoring: The analysis of the

level and the nature of contaminants in solu-

tions, gases, and other waste materials.

119

Typical Photomultiplier Applications and Selection Guide

120

Positron Camera: A scintillation-counter-

type device providing tomographic presenta-

tions based on coincident gamma-ray emis-

sion accompanying annihilation of a

positron and an electron . Medical applica-

tions utilize tracer radio nuclides such as

11C, 13N, and 15O.

Process Control: The measurement of

transmitted or reflected light in continuous

flow processes using solids, liquids, or gases.

Detects flaws, improper marking, and

changes in color and optical density. By

using radioactive sources and scintillators,

photomultipliers can be used for the control

of the weight and thickness of opaque

materials.

Radioimmunoassay (RIA): A technique

that enables the measurement of minute

quantities of substances in biological fluids

as small as 10-12 gram. Compounds are

tagged with radioactive isotopes and are

measured with a liquid scintillation counter

for beta-emitting isotopes or a solid crystal

scintillation counter for gamma-emitting

isotopes.

Radiometry: The measurement of irra-

diance or radiance.

Raman Spectrometry: Measurement of the

wavelength shift of scattered photons from a

highly monochromatic source such as a laser

provides information on molecular structure

and bonding energy.

Scintillation Counting: The measurement of

nuclear radiation by detecting light or single

scintillations emitted from a scintillation

material receiving nuclear radiation.

Severe Physical Environments: Applica-

tions such as oil-well logging, satellites, and

military vehicles subject to severe shock and

vibration.

Thermoluminescent Dosimetry (TLD): Cer

tain phosphors emit light when they are

heated after having been exposed to ionizing

radiation. Devices using this principle afford

personnel protection by determining dosage

levels in medical and biological treatments

and studies. Energy stored in TLD’s is pro

portional to dosage over a very wide range.

Time Measurement: In nuclear experiments

the “time of flight” of nuclear particles is

important. Photomultipliers permit time

measurements down to a fraction of a

nonosecond.

Spectrophotometry

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8"s

1P28 Cs3Sb 8337 9 1-1/8"s

1P28A Cs3Sb 8337 9 1-1/8"s

1P28B K2CsSb 8337 9 1-1/8"s

931A GaAs 0080 9 1-1/8"s

931B GaAs 0080 9 1-1/8"s

4526A Na2KCsSb 0080 10 1-1/2"d

C31034 GaAs 8337 11 2"e

C31034A GaAs 8337 11 2"e

S83063E Na2KCsSb 7056 10 1-1/8"e

S83068E Rb2CsSb 7056 10 1-1/8"e

83089-600 Na2KCsSb 7056 10 1-1/8"e

83101-600 Rb2CsSb B270 10 3/4"e

1 0080, B270 – lime glass; 8337 - Schott, 7056 – borosilicate

2 e – end-window; s – side-window; d – dormer window

TLD

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

S83062E Rb2CsSb 7056 10 1"e

83087-100 Rb2CsSb B270 10 3/4"e

83101-600 Rb2CsSb B270 10 3/4"e

1 B270 – lime glass; 7740 – Pyrex ; 7056 - borosilicate

2 e – end-window

Gamma -Ray Cameras

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

4900 K2CsSb B270 10 3" e

6199 Cs3Sb 0080 10 1-1/2" e

S83010E Rb2CsSb 0080 10 1-1/2" e

S83013F K2CsSb 0080 10 3-1/2" e

S83019F K2CsSb B270 10 2" e

S83020F K2CsSb B270 10 60mm h,e

S83021E K2CsSb B270 10 3" e

S83022F K2CsSb B270 10 2" h,e

S83025F K2CsSb B270 10 3" h,e

S83049F K2CsSb B270 83" e

S83053F K2CsSb B270 860mm h,e

S83054F K2CsSb B270 82" e

S83056F K2CsSb B270 83" h,e

S83069E K2CsSb B270 835x46.5mm, mh

S83079E K2CsSb B270 83" sq

1 0080, B270 – lime glass

2 e – end-window; h – hexagonal; mh – modified hexagonal;

sq - square

Typical Photomultiplier Applications and Selection Guide

121

High Temperature Environments

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

C31000AP Na2KSb 7056 12 2"e

C31016G Na2KSb 7056 10 1"e

C83051 Na2KSb sa 10 1"e

C83060 Na2KSb sa 10 1-1/4"e

C83065 Na2KSb 7056 10 1"e

83103 100 Na2KSb 7056 10 3/4"e

1 sa – sapphire, 7056 – borosilicate

2 e – end-window

Flying-Spot Scanners

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

4552 K2CsSb 0080 9 1-1/8"s

1P21 Cs3Sb 0080 9 1-1/8"s

931A Cs3Sb 0080 9 1-1/8"s

931B K2CsSb 0080 9 1-1/8"s

6199 Cs3Sb 0080 10 1-1/2"e

83087-100 Rb2CsSb B270 10 3/4"e

83090-600 Rb2CsSb B270 93/4"e

83101-600 Rb2CsSb B270 10 3/4"e

S83010E Rb2CsSb 0080 10 1-1/2"e

1 0080 – lime glass; 8337 - Schott

2 e – end-window; s – side-window

Inspection, High Speed

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8" s

4552 K2CsSb 0080 9 1-1/8" s

6199 Cs3Sb 0080 10 1-1/2" e

931A Cs3Sb 0080 9 1-1/8" s

931B K2CsSb 0080 9 1-1/8" s

S83062E Rb2CsSb 7056 10 1" e

S83010E Rb2CsSb 0080 10 1-1/2" e

83087-100 Rb2CsSb B270 10 3/4"e

83090-600 Rb2CsSb B270 93/4" e

83101-600 Rb2CsSb B270 10 3/4" e

1 0080, B270 – lime glass, 7056 – borosilicate

2 e – end-window; s – side-window

Raman Spectroscopy

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

8850 K2CsSb 7740 12 2”e

8852 Na2KCsSb 7740 12 2”e

C31034 GaAs 8337 11 2”e

C31034A GaAs 8337 11 2”e

1 8337 - Schott; 7740 – Pyrex

2 e – end-window

Laser Detection

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

4526A Na2KCsSb 0080 10 1-1/2"d

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

8852 Na2KCsSb 7740 12 2"e

S83063E Na2KCsSb 7056 10 1-1/8"e

83087-100 Rb2CsSb B270 93/4"e

83101-600 Rb2CsSb B270 10 3/4"e

C31034 GaAs 8337 11 2"e

C31034A GaAs 8337 11 2"e

1 0080, B270 – lime glass; 8337 - Schott; 7740 – Pyrex

7056 – borosilicate

2 e – end-window; d – dormer window

Photometry

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8”s

931A Cs3Sb 0080 9 1-1/8”s

931B K2CsSb 0080 9 1-1/8”s

4552 K2CsSb 0080 9 1-1/8”s

1 0080 – lime glass

2 s – side-window

Photon Counting

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

8852 Na2KCsSb 7740 12 2"e

8854 K2CsSb 8337 14 5"e

C31034 GaAs 8337 11 2"e

C31034A GaAs 8337 11 2"e

S83062E Rb2CsSb 7056 10 1-1/2"e

83089-600 Na2KCsSb 7056 10 1-1/8"e

83090-600 Rb2CsSb B270 93/4"e

83101-600 Rb2CsSb B270 10 3/4"e

1 0080, B270 – lime glass; 8337 - Schott; 7740 – Pyrex

7056 – borosilicate

2 e – end-window

Positron Cameras

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

83087-100 Rb2CsSb B270 10 3/4"e

83090-600 Rb2CsSb B270 93/4"e

83101-600 Rb2CsSb B270 10 3/4"e

83102 100 Rb2CsSb 0080 9 1"e

S83062E Rb2CsSb 7056 10 1"e

1 0080, B270 – lime glass, 7056 – borosilicate

2 e – end-window

Typical Photomultiplier Applications and Selection Guide

122

Process Control

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8"s

1P28 Cs3Sb 8337 9 1-1/8"s

1P28A Cs3Sb 8337 9 1-1/8"s

1P28B K2CsSb 8337 9 1-1/8"s

931A Cs3Sb 0080 9 1-1/8"s

931B K2CsSb 0080 9 1-1/8"s

2060 Cs3Sb 0080 10 1-1/2"e

4856 K2CsSb 0080 10 2"e

4900 K2CsSb 0080 10 3"e

6199 Cs3Sb 0080 10 1-1/2"e

S83010E Rb2CsSb 0080 10 1-1/2"e

S83019F K2CsSb B270 10 2"e

S83021E K2CsSb B270 10 3"e

S83049F K2CsSb B270 83"e

S83054F K2CsSb B270 82"e

S83079E K2CsSb B270 83"sq,e

1 0080, B270 – lime glass; 8337 - Schott

2 e – end-window; s – side-window

Severe Physical Environments

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

83101 100 Rb2CsSb B270 10 3/4"e

C31016G Na2KSb 0080 10 1"e

C83051 Na2KSb sa 10 1"e

C83060 Na2KSb sa 10 1-1/4"e

1 0080, B270 – lime glass; sa - sapphire

2 e – end-window

Radiometry

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8"s

1P28 Cs3Sb 8337 9 1-1/8"s

1P28A Cs3Sb 8337 9 1-1/8"s

1P28B K2CsSb 8337 9 1-1/8"s

931A Cs3Sb 0080 9 1-1/8"s

931B K2CsSb 0080 9 1-1/8"s

4526A Na2KCsSb 0080 9 1-1.2"d

2060 Cs3Sb 0080 10 1-1/2"e

6199 Cs3Sb 0080 10 1-1/2"e

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

8852 Na2KCsSb 7740 12 2"e

S83010E Rb2CsSb 0080 10 1-1/2"e

S83062E Rb2CsSb 7056 10 1"e

83087-100 Rb2CsSb B270 10 3/4"e

83101-600 Rb2CsSb B270 10 3/4"e

1 0080, B270 – lime glass; 8337 - Schott; 7740 - Pyrex

7056 – borosilicate

2 e – end-window; s – side-window; d – dormer window

Time Measurement

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

8852 Na2KSb 7740 12 2"e

8854 K2CsSb 8337 14 5"e

S83062E Rb2CsSb 7056 10 1"e

S83068E Rb2CsSb 7056 10 1-1/8"e

83087-100 Rb2CsSb B270 10 3/4"e

83101-600 Rb2CsSb B270 10 3/4"e

1 0080, B270 – lime glass; 8337 - Schott; 7740 – Pyrex,

7056 – borosilicate

2 e – end-window

Scintillation Counting

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

2060 Cs3Sb 0080 10 1-1/2"e

4900 K2CsSb 0080 10 3"e

6199 Cs3Sb 0080 10 1-1/2"e

83103 100 Na2KSb 7056 10 3/4"e

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

8852 Na2KCsSb 7740 12 2"e

8854 K2CsSb 8337 14 5"e

C31000AP Na2KSb 7056 12 2"e

C31016G Na2KSb 0080 10 1"e

C31016H Na2KSb 0080 10 1"e

C31034 GaAs 8337 11 2"e

C31034A GaAs 8337 11 2"e

S83006F K2CsSb 0080 10 5"e

S83062E Rb2CsSb 7056 10 1"e

S83010E Rb2CsSb 0080 10 1-1/2"e

83087-100 Rb2CsSb B270 10 3/4"e

83092-500 Na2KSb 0080 10 1"e

83101-600 Rb2CsSb B270 10 3/4"e

1 0080, B270 – lime glass; 8337 - Schott; 7740 – Pyrex

7056 – borosilicate

2 e – end-window

Radioimmunoassay

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

4856 K2CsSb 0080 10 2"e

4900 K2CsSb 0080 10 3"e

6199 Cs3Sb 0080 10 1-1/2"e

S83068E Rb2CsSb 7056 10 1-1/8"e

S83010E Rb2CsSb 0080 10 1-1/2"e

S83019F K2CsSb B270 10 2"e

S83054F K2CsSb B270 82"e

1 0080, B270 – lime glass; 7056 – borosilicate

2 e – end-window

Typical Photomultiplier Applications and Selection Guide

123

Densitometry

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8"s

1P28 Cs3Sb 8337 9 1-1/8"s

931A GaAs 0080 9 1-1/8"s

931B GaAs 0080 9 1-1/8"s

4552 K2CsSb 0080 9 1-1/8"s

1 0080 – lime glass; 8337 - Schott

2 s – side-window

Colorimetry

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8"s

1P28 Cs3Sb 8337 9 1-1/8"s

931A GaAs 0080 9 1-1/8"s

931B GaAs 0080 9 1-1/8"s

83089-600 Na2KCsSb 7056 10 1-1/8”e

4552 K2CsSb 0080 9 1-1/8"s

1 0080 – lime glass; 8337 – Schott; 7056 – borosilicate

2 s – side-window; e – end-window

Astronomy

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

1P21 Cs3Sb 0080 9 1-1/8"s

1P28 Cs3Sb 8337 9 1-1/8"s

1P28B K2CsSb 8337 9 1-1/8"s

8575 K2CsSb 7740 12 2"e

8850 K2CsSb 7740 12 2"e

8852 Na2KCsSb 7740 12 2"e

C31034 GaAs 8337 11 2"e

C31034A GaAs 8337 11 2"e

1 0080 – lime glass; 8337 - Schott; 7740 - Pyrex

2 e – end-window; s – side-window

Pollution Monitoring

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

4526A Na2KCsSb 0080 9 1-1/2"d

S83063E Rb2CsSb 7056 10 1-1/8"e

8850 K2CsSb 7740 12 2"e

8852 Na2KCsSb 7740 12 2"e

83089-600 Na2KCsSb 7056 10 1-1/8"e

1 0080 – lime glass; 7740 – Pyrex, 7056 – borosilicate

2 e – end-window; d – dormer window

Cerenkov Radiation

BURLE

Type Photo-

cathode

Material

Window

Material1No.

Stages

Dia

meter2

8850 K2CsSb 7740 12 2"e

8854 K2CsSb 8337 14 5"e

S83062E Rb2CsSb 7056 10 1"e

83090-600 Rb2CsSb B270 93/4"e

83101-600 Rb2CsSb B270 10 3/4"e

1 0080, B270 – lime glass; 7740 – Pyrex, 8337 - Schott

7056 – borosilicate

2 e – end-window

Typical Photomultiplier Applications and Selection Guide

124

This page is blank.

Glossary of Terms

and

Appendix

B-

Related to Photomultiplier Tubes

Their Applications

This Appendix contains a glossary of

terms frequently used in connection with

photomultiplier tubes and their applications.

Where suitable definitions were available

from established standards or other reliable

sources, they were used, possibly with minor

modifications. For these cases, the sources

of the definitions are indicated by the

reference number at the end of the defini-

tion. In many cases, however, the definitions

were prepared by the writers of this Manual.

In these latter cases, no source reference is

indicated after the definition.

Absorptance: The ratio of the radiant flux

absorbed in a body of material to that inci-

dent upon it. If the absorptance is a, the

Acceptor: An impurity element in a p-type

semiconductor that may become ionized by

taking an electron from the valence band

and induce conduction by holes. For exam-

ple, boron with a valence of three is a possi-

ble acceptor impurity in silicon (valence,

four).

Afterpulse: A spurious pulse induced in a

photomultiplier by a previous pulse. (112)

Angle of incidence:

The angle between a ray

of light striking a surface and the normal to

that surface.

Angstrom unit,

Å:10-10 meter, or 0.1

nanometer. In the International System of

Units (SI units), the nanometer or microm-

eter is the preferred unit for use in specifica-

tion of wavelengths of light.

Anode: An electrode through which a prin-

cipal stream of electrons leaves the inter-

electrode space. In a photomultiplier, the

anode is operated at a voltage positive with

respect to that of the last dynode and collects

the secondary electrons emitted from the last

dynode. (112)

Anticoincidence circuit: A circuit that pro-

duces a specified output signal when one

(frequently predesignated) of two inputs

receives a signal and the other receives no

signal within an assigned time interval. (112)

Background counts (in radiation counters):

Counts caused by ionizing radiation coming

from sources other than that to be measured.

(112)

Bandwidth: In electrical measurements, the

difference between limiting frequencies in a

frequency band, expressed in hertz (cycles

per second). The noise equivalent bandwidth

is the bandwidth of a rectangular equivalent

spectrum and may be defined as

circuit, A is the maximum absolute value of

angular frequency. (110)

Bleeder:

Resistive voltage divider.

Cage: Part of a photomultiplier including

the dynodes, focusing structure, anode, and

support members.

Candela, cd: The SI unit of luminous in-

tensity .

Cathode: See Photocathode.

CAT-Scanner or CT-Scanner: Computer-

ized Axial Tomography

scanner; a medical

X-ray equipment which provides a

cross-

sectional density map of a patient. In a

typical device, an X-ray fan beam incident

125

Photomultiplier Handbook

on and rotating around the patient is

detected by a large array of scintillation

crystals and photomultiplier tubes.

Cerenkov

radiation:

Radiation generated by

a high-energy charged particle moving

through a dielectric with a velocity greater

than the velocity of light in the dielectric

(i.e., greater than c/n where n is the index of

refraction of the dielectric).

Channel, channel number: In scintillation

counting, a number proportional to pulse

height specifying a generally narrow range of

pulse heights.

Channel multiplier: A tubular

electron-

multiplier having a continuous interior sur-

face of secondary-electron emissive material.

(107)

Coincidence circuit: A

circuit that produces

a specified output signal when and only

when a specified number (two or more) or a

specified combination of input terminals

receives signals within an assigned time inter-

val. (112)

-- --

Collection efficiency: The fraction of elec-

trons emitted by the photocathode of a

photomultiplier that lands on the first

dynode. Or more generally, the fraction of

electrons emitted by one electrode that lands

on the next electrode (dynode or anode).

Color temperature: The temperature of a

black body radiator such that its chromati-

city is the same as that of the light under con-

sideration. (109)

Conduction band: A partially filled energy

band in which the electrons can move freely,

allowing the material to carry an electric cur-

rent. The term is usually restricted to

semiconductors and insulators, where the

conduction band is normally empty and is

separated by an energy-gap from the full

bands below it.

(109)

Count (in radiation counters): A single

response of the counting system. (112)

Counting efficiency (scintillation counters):

The ratio of (1) the average number of

photons or particles of ionizing radiation

that produce counts to (2) the average

number incident on the sensitive area. (112)

Crosstalk: As applied to photomultipliers

used in liquid scintillation counting, light

originating internally in one photomultiplier

and transmitted to another, causing coinci-

dent background pulses.

Crystal: See Scintillator.

Curie,

Ci:

A unit of radioactivity defined as

the quantity of an radioactive nuclide in

which the number of disintegrations per

second is 3.7 x

1010.

(B4)

Current amplification (photomultipliers):

The ratio of (1) the output signal current to

(2) the photoelectric signal current from the

photocathode. (112)

Dark current: That current flowing in the

cathode circuit (cathode dark current) or in

the anode circuit (anode dark current) in the

absence of light or radiation in the spectrum

to which the photomultiplier is sensitive.

Delay line: A transmission line for intro-

ducing signal time delay. (112)

Delta function

(Dirac

Delta Function): A

1, when the integration is

carried out over the full range of the

variable. Pulsed light sources are sometimes

referred to as delta light sources when the

length of the pulse is short with respect to the

response time of the photomultiplier or

detecting instrument.

Detectivity, D: Reciprocal noise equivalent

power, NEP; it is expressed in W -1

Detec-

tivity is a figure of merit providing the same

information as NEP but describes the

characteristic such that the

lower

the radia-

tion level to which the photodetector can

respond, the higher the detectivity. See

Noise

Equivalent Power. (108)

Discriminator,

constant-fraction

pulse-

height: A pulse-height discriminator in

which the threshold changes with input

amplitude in such a way that the triggering

point corresponds to a constant fraction of

the input pulse height. (112)

Discriminator, pulse-height: A circuit that

produces a specified output signal if and

only if it receives an input pulse whose

amplitude exceeds in one case or is less than

an assigned value in another case. (112)

Donor: An impurity element in an n-type

semiconductor that may become ionized by

losing an electron to the conduction band

and induce conduction by electrons. For ex-

ample, phosphorus with a valence of five is a

possible donor impurity in silicon (valence,

four).

126

Dynode: An electrode that performs a

useful function, such as current amplifica-

tion, by means of secondary emission. (107)

EADCI, Equivalent Anode Dark Current

Input:

The input flux in lumens or watts at a

specific wavelength which results in an in-

crease in the anode current of a photomulti-

plier tube just equal to the anode dark cur-

rent.

E2/B: A figure of merit used to evaluate

performance of liquid scintillation counters.

E is the counting efficiency in per cent. B is

the number of background coincident counts

per minute.

Electron affinity,

EA:

The energy, usually

expressed in electron volts, required to move

an electron from the bottom of the conduc-

tion band to the vacuum level.

Electron multiplier: That portion of the

photomultiplier consisting of dynodes that

produce current amplification by secondary

electron emission. (112)

Electron resolution:

The ability of the elec-

tron multiplier section of the photomulti-

plier to resolve inputs consisting of n and

n + 1 electrons. This ability may be expressed

as a fractional FWHM of the

nth

peak, or as

the peak-to-valley ratio of the

nth

peak to the

valley between the nth and the (n+

1)th

peaks. (112)

Electron volt: The energy received by an

electron in falling through a potential dif-

ference of one volt. (108)

Equivalent Noise Input,

ENI:

That value of

input radiant or luminous flux that produces

an rms signal current that is just equal to the

rms value of the noise current in a specified

bandwidth (Usually 1 Hz). See Noise

Equivalent Power. (108)

Exitance: The density of radiant flux emit-

ted from a surface. Radiant exitance is the

integral of radiant flux over all wavelengths

with units of watt per square meter. Lumi-

nous exitance

is the total of all luminous flux

from a surface with units of lumen per

square meter.

Spectral radiant exitance

is the

exitance at a particular wavelength for a

specified wavelength interval; units are watt

per square meter and micrometer. The term

emittance

(now deprecated) is synonomous

with exitance. Sometimes the term exitance

is used to indicate the density of radiant flux

Glossary of Terms

incident upon a surface. It is recommended,

however, that the use of the term exitance be

restricted to emission

from

a surface because

terms such

as irradiance, spectral irradiance,

and illuminance are commonly used to in-

dicate flux density incident on a surface.

Extended Red Multi-Alkali, ERMA: A

designation of a Na2K Sb:Cs photocathode

processed to obtain increased

red-near-

infrared response.

Fall time: The mean time difference of the

trailing edge of a pulse between the 90- and

10-per cent amplitude points.

Fatigue: The tendency of a photomultiplier

responsivity to decrease during operation.

Most commonly, responsivity loss is the

result of a lowering of secondary emission,

particularly in the latter stages of a photo-

multiplier. Recovery may or may not occur

during a period of idleness.

Fermi level:

The value of the electron energy

at which the Fermi distribution function has

the value one-half. (107)

Focusing electrode: An electrode whose

potential is adjusted to control the

cross-

sectional area of the electron beam. (112)

Flying-Spot Scanner:

system for generat-

ing video signals for a television display. In a

typical conception, the scanned raster of a

cathode-ray tube is focused onto a photo-

graphic transparency. The modulated trans-

mitted light signal is directed onto the photo-

cathode of a photomultiplier tube which

provides the electrical video signal.

Footcandle: A unit of illuminance equal to

one lumen per square foot. The SI unit of

il-

uminance, the lux (lumen per square meter),

is preferred. (108)

Footlambert: A unit of luminance equal to

the candela per square meter (nit), is pre-

ferred. (108)

Forbidden band: In the band theory of

solids, a range of energies in which there are

no electronic levels. (109)

Full Width at Half Maximum, FWHM: The

full width of a distribution measured at half

the maximum ordinate. For a normal distri-

bution it is equal to

2(2

ln 2)

1/2

times the stan-

Gain (photomultipliers). See Current Ampli-

fication.

127

Photomultiplier Handbook

Gamma-ray camera: A device used in

nuclear medicine to image distributions of

gamma-ray emitters. In a typical instrument,

gamma rays emanating from tracer elements

introduced into the patient are collimated

and cause scintillations in a single large, thin

sodium iodide crystal. An array of photo-

multiplier tubes views the crystal and pro-

vides addressing information with which an

output image of dots is constructed on a

cathode-ray tube.

Hertz: Hz, the SI unit of frequency

equivalent to cycle per second. (108)

Hysteresis: A borrowed term to describe a

cyclic gain variation in photomultipliers

sometimes observed as a result of insulator

charging and discharging.

Illuminance: The density of the luminous

flux on a surface; it is the quotient of the

flux by the area of the surface when the lat-

ter is uniformly illuminated. The SI unit is

the lux, lumen per square meter. (109)

Irradiance: The density of radiant flux on a

surface; it is the quotient of the flux by the

area of the surface when the latter is

uniformly irradiated. The SI unit is the watt

per square meter.

Lambert’s cosine law: A

law stating that the

flux per solid angle in any direction from a

plane surface varies as the cosine of the angle

between that direction and the perpendicular

to the surface. (107)

Light

pipe: An optical transmission element

that utilizes unfocused transmission and

reflection to reduce photon losses. (112)

Liquid scintillation counter: The combina-

tion of a liquid scintillator and one or more

(usually two) photomultiplier tubes used to

measure or count radioactive disintegra-

tions. The most common application is the

counting of beta rays emanating from a

sample mixed with the liquid scintillator.

Lumen, lm: The SI unit of luminous flux. It

is equal to the flux through a unit solid angle

(steradian) from a uniform point source of

one candela. (107, 108)

Luminance:

The luminous intensity per pro-

jected area normal to the line of observation.

Formerly called photometric brightness or

brightness. (108)

Luminous efficacy: The quotient of the

total luminous flux by the total radiant flux.

It is expressed in lumens per watt. For exam-

ple, the maximum luminous efficacy of a

black body (which occurs at about 6600 K) is

95 lumens per watt. (108)

Luminous efficiency: The ratio of the

luminous efficacy of a given source to the

maximum spectral luminous efficacy. For

example, the maximum luminous efficiency

of a black body (which occurs at about 6600

K) is 95 lumens per watt/680 lumens per watt

= 0.14.

Luminous intensity: The luminous flux per

unit solid angle in the direction in question.

It is expressed in

candelas

(lumens per

stera-

dian). (107)

Lux, lx: The SI unit of illuminance equal to

the flux of one lumen uniformly distributed

over an area of one square meter.

Microchannel plate: An array of small

aligned channel multipliers usually used for

intensification. (107)

Multichannel Analyzer, MCA: See

Pulse-

height analyzer.

Negative Electron Affinity, NEA: A term

referring to an electron emitter whose sur-

face has been treated with an electropositive

material in such a way that the conduction

band minimum lies above the vacuum level.

Nit: The name recommended by the Inter-

national Commission on Illumination for

the unit of luminance equal to one candela

per square meter. Note: Candela per square

meter is the unit of luminance in the Interna-

tional System of Units (SI). (107)

Noise (photomultiplier tubes):

The random

output that limits the minimum observable

signal from the photomultiplier tube. (112)

Noise Equivalent Power, NEP:

The radiant

flux in watts incident on a detector which

gives a signal-to-noise ratio of unity. The

bandwidth and the manner in which the

radiation is chopped must be specified as

well as the spectral content of the radiation.

The most common spectral specification is

for monochromatic radiation at the peak of

the detector response. (Some detector

manufacturers rate their detectors in terms

of an NEP having units of watts Hz

1/2

Assuming that the noise spectrum is flat

within the range of the specification and that

NEP is normally specified for a bandwidth

of one hertz, the two forms of NEP are

numerically equal.) (108)

Glossary of Terms

Noise&signal, (photomultiplier tubes):

The

noise output resulting from the statistical

variation in the signal current itself as con-

trasted with that which may be present when

the detector is in the dark.

Opaque photocathode: A photocathode

wherein photoelectrons are emitted from the

same surface as that on which the photons

are incident. (Also called reflective

photo-

cathode.)

(112)

Peak-to-valley ratio: In a pulse-height-

distribution characteristic, the ratio of the

counting rate at the maximum to that at the

minimum-usually preceding the maximum.

Photocathode: An electrode used for ob-

taining photoelectric emission when ir-

radiated. (112)

Photocell:

A solid-state photosensitive elec-

tron device in which use is made of the varia-

tion of the current-voltage characteristic as a

function of incident radiation. (112)

Photomultiplier, PMT: A phototube with

one or more dynodes between its photocath-

ode and output electrode. (112)

Photon counting: The technique, using a

photomultiplier, of counting output pulses

originating from single photoelectrons.

Photopic vision: Vision mediated essential-

ly or exclusively by the cones. It is generally

associated with adaptation to luminance of

at least 3

candelas

per square meter. (107)

Phototube: An electron tube that contains a

photocathode and has an output depending

at every instant on the total photoelectric

emission from the irradiated area of the

photocathode. (112)

Plateau: (counter): The portion of the

counting-rate-versus-voltage characteristic

curve in which the counting rate is substan-

tially independent of the applied voltage.

(109)

Pulse-height analyzer, PHA:

An instrument

capable of indicating the number or rate of

occurrence of pulses falling within each of

one or more specified amplitude ranges.

(112)

Pulse-height distribution:

A histogram dis-

playing the pulse count versus channel num-

ber as obtained with a multichannel ana-

lyzer, particularly as applied to scintillation

counting.

Pulse-height resolution, PHR: The ratio of

the full width at half maximum of the

pulse-

height-distribution curve to the pulse height

corresponding to the maximum of the distri-

bution curve. In scintillation spectroscopy, it

is customary to state pulse-height resolution

as a percentage. (112)

Pulse jitter: A relatively small variation of

the pulse spacing in a pulse train. In

photomultipliers, pulse jitter is the result of

electron transit time variations. (109)

Pulse width: The time interval between the

first and last instants at which the instan-

taneous amplitude reaches a stated fraction

of peak pulse amplitude. (109)

Quantum efficiency (photocathodes): The

average number of electrons photoelectric-

ally emitted from the photocathode per inci-

dent photon of a given wavelength. (107)

Rad: A unit of absorbed radiation equal to

100 ergs per gram-O.01 J/kg in SI units.

(109)

Radiance: The radiant flux per unit solid

angle per unit of projected area of the

source. The SI unit is the watt per steradian

and square meter, Wsr

(109)

Radiant intensity: The radiant flux pro-

ceeding from the source per unit solid angle

in the direction considered. The SI unit is

watt per steradian. (107)

Reflectance: The ratio of the radiant flux

reflected from a body of material to that in-

cident upon it.

(See Absorptance.)

Reflective photocathode: A photocathode

wherein photoelectrons are emitted from the

same surface as that on which the photons

are incident.

(Also

called

opaque photocath-

ode.)

Rem: Abbreviation for roentgen equivalent

man. (1) In older usage, the dose (absorbed)

of any ionizing radiation that will produce

the same biological effect as that produced

by one roentgen of high voltage x-radiation.

(2) The unit of the RBE (relative biological

effectiveness) dose that is equal to the ab-

sorbed dose in rads times the RBE. (109)

Resistance per square: The resistance of a

square of a thin conductive coating mea-

sured between opposite sides of the square.

The value is independent of the size of the

square.

129

Photomultiplier Handbook

Responsivity: The ratio of the output cur-

rent or voltage to the input flux in watts or

lumens. For example, as applied to photo-

multipliers: radiant responsivity expressed in

mA W

at a specific wavelength or

(108)

Rise time: The mean time difference of the

leading edge of a pulse between the 10- and

90-percent

amplitude points.

Roentgen: A unit of X- or gamma-radiation

exposure such that the associated secondary

ionizing particles produce, in air, ions carry-

ing one electrostatic unit of charge of either

sign per 0.001293 gram of air. This quantity

is the equivalent of 2.58 x 10

-4

coulomb per

kilogram of air.

(109)

Scintillation counter: The combination of

scintillator , photomultiplier, and associated

circuitry for detection and measurement of

ionizing radiation. (112)

Scintillator: The body of scintillator

material together with its container. (107)

Scotopic

vision:

Vision mediated essentially

or exclusively by the rods. It is generally

associated with adaptation to luminance

below about 0.03 candela per square meter.

(107)

Secondary emission: Electron emission

from solids or liquids due directly to bom-

bardment of their surfaces by electrons or

ions. (107)

Secondary emission ratio (electrons): The

average number of electrons emitted from a

surface per incident primary electron.

Note: The result of a sufficiently large

number of events should be averaged to en-

sure that statistical fluctuations are negli-

gible. (107)

Semitransparent photocathode: A photo-

cathode in which radiant flux incident on

one side produces photoelectric emission

from the opposite side. Synonymous with

Transmission-mode photocathode.

(112)

Sensitivity: See Responsivity,

the preferred

term.

Spectral luminous efficacy (radiant flux):

The quotient of the luminous flux at a given

wavelength by the radiant flux at that

wavelength. It is expressed in lumens per

watt. The maximum spectral luminous ef-

is 680 lumens per watt at a

wavelength of 555 nm. (107)

Spectral luminous efficiency (radiant flux):

The ratio of the spectral luminous efficacy

for a given wavelength to the maximum

spectral luminous efficacy. Accordingly, the

spectral luminous efficiency, V(X), is the

ratio V(X)

= K(X)/680 and is identical to the

standard visibility factor of the photopic

human eye and to the y-tristimulus value

tristimulus system. (107, 111)

Spectral radiant intensity:

Radiant intensity

per unit wavelength interval; for example,

watts per (steradian-nanometer). (107)

Stage: One step of a multiplier, as one

dynode stage.

Stem: The portion of a photomultiplier

envelope containing the leads to electrodes.

Steradian: The unit of solid angle which

subtends an area equal to the square of the

radius. (107)

Tea-cup: Descriptive term for an RCA

photomultiplier type having a large

cup-

shaped first dynode.

Time jitter: See Transit-time spread.

Time-to-amplitude converter, TAC: An

in-

strument producing an output pulse whose

amplitude is proportional to the time dif-

ference between start and stop pulses. (112)

Traceability:

Process by which the assigned

value of a measurement is compared, di-

rectly or indirectly, through a series of

calibrations to the value established by the

U.S. national standard. (107)

Transit time:

For a discussion of the several

definitions of this term, refer to the section

“Time Effects” in Chapter 4. Photomulti-

plier Characteristics.

Transit-time spread: The FWHM (full-

width-at-half maximum) of the time distri-

bution of a set of pulses each of which cor-

responds to the photomultiplier transit time

for that individual event. (112)

Transmission-mode photocathode:

A pho-

tocathode in which radiant flux incident on

one side produces photoelectric emission

from the opposite side. Synonymous with

Semitransparent photocathode.

(112)

Transmittance:

The ratio of the radiant flux

transmitted through a body of material to

Glossary of Terms

that incident upon it.

See Absorptance.

Vacuum level:

The minimum potential ener-

gy level an electron must reach to escape en-

tirely from the attraction of a solid or an

atom.

Valence band:

The range of energy states in

the spectrum of a solid crystal in which lie

the energies of the valence electrons that

bind the crystal together. (107)

Venetian-blind dynode:

A descriptive term

for a photomultiplier dynode structure. The

dynode is constructed with a number of

parallel slats with space between permitting

secondary electrons to be directed to the next

stage.

Voltage divider (photomultiplier):

A series

string of resistors across which a voltage is

applied providing an appropriate voltage

drop per stage.

Work function: The minimum energy re-

quired to remove an electron from the Fermi

level of a material into field-free space. (107)

REFERENCES

107. IEEE Standard Dictionary of Electrical

and Electronics Terms,

Wiley-Interscience.

108. RCA Electro-Optics Handbook,

EOH-11.

109. The International Dictionary of

Physics and Electronics,

D. Van Nostrand.

110. Information Transmission, Modula-

tion, and Noise,

Mischa Schwartz, McGraw-

Hill, 1959.

111. Leo Levi,

Applied Optics, Vol. 1,

John

Wiley and Sons, Inc., 1968.

112. An American National Standard,

IEEE Standard Test Procedures for Photo-

multipliers for Scintillation Counting and

Glossary for Scintillation Counting Field.

ANSI

N42.9-1972;

IEEE Std 398-1972.

131

Photomultiplier Handbook

Appendix

C-

Spectral Response Designation Systems

The spectral response of a photomultiplier

tube depends primarily on the chemical com-

ponents of the photocathodes. Differences

in spectral response, however, can result

from variations in the processing of

photocathodes even for the same chemicals.

Some photocathodes are of the “opaque” or

“reflective” type where the photoemission is

from the same side as the excitation. Others,

having essentially the same chemistry but of

the

“semitransparent” type where the

photoemission is from the side opposite

from that of the excitation, may have a

somewhat different spectral response. The

short-wavelength-cutoff characteristic of the

spectral response is the result of the

transmission characteristic of the particular

glass used for the photocathode window.

S-Designation System

In the early

1940’s,

the JEDEC (Joint

Electron Devices Engineering Council) in-

dustry committee on photosensitive devices

developed the “W-system of designating

spectral responses. The philosophy included

the idea that the product user need only be

concerned about the response of the device,

not by how it might be fabricated. And, in

fact, the chemical compositions of some of

the photocathodes and dynodes were con-

sidered proprietary by various manufac-

turers. S-numbers were registered from S-l

through S-40. See Table C-I. Subsequently,

the lack of activity of the industry committee

on photosensitive devices resulted in in-

dependent assignment of codes for spectral

responses. In other cases, the spectral

response was described by a curve and an ac-

tual description of the photocathode com-

position and of the window material.

The following material is a brief account

of the different designation systems and

their relationship and meaning. This infor-

132

mation is of value to the photomultiplier

user because it will help in the interpretation

of the published spectral characteristics data

for a specific tube type regardless of which

response designation system is used.

Numerical System

In 1971 RCA/BURLE introduced a

numerical system of spectral response des-

ignations supplementing and overlapping

the JEDEC S-designation system. The rea-

son for this extended system was the prolif-

eration of spectral responses resulting from

the combinations of different glasses, new

photocathode types, and processing varia-

tions. Table C-II is a catalogue of this 1971

numbering system providing an identifica-

tion and description of each number.

Alphanumeric Coded System

In 1976, RCA/BURLE changed its number

system for spectral-response characteristics to

an alphanumeric combination coded system.

The new designations were combinations of

alphanumerics based on (1) the photocath-

ode material, (2) the window material, and

(3) the photocathode operating mode. Table

C-III provides the code for the spectral

response designation in four columns. The

first two digits in the designation number

(Column I) indicate the photocathode

material; the following alphabetic character

(Column II) indicates the window material;

the next alphabetic character (Column III)

indicates the photocathode operating mode.

Where required, the letter “X” is used as a

suffix to the designation to indicate an ex-

tended response in the red or near infrared.

As an example of the usage of this system,

tube type 931A has a spectral response that

was previously designated as 102 (S-4) in

Table C-II. This tube type has a

Cs3Sb

photocathode, a 0080 lime glass window,

Spectral Response Designation Systems

and a reflection-type photocathode. Its

designation according to the 1976 code

system is 20AR.

Similarly, a tube type having a

Cs3Sb

photocathode, a 0080 glass window, and a

transmission-type photocathode is desig-

nated 20AT. This response was previously

designated 107 (S-l 1).

Current Practices

When the coded spectral response designa-

tion system was devised in 1976, it was an-

ticipated that all the information provided

by the code would be useful to customers in

specifying the type of photomultiplier

needed. Experience has indicated that very

few are sufficiently acquainted with the code

to make good use of it. Instead, most

knowledgeable customers prefer to be in-

formed directly of the nature of the

photocathode and the window. As a result,

therefore, BURLE has discontinued the use of

coded spectral response designations except

for occasional reference to some of the more

common JEDEC S-numbers, which have

become well established. Instead, reference

is made to the photocathode material, the

window material, and any special processing

information. In addition, typical spectral

response and other related data are pro-

vided .

133

Photomultiplier Handbook

Table C-I Spectral Response Designations as Specified by JEDEC

S-Designation

S-l

S-2

S-3

S-4

S-5

S-6

S-7

S-8

S-9

S-10

S-11

S-12

S-13

S-14

S-15

S-16

S-17

Photocathode

Composition Window*

Ag-O-Cs

lime glass

(Obsolete; formerly similar to S-l)

Ag-O-Rb lime glass

Cs3Sb

lime glass

Cs3Sb

Corning 9741

uv transmitting

Ag-O-Rb-Cs borosilicate

Cs3Bi

lime glass

(Obsolete; formerly similar to S-l1)

Ag-Bi-O-Cs

lime glass

Cs3Sb

lime glass

(CdS

a photoconductive crystal)

Cs3Sb

fused silica

(Ge

photovoltaic)

(CdS-CdSe

a photoconductor)

(CdSe

a photoconductor)

Cs3Sb

lime glass

S-18

(Sb-S

photoconductor, camera tubes)

S-19

3Sb

fused silica

S-20 Na2KSb:Cs lime glass

S-21 Cs3Sb

Corning 9741

S-22 (Not used)

S-23

Rb-Te fused silica

S-24

(Not used)

S-25

Na2KSb:Cs lime glass

S-26

S-27

S-28

S-29

S-30

S-31

S-32

S-33

S-34

S-35

S-36

S-37

S-38

S-39

S-40

(InSb

photovoltaic)

(Ge:Au

photoconductive)

(InAs

photovoltaic)

(PbSe

photoconductive)

(Ge:Cu

photoconductive)

(PbS

photoconductive)

(PbS

photoconductive)

(PbS

photoconductive)

(InAs

photovoltaic)

(InSb

photoconductive)

(GaAs

photovoltaic)

(Si

photovoltaic)

(PbSe

photoconductive)

(PbSe

photoconductive)

(Ge:Hg

photoconductive)

*Window may be of material other than that

specified, but it will have equivalent spectral

characteristics.

References: 113, 114

134

Notes

semitransparent or

opaque

semitransparent

opaque with

reflecting substrate

opaque

semitransparent

semitransparent,

processed for ex-

tended red response

Number

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

CsSb

117

K2CsSb

118

K2CsSb

119

Na2KSb:Cs

120

CsSb

121

Cs-Te

122

K2CsSb

123

Cs3Sb

124

Cs3Sb

125

Cs-Te

126

K2CsSb

127

Ag-Bi-O-Cs

128

Ga-As

129

Ga-As-P

130

Na2KSb:Cs

131

Na2KSb:Cs

132

Na2KSb:Cs

133

K2CsSb

134

Ga-As

135

Ga-As-P

136

K2CsSb

Spectral Response Designation Systems

Table C-II

RCA/BURLE

1971 Spectral Response Numbering Code

JEDEC

S-Designation

S-l

S-4

S-5

S-8

S-10

S-l 1

S-13

S-19

S-20

Photocathode

Composition

Ag-o-cs

Cs3Sb

Cs-Bi

Ag-Bi-O-Cs

Cs3Sb

Na2KSb:Cs

K2CsSb

Window

lime glass

Corning 9741 glass

lime glass

SiO2

Borosilicate glass

lime glass

Corning 9823 glass

Pyrex

SiO2

Lime or boro-

silicate glass

Pyrex

Corning 9823 glass

Corning 9741 glass

Pyrex

Sapphire, uv grade

SiO2

Sapphire, uv grade

Corning 9741 glass

LiF

Borosilicate or lime

glass

Corning 9741 glass

Borosilicate or lime

glass

Borosilicate glass

Lime or

borosilicate glass

SiO2

Sapphire, uv grade

Lime glass

Notes

transmitting

uv transmitting

extended red:

ERMA III

extended red:

ERMA III

extended red:

ERMA II

135

Photomultiplier Handbook

Table C-II

RCA/BURLE

1971 Spectral Response Numbering Code (cont’d)

Number JEDEC Photocathode

S-Designation Composition Window Notes

137

138

139

140

141

142

Na2KSb:Cs Corning 9741 glass

extended red:

ERMA II

Na2KSb:Cs

Corning 9741 glass

extended red:

ERMA I

Na2KSb

Borosilicate glass high temperature

In.06-Ga.94-As Corning 9741 glass Type I

In.12-Ga-.88-As

Corning 9741 glass

Type II

In.18-Ga.82-As

Corning 9741 glass

Type III

Table C-III RCA/BURLE 1976 Coded System for Spectral Response Designation

Column I

10 = AgOCs

15 =

AgBiOCs

20 =

CsSb

CsBi

30 =

CsTe

KCsSb

(Bialkali)

40 =

NaKSb

(High

temperature bialkali)

RbCsSb

50 = NaKCsSb (Multialkali)

= NaKCsSb

(ERMA

= NaKCsSb

(ERMA

II)

= NaKCsSb

(ERMA

III)

60 = GaAs

= InGaAs (Type I)

= InGaAs (Type II)

= InGaAs (Type III)

Column

Column III

= 0080 (lime glass) or

D = Dormer-window type

7056 (Borosilicate glass) R = Reflection Type

= 7740 (Pyrex)

= Transmission Type

= 9741

(UV

transmitting

glass)

= 9823

(UV

transmitting

glass)

SiO2

(Fused silica)

= UV-grade Sapphire

LiF

Column IV

X= Extended Response

REFERENCES

113. “Relative spectral response data for

photosensitive devices

(“S”

curves)“,

JEDEC Publication No. 50, Electronic In-

dustries Association, Engineering

Depart-

ment, 2001 Eye Street, N.W., Washington,

D.C. 20006 (1964)

114. “Typical characteristics of

photosen-

sitive surfaces,”

JEDEC Publication No. 61,

Electronic Industries Association,

Engineer-

ing Department, 2001 Eye Street, N. W.,

Washington, D.C. 20006. (1966)

136

Spectral Response Designation Systems

Appendix

D-

Photometric Units and Photometric-to-Radiometric Conversion

Photometry is concerned with the mea-

surement of light. Because the origins of the

photoelectric industry were associated with

the visible spectrum, the units first used for

evaluating photosensitive devices were pho-

tometric. Today, however, even though

many of the applications of photosensitive

devices are for radiation outside the visible

spectrum, the photometric units are still re-

tained for many purposes. Because these

units are based on the characteristics of the

eye, this discussion begins with a considera-

tion of some of these characteristics.

CHARACTERISTICS OF

THE EYE

The sensors in the retina of the human eye

are of two kinds: “cones” which predomi-

nate the central (or foveal) vision and

“rods” which provide peripheral vision. The

cones are responsible for our color vision;

rods provide no color information but in the

dark-adapted state are more sensitive than

the cones and thus provide the basis of dark-

adapted vision. Because there are no rods in

the fovea1 region, faint objects can more

readily be observed at night when the eye is

not exactly directed toward the faint object.

The response of the light-adapted eye (cone

vision) is referred to as the Photopic eye

response; the response of the dark-adapted

eye (rod vision) is referred to as Scotopic eye

response.

Although characteristics of the human eye

vary from person to person, standard

luminosity coefficients for the eye were

defined by the Commission Internationale

d’Eclairage (International Commission on

Illumination) in 193 1. These standard C . I. E.

luminosity coefficients for photopic vision

are given in Table D-I. They represent the

relative luminous equivalents of an

equal-

energy spectrum for each wavelength in the

visible range, assuming foveal vision. An ab-

solute

“sensitivity” figure established for

the standard eye relates photometric units

and radiant power units. At 555 nanometers,

the wavelength of maximum sensitivity of

the eye, one watt of radiant power cor-

responds to approximately 680 lumens. The

quotient of the luminous flux at a given

wavelength by the radiant flux at that

wavelength is referred to as the Spectral

Luminous Efficacy ,

D-l

Various determinations of the maximum

value, K (555 nanometers), have varied

somewhat from the nominal value of 680

lumens per watt.

For the dark-adapted eye, the peak sen-

sitivity increases and is shifted toward the

violet end of the spectrum. A tabulation of

the relative scotopic vision is also given in

Table D-I. The peak luminosity for scotopic

vision occurs at 511 nanometers and is the

equivalent of 1746 lumens/watt. Fig. D-l

shows the comparison of the absolute

luminosity curves for scotopic and photopic

vision as a function of wavelength.

The sensitivity of the eye outside the

wavelength limits shown in Table D-I is very

low, but not actually zero. Studies with in-

tense infrared sources have shown that the

eye is sensitive to radiation of wavelength at

least as long as 1050 nanometers. Fig. D-2

shows a composite curve given by Griffin,

Hubbard, and

Wald115

for the sensitivity of

the eye for both foveal and peripheral vision

from 360 to 1050 nanometers. According to

Goodeve116 the ultraviolet sensitivity of the

eye extends to between 302.3 and 312.5

nanometers. Below this level the absorption

137

Photomultiplier Handbook

Table D-I

Relative Luminosity Values for Photopic and Scotopic Vision

Wavelength

(nm)

410

420

430

450

460

470

480

490

500

510

520

530

540

550

560

570

580

590

610

620

630

640

650

660

670

680

690

700

710

720

730

740

750

760

770

(B>3 cd m-2)(B<3

10-5cdm-2)

0.00004

0.00012

0.0004

0.0012

0.0040

0.0116

0.023

0.038

0.060

0.091

0.139

0.208

0.323

0.503

0.710

0.862

0.954

0.995

0.952

0.870

0.757

0.631

0.503

0.381

0.265

0.175

0.107

0.061

0.032

0.017

0.0082

0.0041

0.0021

0.00105

0.00052

0.00025

0.00012

0.00006

0.00003

0.0003

0.0008

0.0022

0.0055

0.0127

0.0270

0.0530

0.0950

0.157

0.239

0.339

0.456

0.576

0.713

0.842

0.948

0.999

0.953

0.849

0.697

0.531

0.365

0.243

0.155

0.0942

0.0561

0.0324

0.0188

0.0105

0.0058

0.0032

0.0017

0.0009

0.0005

0.0002

0.0001

138

Photometric Units and Photometric-to-Radiometric Conversion

of radiation by the proteins of the eye lens

apparently limits further extension of vision

into the ultraviolet. Light having a wave-

length of 302 nanometers is detected by its

fluorescent effect in the front part of the eye.

WAVELENGTH-NANOMETERS

92CS-

32467

Fig. D-7

Absolute luminosity curves for

scotopic and photopic eye response.

PHOTOMETRIC UNITS

Luminous intensity (or candlepower) de-

scribes luminous flux per unit solid angle in a

particular direction from a light source. The

measure of luminous intensity is the fun-

damental standard from which all other pho-

tometric units are derived. The standard of

luminous intensity is the candela; the older

term candle is sometimes still used, but

refers to the new candle or candela.

The candela is defined by the radiation

from a black body at the temperature of

solidification of platinum. A candela is

one-

sixtieth of the luminous intensity of one

square centimeter of such a radiator.

-PERIPHERY

-8

-10

300 500 700 900

1100

WAVELENGTH

NANOMETERS

92CS-

32468

Fig.

D-2

Relative spectral sensitivity of the

dark-adapted foveal and peripheral retina.

A suitable standard for practical photo-

electric measurements is the AJ2239 cali-

brated lamp, which operates at a current

of about 4.5 amperes and a voltage of 7 to 10

volts. A typical lamp calibrated at a color

temperature of 2856

provides a luminous

intensity of 55 candelas. The luminous inten-

sity of a tungsten lamp measured in candelas

is usually numerically somewhat greater than

the power delivered to the lamp in watts.

A color temperature of 2870 K served as

the basic test standard in this country for

about 30 years. A change had been made to

agree with C.I.E. illuminant A, a more wide-

spread standard that at first required a color

temperature of 2854 K, but has more recent-

ly been adjusted to 2856

to accommodate

to the international practical temperature

scale of

1968.

Unknown PMTHandbook

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