A simple gain control system for scintillation counting systems in space experiments

NUCLEAR INSTRUMENTS AND METHODS IO8 (I973) I 4 7 - I 5 5 ;

A SIMPLE

GAIN CONTROL

© NORTH-HOLLAND PUBLISHING CO.

SYSTEM FOR SCINTILLATION

COUNTING

SYSTEMS

IN SPACE EXPERIMENTS I. ARENS and B. G. TAYLOR ESRO, Domeinweg, Noordwijk, The Netherlands

Received 15 November 1972 An automatic gain control (AGC) system for use with large area plastic scintillation detectors has been developed and tested. The reference source, called an alpha lamp, consists of a piece of NaI(TI) scintillator doped with alpha emitting 241Am and is encapsulated in an AI housing with glass window. It is optically coupled to the scintillator. The reference pulses obtained are of high level and have different time characteristics compared

to true event pulses, and are distinguished by means of a pulse length discriminator. The design of the complete electronic system of the AGC is described as well as calculated and measured performances, and temperature compensation. Remaining problems connected with alpha lamps of a different scintillation material offering easier temperature compensation are discussed.

1. Introduction

for a c o u n t e r system in which (see fig. 1) a plate o f N u p l e x 3 scintillator 220 by 220 m m 2 and 4 m m thick

M a n y factors can influence the p e r f o r m a n c e o f scintillation c o u n t i n g systems in satellite b o r n experiments over their o p e r a t i o n a l lifetime o f several years. T h e aging o f p h o t o m u l t i p l i e r s and the d e g r a d a t i o n o f optical surfaces and couplings are long t e r m effects. S h o r t term effects include gain changes due to extreme t e m p e r a t u r e variations i n t r o d u c e d by eclipsing and due to excessive i l l u m i n a t i o n d u r i n g passages t h r o u g h the r a d i a t i o n belts or d u r i n g solar flare events. V a r i a t i o n s in p e r f o r m a n c e can in some instances be o v e r c o m e by careful c a l i b r a t i o n and folding the calib r a t i o n function into the o u t p u t d a t a d u r i n g analysis, or by extensive control d u r i n g orbital operation. H o w e v e r , in the present a p p l i c a t i o n the scintillation c o u n t e r is p a r t o f an o n - b o a r d decision m a k i n g system d e m a n d i n g " r e a l t i m e " c o n t r o l o f performance. T h e stimulus for any gain c o n t r o l system should p e r m i t as much as possible o f the scintillation counter/ p h o t o m u l t i p l i e r system to be regulated. The source should be stable against ageing and e n v i r o n m e n t a l conditions, n o t interfere significantly with the c o n d u c t o f the experiments a n d p r o d u c e signals which can be recognised by the system against a b a c k g r o u n d o f noise and true signals in the scintillator. As is usual for all satellite experiments stringent limitations are placed on weight, power, reliability, mechanical integrity etc. A n u m b e r o f solutions for o n - b o a r d gain control systems have been described in the literatureS-6), but n o n e was considered suitable for the present application. 2. The application and method

The gain c o n t r o l system described here was developed

Fig. 1. BI scintillator of the COS-B Triggering Telescope with adiabatic light guides. One reference source (alpha lamp) is inserted in the center of the scintillator, and one photomultiplier is mounted. The metal sheet used as a mounting aid is not needed in the satellite. 147

148

I. ARENS AND B. G. TAYLOR

was viewed by two RCA C7151Q photomultipliers via two adiabatic strip light guides of plexiglass 17271°). This system forms part of the Triggering Telescope of the COS-B Extraterrestrial Gamma Ray ExperimentT) *. One purpose of this counter is to permit the identification of events in which the two relativistic electrons, produced in the 7--,2e pair production process in the spark chamber, penetrate the counter, among a far greater number of single particle events. On telecommand, only two-particle events may be allowed to trigger the spark chamber. The gain control system is incorporated to ensure that the discrimination against single particle events remains stable with time. In response to discrete energy gamma rays from a calibration source Nuplex 3 gives rise to a broad Compton distribution which is not suited for stabili* "The Caravane Collaboration". This group consists of: Cosmic-Ray Working Group, Kamerlingh Onnes Laboratorium, Leiden, The Netherlands; Laboratorio di Fisica Cosmica e Tecnologie Relative del CNR, lstituto di Scienze Fisiche dell'Universit& di Milano, Italy; Max Planck Institut fiir Physik und Astrophysik, Institut fiir Extraterrestrische Physik, Garching near Mtinchen, Germany ; Service d'Electronique Physique, Centre d'Etudes Nucl6aires de Saclay, Gif-sur-Ivette, France; Space Science Department, European Space Research and Technology Centre, Noordwijk, The Netherlands.

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sation purposes, but monoenergetic particles like the alpha particles of 241Am will produce a narrow peak suitable as a reference. However the scintillation efficiency of Nuplex 3 for alpha particles is only about 7% of the beta efficiency and the peak of the 5 MeV alpha particles would occur in or below the energy region where most true pulses are expected. As a result discrimination between true and reference signals would be very difficult. Therefore a small separate scintillator of Nal(Tl) absorbing the energy of the alpha particles and generating intense light flashes is optically coupled to the main Nuplex scintillator. The sources, called hereinafter alpha lamps, consist of an aluminium housing with glass windows containing the NaI(TI) scintillator doped with Z¢lAm to give countrates of approximately 500 per second. The alpha lamps, one per photomultiplier, are located in the center of the scintillator, each emitting towards its photomultiplier tube. Charged particles penetrating the scintillator give pulses typically 10 ns wide, while the alpha lamp yields signals with a decay time constant of approximately 250 ns. The discrimination between true and reference events is performed utilising a pulse length discriminator, shown as one of the blocks in fig. 2. It is a combination of a fast, low level discriminator and a pulse length selector both made of MECL integrated

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SIMPLE GAIN CONTROL SYSTEM circuits. Every second of its output signals is used to gate the output of the level discriminator of the gain control system. This level discriminator compares the pulse height of an analog signal with a threshold which may be temperature controlled. The gating is necessary to permit only signals from the reference source to set the control flip flop. The intermediate signals of the pulse length discriminator reset the flip flop so that its duty cycle is a measure of the average amplitude of the reference signals, but does not depend on the total countrate of the reference source. Finally the duty cycle signal is averaged by the integrator, amplified, and applied to the control input of the high voltage (hv) supply. Details of the circuits are shown in fig. 3a and 3b. The output of the pulse length discriminater could be used to block the normal measuring electronic system for events with long decay times to keep the measurements free from AGC interference. However, in the present design this is unnessary as the time the detector is busy with gain control signals is only 0.5 per mille (500 x 1 ps of 1 s), which is comparable with the expected busy time caused by true events (104 x 50 ns) at absolute maximum counting rate. The solution adopted requires that the light emission spectrum of the alpha lamp is matched with the spectrum of the scintillator to be stabilised. Poor matching may result in erroneous gain corrections, if for example the photocathode efficiency changed in the spectral region of the reference light but not of the Nuplex emission. NaI(TI) and CsI(Na) are suitable materials for the alpha lamp, since they emit in the region of 400 nm 8'9) which is the same as for plastic scintillators1°). CsI(TI) is reported to emit light of much longer wavelengthsS). The intensity of the prompt scintillation light of

151

Nuplex, when irradiated with constant energy does not have a significant temperature dependance. However the same is not true for CsI(Na), Csl(T1) and NaI(TI) as shown in fig. 49'tx). If these temperature effects are not compensated, the pulse height of the regulated Nuplex scintillator will show reciprocal temperature functions. CsI(Na) exhibits a linear temperature dependance over the range of interest for satellite borne experiments permitting simple compensation, whereas the non-linear dependance of NaI(T1) requires a more sophisticated compensation. Unfortunately, CsI(Na) is not commercially available with radioactive dopants at present and placing it in contact with a separate source of alpha particles has proven to be difficult because of uncontrollable surface effects. The temperature behaviour of an alpha lamp of NaI(TI) doped with 241Am has been determined 9) confirming the overall shape of the curve of fig. 4, which was obtained for gamma ray exitation. However, differences in temperature behaviour between samples were found. 3. Calculation of performance and results obtained 3.1. REGULATION FACTOR

For discussion it is useful to simplify the system to a model shown in fig. 5 of a closed loop amplifier where detector and alpha lamp are represented by a feedback circuit. The model comprises the analog 120

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152

I. A R E N S A N D B. G. T A Y L O R

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SIMPLE GAIN

CONTROL

electronics (including the A G C discriminator and logic) converting the average input pulse height to an output duty cycle, a series of amplifiers, whose output is the high voltage monitored by the housekeeping output and the feedback circuitry. There is also a noise source at the input representing the statistical fluctuations of the pulse rate. The feedback circuit is particularly non-linear. In a small range of the output duty cycle (representing a dc voltage of 0.6 V at the input of the integrator) we have linearity, and the feedback appears as an attenuator as shown in the graph of fig. 5. The slope of the curve was calculated as./' = ~o assuming a 20% fwhm Gaussian distribution of the reference pulses which is realistic for the alpha lamps used so far and a log-log slope of 9 = 6 of the photomultiplier gain (taken from the RCA data sheet). Thus we may say that the feedback f a c t o r f i s ~g while the open loop gain A' of the amplifier shown in the diagram is 2700. The closed loop gain is then given by: A' A = - - ~ l+fA'

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for 9 = 6.

In practice the regulation factor was tested by attenuating the photomultiplier output and recording the pulse height spectra of the reference pulses with a 400 channel analyser. No difference of the spectra could be detected, which indicates that F was greater than 200. 3.2. TIME RESPONSE

The circuit is essentially an integrating regulator, therefore a step function input of gain change will result in a linear time response of the hv, changing to an exponential behaviour. The time function of the integrator output follows the formula t

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153

SYSTEM

linear behaviour for step changes of gain is seen in fig. 6, which also shows the ripple of the hv caused by statistics. The ripple r can be calculated with the feedback model of the control circuit. Assuming first no feedback, the ripple is caused by the statistical error of the number of pulses being produced during the averaging time interval R2 C. The relative error is therefore ( n R 2 C) - ~ ,

n being the countrate of useful pulses. The input of the integrator will have the error r o = Ui,(nR 2 C) -~.

In a closed loop this ripple will be amplified by f - l , f being the feedback factor, r = U i n f - X ( n R 2 C ) -~.

For U~. = 0.6 V, n = 250 s - l , R E = 10 6 Q , C = = 20 pF, one obtains r = 0.43 V. The estimated rms ripple from the record, shown in fig. 6, is in reasonable agreement with prediction. With the formula for r one can predict the influence of various parameters on the ripple. Obvious means of reducing the ripple are increasing the feedback capacitance C and the countrate n, but an increase of n is not desirable because of increased busy time for A G C pulses. An increase of the value of resistor R2 would increase the overall gain, which would also be desirable for a higher regulation factor F. Practice shows, however, that too high a gain causes instability because of the non-linear feedback characteristics. On the other hand, a low value of R2 is desirable to permit the use of a tantalum capacitor for C for a higher capacitance at the expense of a higher leakage current. The input voltage should not be made too small compared with the offset voltage of the operational amplifier. The feedback factor f influences both regulation factor F and ripple r. Both are better for narrower spectra from the alpha lamp, meaning higher f v a l u e s . 3.3. CAPTURE RATIO The present system is captured in a voltage range from 1250 to 2400 V. Because the photomultiplier should be operated below 2000 V, this corresponds to a dynamic amplitude range of approximately 20. There are two possible self-locking modes of the system, one at high and one at low voltages. At high voltages true signals from the Nuplex detector may exceed the time threshold, but the amplitudes of these pulses may be too low to exceed the A G C level discriminator. If the rate of these pulses

154

I. ARENS AND B. G. TAYLOR

will be comparable to the reference rate, the system sees too low a duty cycle and will produce a correction to even higher voltages. To avoid this effect the threshold of the timing discriminator should be carefully set. At low voltages pulses from the pulse length discriminator are missing and the control flip flop is not reset. However, this is avoided by initialising the flip flop correctly when switching on the A G C system. 3.4. TEMPERATURECOMPENSATION A simple silicon diode is used to compensate for

the temperature dependance of the light output of the alpha lamp. The diode voltage drop is used to control the comparator level of the A G C discriminator. Firstly, the temperature behaviour of the uncompensated system was tested and a CsI(Na) alpha lamp with a plated source of Z4~Am in contact with the scintillator, was used as it was the only one available at this time. The spectra of fig. 7 show the distributions of pulses in the Nuplex scintillator, using a source of beta particles (9°Sr) and selecting those crossing the [ I

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SIMPLE GAIN

CONTROL

detector by a coincidence with a separate N uplex detector placed underneath the beta source. The peak of the reference source was in fact kept at constant position, but is not shown because the reference signals of course do not fulfil the coincidence condition. The b r o a d peak o f about 60% fwhm is due to the fact that only one photomultiplier tube on the counter operated during the test. One would expect that the relative peak position would follow the reciprocal of the temperature behaviour shown in fig. 4, for CsI(Na). However, it was found that the temperature sensitivity of the actual pulser was higher at low temperatures and lower at high temperatures, than indicated by this figure. The influence o f the crystal-source contact may be a reason for this. After compensation o f the temperature effect with the existing sensor g o o d stability was reached between - 2 5 °C and + 2 5 °C, but not at higher temperatures. The instability is lower than 2% between - 2 5 ° C and + 2 5 ° C and about 5% for a temperature change from + 25 °C to + 50 °C. With a more sophisticated temperature compensator, an overall stability o f less than 2% deviation over this temperature range would p r o b a b l y be achieved.

4. Future developments For the system to be flown, a single alpha lamp using NaI(TI) doped with 241Am will be contained in a housing with two opposed windows in order to minimise its dimensions, the number o f reference pulses produced and to eliminate mutual interference between the two halves o f the scintillation counter system.

SYSTEM

155

To avoid too complex temperature compensation systems, the production o f CsI(Na) doped with Z41Am should be investigated. The long term stability o f alpha lamps will also be studied. The authors wish to thank Mr G. Englert for design of the A G C logic and Mr E. Leimann assistance during the temperature tests. They indebted to D r E. A. Trendelenburg for making work possible.

the for are this

References 1) j. R. Gilland and L. Ried, IEEE Trans. Nucl. Sci. NS-16

(1968) 277. ~) G. R. Streeter and G. A. Guenther, IEEE Trans. Nucl. Sci. NS-17 (1970) 1724. 3) E. L. Chupp, P. J. Lavakare and A. A. Sarkady, 1EEE Trans. Nucl. Sci. NS-16 (1968) 309. 4) D. J. Forrest, P. R. Higibie, L. E. Orwig and E. L. Chupp, Nucl. Instr. and Meth. 101 (1972) 567. ~) C. Bacci, V. Bidoli and R. Baldini-Celio, Nucl. Instr. and Meth. 57 (1967) 100. 6) R. Leydig and R. B. Perkins, Los Alamos Scientific Laboratory, Contract W-7405. 7) H. C. v.d. Hulst et al., New techniques in space astronomy (Reidel Publ. Co., Dordrecht, 1971). 8) j. B. Birks, The theory and practice o f scintillation counting (Pergamon Press, New York, 1964). 9) j. Menefee, Y. Cho and C. Swinehart, IEEE Trans. Nucl. Sci. NS-15 (1967) 464. 10) Catalog Nuplex Kunststoff Szintillatoren (R6hm GmbH, Darmstadt, West Germany, 1970). 11) Datasheets for scintillators (Harshaw Chemicals Co., Cleveland, Ohio, 1965).