Use of the photomultiplier FEU-12 in scintillation spectrometry

J. Nuclear Energy II, 1957. Vol. 4. pp. 358 to 365.

Pergamon Press Ltd.. London

USE OF THE PHOTOMULTIPLIER FEU-12 SCINTILLATION SPECTROMETRY” Y. A. NEMILOV, V. M. OVCHINNIKOV, A. N. PISAREVSKY, and

IN

E. D. TETERIN

Abstract-The spectral properties and certain other characteristics are given for the FEU-12, a photomultiplier having Venetian-blind type dynodes, developed by G. S. WILDGRUBE and collaborators. INTRODUCTION IN recent years the importance of scintillation counting as an experimental technique

has increased considerably. its application has been especially wide in the field of nuclear spectroscopy. Photomultipliers which are to be used in scintillation spectrometers have to satisfy the following requirements: 1. A high degree of amplitude resolution, which is ultimately determined by : (a) a high effective photocathode sensitivity,? (b) the spectral sensitivity of the photocathode conforming to the luminescence spectrum of the phosphor used. 2. Adequate general sensitivity. 3. Constancy of the general sensitivity of the photomultiplier in continuous and in intermittent use. 4. Rapid attainment of stable operating conditions after switching on. 5. Linearity of the response characteristic over a wide range of light levels. 6. Low inherent noise. 7. Pentode-type input and output characteristics. 8. The general sensitivity of the photomultiplier being independent of load within wide limits. 9. Minimal influence of intense radioactive radiations on the general sensitivity. 10. Minimal sensitivity to external magnetic fields and a low order of microphony. 11. High overload capacity. The combination of all these requirements amounts to a most exacting specification, which by no means all photomultiplying systems fulfil satisfactorily. For example, the widely used photomultiplier FEU-19, while possessing a high general sensitivity, can only give satisfactory discrimination within a limited energy range of the radiation studied, even when selected photocathodes of high quantum efficiency are used and the voltages are taken from a special potential-divider arrangement. Furthermore, it hardly satisfies any of the remaining requirements. For these reasons its field of application in scintillation spectrometry is rather narrow. The inadequacies of the FEU-19 are the natural consequence of using antimony-caesium emitters as the multiplying surfaces. The suitability for gamma-ray scintillation spectroscopy of the photomultiplier type FEU-12, developed in G. S. WILDGRUBE’S laboratory, is examined below in the light of the above requirements. * Translated by G. C. PETTITT from Atomnaya Energiya 1, No. 4, 51 t By effective photocathode the first dynode.

sensitivity is understood

358

(1956). the overall sensitivity measured

at the output of

Use of the photomultiplier

FEU-12 in scintillation spectrometry

CONSTRUCTIONAL

359

DETAILS

The FEU-12 has a semi-transparent photocathode of the end-window type, Behind the photocathode follows the focusing system, of which 50 mm in diameter. one of the electrodes may be used for cutting off the electron stream. Next comes a multiplying system of the “Venetian-blind” type consisting of twelve dynodes : the electrons follow linear paths in the FEU-12. The multiplying system is completed by a reflector-type collector. The envelope of the multiplier is fitted with a standard cathode-ray tube base, and the maximum diameter of the tube is determined by this diameter. Leads from the photocathode, modulator, and collector are brought directly out through the glass, and being of the stud type, do not result in any increase in dimensions. All insulation and supports of the dynodes within the bulb are of ceramic. The exceptional rigidity of the whole system distinguishes the FEU-12 favourably from multipliers of other types. The FEU-12 is produced either with an antimony-caesium or with a bismuth-silvercaesium photocathode. The dynode system is stamped from a special alloy which is activated in the production process. A general view of the photomultiplier is shown in Fig. 1. SOME

STATIC

CHARACTERISTICS

The most important static characteristics of a photomultiplier are : (1) The light characteristic; (2) The current-voltage characteristics of the first and last stages; (3) The spectral characteristic of the cathode; (4) The uniformity of response over the photocathode surface, as measured at the output ; (5) The overall sensitivity of the photocathode.

0

5 *

10

15

I

I

I

20

25

30

Light flux @ FIG. 2.-The

c mlm

static light characteristic of an individual FEU- 12.

Other things being equal, the static light characteristic gives an indication of the dynamic range which can be covered during measurement without altering the supply conditions of the photomultiplier. The static light characteristic of the FEU-12 is shown in Fig. 2. It is linear up to mean output currents in excess of 50 mA. Such currents can be taken for protracted periods.

FIG. I.-The

FEU-12.

p. 358

360

Y. A. NEMILOV, V. M. OVCHINNIKOV, A. N. PISAREVSKY,and E. D. TETERIN

In Figs. 3a and 3b are shown the current-voltage characteristics of the first and last stages, which will be seen to have a well-defined pentode character. The former reaches saturation at 25 V, the latter at 40 V, for all the specimens investigated. The pentode nature of these characteristics excludes the undesirable phenomena which usually ,arise in the case of multipliers having characteristics with sharply defined maxima. In the case of the first stage, it is ensured by the focusing system ul

Photo-multiplier

No. N99

Light flux = 3.1 x lo-’ Im

Light flux = 2 x 10e4 Im *O-~O_~_-O-O-Q--o---c

FIG. 3.-(a)

V ua Current-voltage characteristic of the first stage. (b) Current-voltage characteristic of the output stage of the FEU-12.

used for photoelectron collection ; for the output characteristic it is determined by the reflector-type collector. In Fig. 4 are shown the averaged spectral characteristics of antimony-caesium and bismuth-silver-caesium photocathodes for the multipliers investigated. The characteristic of the Bi-Ag-Cs extends to a longer wavelength spectrum range than that of the Sb-Cs, and it has a wider response band. The uniformity of the photocathode was examined at a spectral peak of an alpha emitter for three specimens of the multiplier. For this purpose a small crystal of CsI(T1) was used, 2.5 mm diameter x 1.5 mm high. On it was placed an alphaemitter which was effectively a point source. For the worst of the three specimens the sensitivity variation over the area of the photocathode did not exceed lo%, while over the two-thirds of the area nearest the tube axis the variation was only 5%. The sensitivity of the photocathodes varied in the samples under investigation from 30 to 80 ,uA lumen- l. Of twenty-five specimens examined, 80 % had a sensitivity

Use of the photomultiplier

FEU-12 in scintillation spectrometry

361

in excess of 45 ,uA lm-l when tested with a type A light source with colour temperature 2848”. Current amplification, for the tubes tested, was found to lie between IO5 and 4 x 106, when a supply voltage of 1600 was uniformly distributed between the electrodes. If necessary, the amplitication coefficient can be raised to 106-10’ by increasing the voltage, which can be taken up to 2000-2500 without damage. To conclude the examination of the static characteristics, it may be noted that those parameters, determined by the geometry of the multipliers, were virtually identical in all the specimens investigated.

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? 40-

x 20-

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I 400

500

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700

It FIG.

USE

4.-Averaged OF

THE

_

800 pm

spectral characteristics of (1) Sb-Cs and (2) Bi-Ag-Cs photocathodes. FEU-12

IN

CHOICE

A SCINTILLATION OF

SUPPLY

SPECTROMETER

SYSTEM

The multipliers have been used in several varieties of gamma-scintillation spectrometer (single, pair, etc.). However, the functioning of a photomultiplier is most clearly demonstrated when it is used in a spectrometer of the single type, and only this type will be considered. Crystals of NaI(T1) and CsI(T1) were used. In the following table the relative signal-strengths for crystals and photocathodes of both types are shown. The figures are averaged results for crystals of medium quality and for different multipliers. In some cases there were variations of up to 50 % from these values. Photomultipliers with Bi-Ag-Cs photocathodes can be used in conjunction with phosphors whose luminescence spectrum is shifted into the green region of the spectrum. The correct choice of supply system of a photomultiplier is sufficiently important to warrant special consideration. For example, it is decisive in the adjustment of the FEU-19. An important merit of the FEU-12 is that it demands no special distribution of the supply system and it is possible to work with an equal voltage between all electrodes. The dependence of amplitude resolution on the supply system was investigated for several photomultipliers. The results show that the best resolving power is obtained with a voltage ratio between electrodes of 2-3 : 1 : , . . 1 (Fig. 5). For all the tubes examined this arrangement appeared to be the optimum from the point of view of resolving power and signal/noise ratio. No special individual adjustments of supply system were called for at all.

362

Y.

A. NEMILOV,V. M. OVCHINNIKOV,A.

N.

PISAREVSKY,

and

E. D. TETERIN

Fig. 6 shows the results of tests for linearity of response in conjunction with a crystal of CsI(T1). A similar result was also obtained with a crystal of NaI(T1). There was no observable deviation from linearity in any of the tubes tested.

I-+

FIG. 5.-Diagram I-photocathode,

showing the electrode arrangement and the voltage distribution between dynodes. R,=2-3,R,=R,=...=R,,=l 2-focusing electrodes, 3-Venetian-blind type dynodes, 4-collector.

The signal amplitude from the multiplier attained 40-50 V, and with some specimens it was possible to obtain signals of up to 100 V by increasing the supply voltage to 2000-2400. The resolution of the multipliers was tested in conjunction with a crystal of NaI(T1). For all multipliers having a photocathode sensitivity higher than 45 ,uA lm-l In

r

f 30-

No.

Photomultiplier

N78

.-9 -c, 2 20 -’

EY FIG. L-Response

MeV

of the FEU-12 in conjunction with CsI(TI).

the resolving power, as measured by the width of the spectral peak of Cs137at halfheight, was not worse than 13 %. For a good-quality crystal of NaI(T1) and a multiplier with Sb-Cs photocathode (E = 54 ,uA/lm-I) a resolving’power of 8 % was obtained for the Cs13’line. In Figs. 7 and ,8 are shown the gamma-spectra of several isotopes taken with crystals of average quality and photocathodes of average sensitivity. Writing A for the amplitude resolution and E for the gamma-ray energy, the law A NE* is well borne out for an energy range within which the crystal dimensions do not too seriously affect the resolving power. The relation A .- E*, where E is the

363

Use of the photomultiplierFEU-12 in scintillationspectrometry

photocathode sensitivity, is realized for multipliers with photocathodes of similar spectral characteristics. Conformity with this latter equation bears witness to the high degree of geometrical uniformity in the different specimens. ,1.12

100 FIG. 7.-They

INHERENT

spectrum

90

MeV

80 70 60 50 40 Discrimination threshold -

of Zn 65 , taken with a CsI(T1) crystal 2.0 cm.

30

3.0 cm and height

of diameter

NOISE.

STABILITY OF PARAMETERS. SENSITIVITY TO ELECTROMAGNETIC FIELDS With the optimum supply system indicated in Section 4, the threshold of virtually complete exclusion of noise corresponded to the height of a pulse which would represent an energy release in a crystal of NaI(T1) in no case exceeding 15 keV. For the 1.17MeV~ ,,

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1 60

50

40

Discrimination FIG. 8.-Peaks

30

w.,

20

threshold

from the y spectra of Co 0°, ZnG5, and Cs13’, taken with a NaI(T1) crystal diameter 3.0 cm and height 1.2 cm.

of

majority of the specimens the threshold lay below 10 keV, while for a few it amounted to 4-5 keV. All measurements were taken within a metallic screen with the positive side of the EHT earthed. If such a metallic screen at photocathode potential is put on to the front part of the bulb, then the threshold of exclusion of noise is lowered by a factor of approximately 1.5. The effect of the screen is especially strongly felt when the supply voltages are raised. With voltages of over 2000, the threshold goes up, and the signal/ noise ratio is worsened.

364

Y. A. NEMILOV,V. M. OVCHINNIKOV,A. N. PISAREVSKY,and E. D. TETERIN

A particularly essential requirement of a multiplier is its stability of functioning and maintenance of parameters from one cycle of measurements to the next. Many photomultiplying systems do not stand up to this, perhaps the most severe test of all. Fig. 9 shows the stability curve of the FEU-12 for the position of the spectral peak from Cs i3’, taken over a period of eight hours of uninterrupted functioning. Photomultiplier . .

No. N99 .

.

.

.

> ‘3 c, 0.8 a, k 0.70 k 0*6d 0.5 0

I

I

I

I

I

I

1

2

3

4

5

6

~

Time in months

FIG. 9.-Stability

of the FEU-12 under continuous operation.

Before each measurement the recording system was calibrated by pulses of standard amplitude. Deviation from the original position of the spectral peak does not exceed l-1*5%, and no definite tendency towards one particular side is observable. A similar stability of functioning was characteristic of all samples investigated.

Photomultiplier

50 45

No. N99 .

d-79

.

B-.-

.

40 35 30 251

1

1

1

1

2

3

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1

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I

I

5

6

7

8

9

10

time

FIG. lO.-Stability

I ;,

12 hr

of the overall sensitivity of the FEU-12.

The parameters of the FEU-12 have a high degree of permanence. Meticulous observation of a randomly chosen specimen over a period of five months gave the results presented in Fig. 10. Similarly, in the course of prolonged work with other specimens, no variation in the general sensitivity in excess of 5-10’% was observed. It should also be noted that many specimens were subject for long periods to strong sources of radioactive radiation. In view of the absence of sharp focusing, the electron beam of the FEU-12 is less [sic] subject to the influence of external electromagnetic fields than systems with trough-shaped emitters. The multipliers function without damage in the direct proximity of a cyclotron magnet in a screen consisting of two layers of 0.5~mm-thick transformer steel and permalloy.

Use of the photomultiplier

FEU-12 in scintillation spectrometry

365

The FEU-12 is little affected by radioactive radiation. This assertion follows from the fact that within the limits of the permissible current in the potential divider, gamma rays and neutrons in a neighbouring energy range exerted no influence on the position of lines measured in the energy scale. No special study was made of the transitory conditions obtaining after connection of the EHT. However, no appreciable change in the counting rate in any differential channel of the energy scale was observed after a delay of 2-3 min. SUPPLEMENTARY

REMARKS.

FURTHER

POSSIBILITIES

In collaboration with B. V. RWAKOV of N. A. VLASOV’S laboratory, the authors measured the rise time of the FEU-12 output pulse by the method of delayed selfcoincidences. The result was 1.5-2 x lo-* sec. Among the unexplored possibilities of the instrument, which in our opinion should lead to a still greater improvement in its resolving power, the following may be indicated :(a) Displacement of the maxima in the spectral characteristics of the photocathodes to a shorter wavelength region of the spectrum; and (b) modernization of the first section so as to enhance its collection efficiency. L CONCLUSIONS 1.

The FEU-12 is suitable in all respects for use in scintillation spectroscopy. 2. It appears to be particularly suitable for work with large radioactivities or with short half-lives, and also for work against a background of strong activity in neighbouring energy regions. 3. The high stability of the instrument and the permanence of its characteristics are worthy of special mention. The authors take pleasure in thanking G. S. WILDGRUBE for putting at their disposal both the photomultipliers and certain materials called for by the tests. REFERENCES 1. CHECKIKN. O., FAINSTEIN S. M., and LIFSHITS T. M. (1954). 2. BIRKSJ. Scintillation Counters. 3. Nucleonics 12, No. 3 (1954).

Electron Multipliers. Gostekhizdat, Moscow