Two types photomultiplier voltage dividers for high and changing count rates

Nuclear Instruments and Methods 174 (1980) 585-591 © North-Holland Pubhshing Company

TWO TYPES OF PHOTOMULTIPLIER VOLTAGE DIVIDERS F O R HIGH AND CHANGING COUNT RATES W.L. REITER and G. STENGL Institut fur Radiumforschung und Kernphysik der Osterr Akademie der Wtssenschaften, Boltzmanng. 3, A-1090 Vtenna, Austria Received 18 September 1979 and in revised form 4 February 1980

We report on the design of two types of voltage distribution circuits for high stabihty photomultiplier operation. "Type A" voltage divider is an ohmic voltage divider with high bleeder current (up to 10 mA) and the resistor chain split at one of the last dynodes, usually the dynode where the analog signal is derived from. This simple constructive measure improves the stab~ty of the dynode voltage by a factor of 5 compared with an unsplit conventional resistor chain. "Type B" is a novel active voltage divider using cold cathode tubes as regulating elements. This voltage dlvtder exhibxts excellent temperature stability (about 104[°C). With "type B" an equal stabdity compared with conventional ohmic dividers can be achieved at a bleeder current smaller by one order of magnitude. Of course both concepts, "type A" and "type B", can be combined.

1. Introduction

dynodes [4]. In this case an active stabilizing system of the detector gain has to be employed [5].

It is a well-known fact that the gain o f photomultipliers depends on the counting rate, the lurmnosity and the duration of the scintillations [1,2]. To eliminate changes in gain and transit time o f a photomultiplier the interdynode voltages have to be kept stable if the current drawn b y the photomultiplier changes velth counting rates. A variety o f techmques has been proposed to maintain a constant gain of the device [31. This note reports on two types o f voltage dividers: Type A : A simple ohmic voltage divider working at a high bleeder current with a resistor chain split at one o f the last dynodes, as a modification o f the concept of the conventional low ohmic voltage divider. Type B" An active voltage divider stabilizing the interdynode voltages using cold cathode tubes. A low bleeder current is sufficient for this type o f divider, thus avoiding undesired heating-up effects of the photomultiplier caused by high-current dividers at an equal stability in multipher gain. In comparison with other active bases (using e.g. zener diodes or transistors in an emitter follower configuration) this type exhibits a very small dependence on temperature o f its performance. In spite o f constant dynode voltages nevertheless the gain of a photomultlplier may not remain constant because of interior effects of the photomultiplier such as rest-gas effects and ageing o f the

2. The 'type A ' voltage divider The concept of the sprit resistor-chain divider is shown in fig. 1. The branching is done at the dynode at which the analog signal is deduced. The distribution of the bleeder current is as follows: the greater part o f it is applied to the last dynodes (branch 2) and the rest to branch 1. Voltage dividers of this type have been designed for the 14-stage RCA 8854 and the 12-stage RCA 8850 photomultiplier. Fig. 2 gives the detailed circuit o f the divider for the tube RCA 8850. The bleeder current was 10 m A at 2200 V: 3 m A had been gwen to the branch c a t h o d e - 1 0 t h dynode, 7 m A to the branch 1 l t h d y n o d e - a n o d e . All resistors are metal film (1 W, T = 25 ppm/°C). The voltage distrlbutlon was linear ( " B " proposed by RCA). In fig. 3 the variation o f the gain o f the RCA 8850 photomultiplier with counting rate measured at the 10th dynode is given. The scintillator was a 2 " X 1~" NaI(T1) crystal, as a radiation source 6°Co was used. The gain variation was determined by the peak shift o f the 6°Co photopeaks m the pulse height spectrum measured with a multichannel-analyzer normahzed to the counting rate which produced no further gain variation. Fig. 3a gives the relative gain variation due 585

W L. Reiter, G Stengl / Photomulttplier voltage dividers

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shafts of the 6 ° C o photopeaks are only due to effects resulting from the voltage divider functmn. Subtracting this gain variation from that induced by an equivalent radiation load yields the contribution of the photomultiplier tube to total gain shaft. The long term variation of the 10th dynode-anode potential was 5 m V of 270V (18.5 ppm) during 10h at a counting rate of 500 Hz 6°Co. (This is within the specification of the hv-power supply m use.) With the assumption of a linear voltage dlsmbutlon and neglecting the currents of the first 9 dynodes the variation of the potential of the 10th dynode in respect to the anode as a function of the current on the anode AUa0-A =f(iA) at constant bleeder current ig for the spht resistor-chain divider ('type A') as given by the expression

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to effects coming from the divider, fig. 3b ts attributed to the gain variation caused by the multiplier itself, exhabiting two components' a fast variation up to 5 kHz (10/aA anode current) and a slow one up to 30 kHz (60 /aA anode current). To distinguish between gain variations caused by the photomultlplier and by the voltage divider in addition to the shifts of the photopeaks the voltage between the dynode under consideration and the anode was determined by a differential voltmeter with and without a 6°Co-load applied to the detector. Without applying radiation the voltage drop was simulated using a bleeder-resistor of appropriate value according to the dynode currents from dynode to anode. Hence the

3. The 'type B' voltage divider Cold-cathode tubes have been found to be simple constructwe elements to fix the mterdynode voltages in keeping the dynode current constant, Cold-cathode tubes SR 125 manufactured by Cerberus have been used, their ignition voltage is 125 V at a current of 0.1 mA. The essential quality of these tubes is their temperature stability. A temperature coefficient of 4 × 10-s/°C has been determined. Referring to the potential of the 10th dynode a temperature coefficient of 1.5 × 10-4[°C was measured. The pnnciple of the current regulating cold-cathode tube divider is given in fig. 5 for the 12th dynode. An increase of the anode current i A changes the current at resistor R12 and thus the potential of the 12th dynode by 2xU which is approachang the anode potential. For stable operation conditions (i.e. ignition voltage) of the cold cathode tube a change 2xU in the potential at the resistor Rc is caused. As a

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W.L. Relter, G. Stengl /Photomultipher voltage diwders

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consequence the current in R c changes by Aic = A U / R c (typical values are: A U = 1 V and R c = 100 k~2, Aic = 10 /aA), creating a potenUal difference m the opposite direction. So, the device is regulating the

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dynode voltage at the potential o f the according dynodes. Fig. 6 gives the clrcmt of a voltage divider with stabilisation applied to the last three dynodes. The voltage distribution of ttus divider (bleeder current 1.2 m A at - 1 8 0 0 V) used with the photomultiplier RCA 8850 is given in table 1. Applying an ohmic load to the divider the stability o f the 12th dynode has been determined to be 500 ppm//~A and 260 ppm//~A at the 10th dynode. In the same way as for 'type A ' voltage dividers the gain

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W.L Retter, G. Stengl / Photomultipher voltage dividers

590

Table 1 Voltage distribution for 'type B' voltage divider. Cathode-toanode voltage. -1800 V, Bleeder current: 1.2 mA

and 7 particularly at count rates exceeding 25 kHz may be attributed to the lrreproducibllity of photomultipher performance after applying high count rates [6]. The variation o f the potential o f e.g. the 12th dynode with a load applied to the multiplier can be written as

lnterdynode voltages (V) C cathode D dynodes Voltage A anode C-D1 D1-D2 D2-D3 D3-D4 D4-D5 D5-D6 D6-D7 D7-D8 DS-D9 D9-D10 D10-Dll D11-D12 D12-A

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Comparing an ohmic divider with the 'type B' divider the ratio R~ to R13 is a characteristic value. It has to be noted that the resistor R C can be kept small In respect to an ohmic divider, especially with a 12-stage multiplier and the analog signal derived from the 10th dynode.

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W.L. Reiter, G Stengl / Photomultiplier voltage dividers

4. Conclusion Gain variations of photomultlpliers with changing count rates are severe problems for the expenmentor especially with measurements having fixed thresholds such as in neutron spectrometry work. The effects are twofold: changes m gain coming from the tube itself and those which are caused by the voltage divider. This note reports on two types of voltage &vider circuits which by simple constructive means eliminate shortcomings of simple divider chains. The voltage divider wxth a split resistor chain and a high bleeder current ('type A') improves the stability of the dynode voltages by a factor of 5 if compared with a conventional unspht resistor chain. With a high bleeder current (about 10 mA) this voltage divider is advantageous at high count rates. Because of the high current this type of dwider is less desirable with counter arrays and might exceed the permissible limit of heat in some experimental set-ups. The active voltage divider ('type B') with coldcathode tubes as regulating elements shows excellent stability for changes of the ambient temperature (about 10-4/°C). The bleeder current of this divider can be kept low ( 1 - 2 mA) giving a stability of the performance equivalent to an ohmic divider with a bleeder current about one order of magnitude higher. It should be mentioned that this &vider is operating with a fixed voltage distribution which might be vaned by changing the corresponding resistors. (This constitutes no drawback as most spectrometer systems are operating at a fixed high voltage.) The variations of the dynode potentials are reduced to about 10% depending on the value of

591

resistor R c if compared to a simple divider chain with the same bleeder current. These features make this voltage divider favourable to be used in counter arrays at medium high count rates. Both concepts, 'type A' and 'type B' can be combined for very high stability requirements. Voltage dividers of 'type A' and 'type B' have been designed and operated for the RCA 8850, 8854 and XP 2041 photomultipliers used with NE 213 liqmd scintillator spectrometers for neutron detection in experiments at the Swiss Institute for Nuclear Research (SIN) [7]. The authors would like to express their gratitude to Prof. H. Vonach for Ins continuous support and to Dr. R. Nowotny for helpful discussions.

References [1 ] H. Schnexder and C. Wemgart, Nucl. Instr. and Meth. 40 (1966) 305. [2] R.B. Galloway and D.G. Vass, Nucl. Instr. and Meth. 49 (1967) 55. [3] H. Jung and M. Brullmann, Nucl. Instr. and Meth. 65 (1968) 178, ref. 2 and the literature quoted there. [4] H. Jung, Ph. Panussi and J. Janecke, Nucl. Instr. and Meth. 9 (1960) 121. [5] W L Relter and G. Stengl, Nucl. Instr. and Meth. 169 (1980) 469. [6] G.A. Akapdzhanov et al., Nucl. Instr. and Meth. 161 (1979) 247, H Schneider and C. Wemgardt, Nucl. Instr. and Meth. 40 (1966) 305. 171 Experiment R-73-04.1, SIN Physics Report No. 2 (Dec. 1977).