Photomultiplier Gain Stabilization CircuitM. Ageno and C. Felici Citation: Review of Scientific Instruments 34, 997 (1963); doi: 10.1063/1.1718673 View online: http://dx.doi.org/10.1063/1.1718673 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/34/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in New method for gain control of photomultipliers Rev. Sci. Instrum. 47, 152 (1976); 10.1063/1.1134468 An accurate method of photomultiplier gain determination Rev. Sci. Instrum. 45, 1071 (1974); 10.1063/1.1686812 Automatic Gain Control for Photomultiplier Tubes Rev. Sci. Instrum. 35, 1560 (1964); 10.1063/1.1719208 Precision Photomultiplier Gain Stabilization Rev. Sci. Instrum. 27, 240 (1956); 10.1063/1.1715535 Stabilization of Photomultiplier Tubes Rev. Sci. Instrum. 23, 770 (1952); 10.1063/1.1746171

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

129.105.215.146 On: Sat, 20 Dec 2014 20:34:20

THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 34, NUMBER 9 SEPTEMBER 1963

Photomultiplier Gain Stabilization Circuit

M. AGENO AND C. FELleI

Physics Laboratory, Istituto Superiore di Sanita., Rome, Italy

(Received 22 May 1963)

A simple method is described for the gain stabilization of a photomultiplier in which the stabilization system does not interfere with the fast pulse spectrum to be measured. The functioning principle consists in sending a low fre.quency (77 Hz) and very low intensity light pulse, which gives a modulation of the photomultiplier thermionic nOise, on the photocathode together with the scintillation light. The modulation frequency is then selected on the anode by a s~ecial filter. The signal obtained in this way is used to trigger a feedback circuit, which makes the photomultiplier gain constant by suitably varying its voltage.

1. INTRODUCTION

ONE of the difficulties that confronts us when we wish to make accurate measurements of the pulse amplitude

distribution in a scintillation counter depends on the fact that the gain of a photomultiplier is usually not very stable. In practice slow alterations often occur in the emit­ting surface and cause variations of the multiplication coefficient and consequently a long term drift of the total gain. Moreover, as it has often been pointed out, the total photomultiplier gain varies with the pulse rate. l -4

In order to avoid these disadvantages several photo­multiplier gain stabilization circuits have been suggested by several authors. Such circuits are usually based on the following principle. A reference line is used, consisting of a train of pulses which are superimposed on the spectrum of pulses under observation (scintillations produced either by 'Y rays or high-speed particles in a phosphor crystal). The various stabilizer types suggested differ from one another either in the nature of the reference line or in the system used to keep the amplitude of its pulses constant.

The choice of a 'Y line,5 as a reference line, has the dis­advantage of introducing an appreciable alteration in an extensive region of the spectrum under observation, caused by the continuous distribution of pulses due to the Comp­ton effect in the crystal. From this point of view perhaps, the use of an auxiliary source emitting monoenergetic a particles5 ,6 as a reference line would be more satisfactory, but this method also has its drawbacks since it does not allow an easy adaptation of the amplitude of the reference line to the spectrum zone under observation. It therefore makes useless a certain region of the spectrum in which measurements cannot be made, nor can a distribution of unknown pulses be superposed on the reference line, since this could presumably distort the line profile and cause the reference to shift in an uncontrollable way.

~ R. L. Cald,,:ell and S. E. Turner, Nucleonics 12, 47 (1954). . P. R. Bell m Beta and Gamma-Ray Spectroscopy, edited by K.

Slegbahn (North-Holland Publishing Company, Amsterdam, 1955), p.147.

3 H. Jung, P. Panussi, and J. Janecke, Nucl. Instr. & Methods 9, No.2, 121 (1960).

4 R. Chery, J. Phys. Radium 21, 679 (1960). • H. de Waard, Nucleonics 13, 36 (July 1955). 6 S. A. Scherbatskoy, Rev. Sci. Instr. 32, 599 (1961).

997

Other authors7 suggest the use of an artificial line as the reference line, obtained by means of a pulsed luminous source. In this case, it is easy to vary the position of the line with respect to the spectral region under observation. As a reference line, a constant light source has also been used8 ; the voltage drop, produced in a resistance by the photomultiplier anode current, is compared in this case with the voltage of a Zener diode and the error signal is used to regulate the high voltage. The disadvantage of this method is that it requires the use of a de amplifier, the stability of which limits the efficiency of the apparatus. Furthermore, the introduction of a continuous component increases the statistical fluctuations of the number of photoelectrons and, therefore, influences the pulse spec­trum under observation, viz. the dispersion increases and the resolving power decreases.

Recently Marlow9 built a photomultiplier voltage stabi­lizer, which is an improved type of the de Waard stabi­lizer. The most interesting innovation is the use of a Raysistor to vary the amplitude of the phototube output signals. In this way, the high voltage applied to the photomultiplier is kept constant and thereby variations of the electron transit times are not introduced. This is especially important when the photomultiplier is included in a coincidence system with a high resolving power.

2. THE NEW METHOD

All the versions mentioned above leave still unsolved the problem of how to realize a gain stabilization which inter­feres as little as possible with the detection of the light pulses coming from either a scintillation or a Cerenkov counter.

We faced this problem two years ago and the solution we suggested was presented at the 8th Internati~nal Elec­tronics Congress in Rome, June 1961,10 A brief description of the apparatus, under test at that time, has also been

7 S. Haun and D. Kamke, Nucl. Instr. & Methods 8,331 (1960). 8 F. P. G. Valda, Nuc!. Instr. & Methods 10, 234 (1961). 9 K. vy. Marlow, Nucl. Instr. & Methods 15, 188 (1962). 10 Ath dell' 8° Congresso Internazionale per l'Elettronica Roma,

19-24 giugno 1961.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

129.105.215.146 On: Sat, 20 Dec 2014 20:34:20

998 M. AGENO AND C. FELlCl

FIG. 1. Block diagram of the circuit stabilizer.

publishedY Subsequently, a stabilization system, essen­tially identical to this, has been described in the same brief way, by Rijks.12

However, a year's experience has shown some disadvan­tages of the original version of our apparatus which have led us to introduce some important modifications. In this paper it is our purpose to describe the latest version, which has the required properties, as a long series of tests has shown.

The apparatus is based on the following idea. Introduc­ing in the light guide a suitable luminous signal, it is possible to superpose on the natural photomultiplier back­ground a low frequency component of convenient am­plitude. This component appears amplified at the anode, and it is possible to single it out from all the other back­ground components by means of a tuned amplifier. Suit­ably rectified, the tuned amplifier output is then compared with a reference voltage and the error signal is employed to vary the photomultiplier supply voltage, so that the gain is always brought back to the same constant value. The advantage of this method in comparison with the preceding ones is that a low frequency modulation of the photomultiplier background causes only the slightest dis­tortion in the observation of the much faster pulses coming from the scintillator. If we take into account the essen­tially discontinuous character of the photoelectric emission, we can see that the background modulation produces a change not confined to one frequency only, but which is necessarily distributed all along the spectrum. Such a change, however, can be neglected in the range of fre­quencies concerning the scintillator fast pulses.

On the other hand, the fast electronics, meant to handle these pulses, is not capable of transmitting signals having a frequency of some tens of cycles; thus it works automati­cally as a rejection filter.

The block diagram of the whole apparatus is shown in

11 M. Ageno and C. Felici, Serie di apparecchi transistorizzati per l'elaborazione degli impulsi di contatori a scintillazione, Reports 1st. Sup. Sanita, Lab. di Fisica, ISS 61/30.

12 H. J. Rijks, Nucl. lnstr. & Methods 14, 76 (1961).

Fig. 1, while Fig. 2 shows the detailed electrical diagram. Even if both diagrams need no great explanation, it is nevertheless useful to add further considerations on the individual parts of the circuit.

(a) Generator of the Periodic Luminous Signal

The light source consists of a small hydrogen thyratron, Z 70 U, having a turn-on luminous intensity guaranteed constant within 1%. The trigger consists of a 77-Hz phase­shift oscillator, followed by a Schmitt-type discriminator, which is used to transform the sine wave provided by the oscillator into a square wave, more appropriate to control the thyratron firing.

Obviously, the light source stability fixes an upper limit to the photomultiplier stability. With the solution we adopted, this limit is 1%, but, if necessary, it would not be difficult to improve this situation appreciably. A constant light source can be obtained, for instance, exciting to saturation a long life phosphor by means of a sufficiently intense radioactive source. This source can be easily made periodical by shielding it, at regular intervals, with a suitable mechanical apparatus.

A 77-Hz frequency has been selected in order to make it distinct from the frequencies present in the pulses to be handled and, at the same time, to suffer no interference from the local power frequency. In this way we are not confronted with such problems as shielding and elimination of ripple during the amplification of the photomultiplier anode output signals.

(b) Signal Amplification at 77 Hz

Figure 3 shows the photomultiplier thermionic noise with and without the 77-Hz modulating signal. A Tektronix oscillograph has been used (horizontal, 5 X to-3 sec/ div ; vertical, 0.1 V/div). In order to extract the 77-Hz sinu­soidal component from the background, it is not con­venient to use a tuned amplifier (as in the first version of our stabilizer). The reason is that any frequency shift of the luminous signal is converted, in this case, into a detect­able variation of the output amplitude. Therefore, in order to avoid this inconvenience, we have replaced the tuned amplifier by a band filter, having a very narrow band centered at 77 Hz, followed by an ordinary feedback amplifier. Figure 4 shows the frequency response of the filter we have employed.

(c) Error Signal Formation and Regulation of the High Voltage

The remaining part of the circuit needs little explana­tion. The output sine-wave signal, rectified by a four-diode bridge and smoothed, is compared, by means of a differ­ence amplifier, with a reference voltage (supplied by an

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

129.105.215.146 On: Sat, 20 Dec 2014 20:34:20

PHOTOMULTIPLIER GAIN STABILIZER

4ISK N,5 4ISK~---H

P-O:>

150K

a)..r ~

~OK h

. 220'; I OIIt! H.V. ) <:> 0 -®

In Out OIIt ..L (H. Y.I ':;' ( fut (77Hz -= -- Si9lA1 signlll ----- --- ~

In' ...... f.M.

'lte'6C1I6 +300

1/l

lK

v,-f>AJ(S

77 Hz osciltil:ar can-;;;-"'J#'4

1k;,;'

Y,.- [elf

+~yo-~----------4r---'--~------------~---

-150Y

FIG. 2. Electrical diagram of the circuit stabilizer.

999

1..I 150V

plug for ZroU

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

129.105.215.146 On: Sat, 20 Dec 2014 20:34:20

1000 M. AGENO AND C. FELlC[

FIG. 3. Thermionic noise of the photomulti­plier: (a) not modu­lated; (b) modulated by the 77-Hz signal (hori­zontally SX 10-3 sec/div, vertically 0.1 V /div).

8SA2). The error signal is then fed onto the grid of a tube in series with the high voltage supply between its positive terminal and the ground. However, the divider feeding the different dynodes is put between the ground and the negative terminal of the same supply. Thus the voltage applied to the photomultiplier varies until the error signal vanishes so that the 77-Hz sine signal extracted from the photomultiplier background, and thereby its amplification, is kept constant.

Blocking condensers have been set in several points of the circuit, in order to make the system sensitive only to very slow amplification changes, eliminating all the short period variations. The response time is of the order of 100 sec.

Of course the solutions we have adopted are not the only possible ones and in several cases they can be replaced, with some advantage, by others. For instance, when it is not convenient to vary the electron transit times from the photocathode to the photomultiplier anode (and this

FIG. 4. Frequency response of the band filter.

occurs, as already mentioned, if the applied voltage varies), the potential difference between photocathode and first dynode may be kept constant, and the regulating circuit used to vary only the potential difference between first dynode and anode. }\fost of the transit time variation occurs between photocathode and first dynode.

The circuit described in the preceding paragraphs has been built so that it is possible to insert it between any conventional stabilized high voltage supply and the photomultiplier.

The inserting procedure is as follows:

(1) The high voltage supply is regulated to provide an output voltage about 300 V higher than the average one to be applied to the photomultiplier.

(2) The stabilization circuit is inserted and the input attenuator is varied (Fig. 2) until we obtain the required voltage on the photomultiplier. This voltage can be read

FIG. S. Efficiency of the stabilization circuit. Horizontally, position of the peak corresponding to the 667-keV line of the CS137 , spectrum, without stabilization. Vertically, position of the same peak after inserting the stabilizer.

on an instrument (set on the stabilizer panel) in parallel with the high voltage output.

(3) The difference amplifier balance is obtained by cor­recting the high voltage supply output. (This balance is controlled by means of an instrument on the stabilizer panel.)

When the voltage required on the photomultiplier can­not be obtained using only the attenuator, it is necessary to vary the current in the Z 70 lJ by means of the relevant poten tiometer.

The apparatus has been tested for a long time in con­nection with a PlO type stabilized power supply (Italelet­tronica) and an RCA 6810 A photomultiplier which is part of a 200-channel '}'-ray scintillation spectrograph.

As operated, the average voltage supplied by the PlO stabilized power supply was 2400 V, while the average potential difference between the terminals of the photo­multiplier divider was 2100 V. The average drop in the

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

129.105.215.146 On: Sat, 20 Dec 2014 20:34:20

PHOTOMULTIPLIER GAIN STABILIZER 1001

valve in series was therefore 300 V and the regulation range provided was ±100 V around the mean value.

The regulating circuit efficiency has been tested many times in the following way. A series of measurements was made on the CS137 "{ spectrum without the regulating circuit and artificially decreasing the photomultiplier gain by means of a small magnet placed near to it, at different dis­tances, exactly measured and reproducible. Each measure­ment was then repeated after inserting the regulator. In each case the position of the peak corresponding to the total absorption by the crystal of the 667-keV "{-line quanta

THE REVIEW OF SCIENTIFIC INSTRUMENTS

was graphically established. The results obtained are col­lected in Fig. 5. Each point of the diagram corresponds to a well-determined position of the magnet; horizontally we have given the order number of the channel to which, without stabilization, the cesium peak maximum belongs, vertically the same quantity is given after inserting the stabilizer.

As can be seen, a variation of the photomultiplier gain causing, without stabilization, the cesium peak to shift along 50 channels, is compensated by the stabilizer so that the actual shifting of the cesium peak cannot be resolved.

VOLUME 34, NUMBER 9 SEPTEMBER 1963

Nanosecond Scintillation Coincidence Spectrometer System with High Reliability*

L. S. BELLER

Atomics International, a Division of North American Aviation, Inc., P. O. Box 309, Canoga Park, California

(Received 26 March 1963; and in final form, 2 July 1963)

A low-energy scintillation coincidence system capable of few-nanosecond resolution and vt;ry low timing jitter is described. Saturating circuit techniques are used to make the performance nearly independent of tube parameters, resulting in stable and reliable performance over periods of months.

INTRODUCTION

THIS paper describes a complete scintillation coin-cidence spectrometer system designed for use in a

new method of U238 activation analysis,1 which uses the fact that one branch of the beta decay of Np239 leads to a 0.193-,usec gamma-emitting2 state in PU239. A delayed beta­gamma coincidence measurement allows this decay to stand out distinctly from a large background of mixed fission-product activity. The sharpest possible rejection of noncoincident events is needed, as well as stability for periods of weeks or months in the presence of high single­channel input rates.

In obtaining the required stability, the following ap­proaches were used: (1) At each point, detailed circuits and circuit configurations giving the best stability were chosen from among the competing types. (2) The sensitivity was made large enough so that the fast coincidence unit detects pulses arising from single-electron events at the photocathode. (The system therefore detects the first photoelectron from each radiation event; the timing and stability then depend primarily on. the scin­tillator-photomultiplier combination, and not on electronic stability.) (3) In order to guarantee this sensitivity, high gain, saturating digital techniques were used in all timing circuitry, amplitude dependent circuits being avoided as much as possible. All circuits are overdriven so that, ideally at least, all stages are limiting amplifiers. Changes

* Work sponsored by the U. S. Atomic Energy Commission. I L. S. Beller, Trans. Am. Nucl. Soc. 4, No.1, 28 (1961). 2 D. Strominger et al., Rev. Mod. Phys. 30, 585 (1958).

in active parameters with age then affect only the amount of overdrive, which in turn affects performance only to second order. (4) Finally, special attention was paid to the

FIG. 1. System block diagram.

LItlEAR AJMIUFIER

1l1li.£ CHANNEL

PHA·

r---'--..., I 1

'---,--.... 1 r---'--..., I

I I

'----...-....... I ,.-....... -1...---., ... __ -' ,

I·~::'I I----'i

r---""----,I~I L-Pr~

I I

,-----.1

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

129.105.215.146 On: Sat, 20 Dec 2014 20:34:20