McMillan, J E

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29th International Cosmic Ray Conference Pune (2005) 5, 399402A high voltage current monitor for measuring photomultiplier anodecurrentJ.E.McMillanDepartment of Physics and Astronomy, University of Shefeld, Shefeld, South Yorkshire, S3 7RH, Great BritainPresenter: J.E.McMillan (j.e.mcmillan@sheffield.ac.uk), uki-mcmillan-J-abs1-og27-posterA system is presented which is capable of monitoring currents in the microamp region at a voltage of upto 2.8kV. This can be used for measuring the anode current of a photomultiplier when operated with thephotocathode grounded.1. IntroductionPhotomultipliers are susceptible to damage caused by heating of the last dynodes if the average DC anode cur-rent is allowed to exceed about 100   A. As part of a test facility intended to prematurely age photomultipliers,it was required to monitor anode current while exposing the photomultipliers to elevated light levels. Theseongoing aging studies are intended to provide estimates of longevity of photomultipliers used in long-termparticle astrophysics experiments as well as in medical imaging applications.Measurement of the anode current with the anode at a voltage close to ground and the photocathode at ahigh negative voltage is trivially simple. However, with the photomultiplier operated with the photocathodegrounded and the anode at high voltage, direct measurement becomes problematic. Grounded photocathodeoperation is usually preferred, since it results in lower noise operation. Similar considerations apply whenmonitoring photomultipliers used for air cherenkov or air fluorescence detectors.Frishman and Akerlof [2] used a simple voltage-to-frequency converter (VFC) to routinely monitor the anodecurrent of photomultipliers on an air cherenkov telescope. The VFCs were mounted close to the photomultiplierand the frequency was measured remotely with a multichannel scaler unit. This system was not capable ofoperating with the anode at high potential but demonstrated the use of VFCs to generate a telemetry signalwhich could be remotely monitored.For the Pierre Auger air fluorescence detector, Argiro` et al. [1] developed a a circuit based on optocoupledcurrent mirrors, which was the first workable solution to the problem of measuring the anode current at highvoltage. The output was in the form of a DC voltage proportional to the anode current. This voltage must eitherbe converted to digital form at the photomultiplier base or it must be transmitted some distance and digitizedremotely.Kliefges et al. [3], also working on the Pierre Auger air detector, rejected the current mirror as too complexand instead proposed a statistical analysis of the signal fluctuations as a measure of the anode current. Thecalculations for this were performed using part of a field programmable logic array which was already availableas part of the experiment. This method appears to depend on knowledge of the photomultiplier gain and on theconstancy of the gain over time, neither of which were realistic for the present requirement.2. Anode current monitoring using a VFCThe method adopted was essentially an extension of the technique of Frishman and Akerlof [2]. The currentpassing through the anode resistor of the photomultiplier gives rise to a voltage which is used to control a volt-age to frequency converter. The output frequency is transmitted from the photomultiplier base and measured400 J.E.McMillan+5V0V10Vrms 20kHz sineto scalerC4 & C6 are Murata DSS310EHTPMT anodeV/FU1AD65447631825+VINCTCTRTFOUT+VSCL-VSC1470pFR11kAnode Resistor9k1R2390R4220kL2100nHL1100nHC2100nFU2ISP815X1243- +D1T1PT4E+C3220uF/40VC622nFC422nFU3UA78L05ACINCOMO U TD31N6263+C510uF/16VD21.5KE6.8CAR5220kD41N6263C7100nFFigure 1. schematic of voltage - freq converterremotely with a scaler. By passing the output signal through an optoisolator and operating the whole of theVCF circuit at high voltage, the anode current can be measured without damaging the VCF.The circuit is shown in figure 1. The VFC used was an Analog Devices AD654, providing a better engineeredsystem than the 555-based circuit used by Frishman and Akerlof [2]. The photomultiplier dynode chain wasan Electron Tubes C638, a standard printed circuit board constructed with surface mount components, whichcan be configured for a variety of photomultipliers with only minor alterations. The standard anode resistorprovided on the C638 board is 100k  . Use of this value would have made the current monitor excessivelysensitive, so it was reduced to 9.1k  by soldering an additional 10k  in parallel. This is illustrated at thebottom left of the schematic together with its connections to EHT and to the photomultiplier anode. It wasconnected to the VCF via two 100nH inductors (L1,L2) and two 220k  resistors (R4,R5). These coupledthe DC voltage to the input pins of the VCF, while ensuring that the presence of the VCF did not affect thephotomultiplier output pulses. This remained true, even when used with fast photomultipliers of the 9813 type,having risetimes of 2ns. These inductors and resistors were mounted as close as possible to the anode resistoron the C638 board.In the prototype, the remaining components were mounted on a separate printed circuit board with two shortflying leads to the coupling resistors. Care was taken to decouple the VCF power rails adequately and clampdiodes were provided to ensure that the input to the VCF could not exceed the rail voltages in the event ofexcessive current flowing through the anode resistor.A high voltage current monitor ... 40110Vrms20kHz sine+24V+5V-5.2V-24Voutput0VU1 ICL8038245678101112S W O U TDCFADJDCFADJV+FMBIASFMSINTCV-S W A D JR14k7C13n3FR24k7R382kC2100nF+-U2LM388610931,57 4 8R410kR51kR61kC31uFR722kR827kC44u7FC54u7FC6100nFC7100nFFigure 2. schematic of power supply3. Powering the VCFThe optoisolated VCF circuit floated to high voltage is only half of the design. The circuit requires power tooperate, admittedly not a great deal, but no configuration could be found in which it was powered directly fromthe photomultiplier EHT supply. With a +5V power supply and the optoisolator specified, the circuit requiredaround 6mA. For initial experiments the VCF was battery powered and this remains a possibility. However,it was intended to use the circuit over a period of many months and batteries remain unreliable under suchconditions. DC/DC converters were rejected since current models produce a large level of switching noisewith harmonics extending into the 100MHz region. They are incompatible with typical photomultiplier anodepulses of a few millivolts and durations of a few nanoseconds.Instead it was decided to provide power using a classical linear power supply. This consisted of a transformerwhich provided the required high voltage isolation between its primary and secondary windings, followed bya bridge rectifier, smoothing capacitors, a three-terminal regulator and further decoupling components, whichare illustrated in the top half of figure 1. While it would have been possible to send mains voltage to the circuitand to find stepdown transformers with sufficient isolation, this was rejected both on the grounds of safety andbecause available mains transformers were all too bulky.Small high-voltage isolation transformers with 1:1 turns ratio are widely available and experiments were per-formed on a range of these. The chosen transformer was a PT4E from Oxford Electronic Devices (OED),intended for use as a pulse transformer in thyristor and triac trigger control applications. It was specified tohave a bandwidth of 3kHz to 1MHz and 2.8kV voltage isolation. Its physical size was 	 mm.Attempts to operate it (or any of the other transformers tested) at 50Hz were futile, as at this frequency it wasextremely inefficient and saturated easily. Instead the supply frequency was raised to 20kHz where it workedperfectly. The result was an AC/DC power supply operating perfectly with 2.5kV DC offset between input andoutput and less than 1mV ripple and noise on the 5V output line.Clearly 20kHz is not a standard supply frequency, so this was generated using an ICL8083 function generatorchip driving a LM3866 power amplifier module, as shown in figure 2. These were housed in a standard NIM402 J.E.McMillanmodule and derived their power from the standard crate power connector. The +5V and -5.2V rails markedon figure 2 were derived from the +12V and -12V crate supplies via three-terminal regulators. Eight currentmonitors could be driven from the unit simultaneously, being connected using twisted pair ribbon cable.The returning signals from the optoisolators were also buffered in this unit before being fed to a Hytec EC727multichannel scaler module. This is a 32-channel Camac scaler, the other channels of which were already in useto monitor photomultiplier noise rates. This buffering consisted of reconstructing the VFC output waveform bypassing the signal through a Schmitt trigger, then converting to differential ECL required by the scaler module.A simple computer routine was used to convert the frequency measured back to anode current, using a linearfunction based on the calibration data.4. ConclusionsThe circuit, as constructed, measured low currents at high voltage without introducing noise into the circuitbeing measured. It was fully linear over the required range of 1–100   A. While it was designed for the specificpurpose of monitoring photomultiplier anode current, it would be useable wherever DC currents at high voltageneed to be monitored. Higher currents can easily be accommodated by reducing the value of the sensing resistorfrom 100k  , while lower current measurements can be attained by increasing this resistor. The input resistanceof the AD654 is specified as 240M  , so the sensing resistor can be increased to 10M  without problems, givinga range of 10–1000nA.The limitations on the maximum voltage the circuit can be operated at are determined by the 1:1 isolationtransformer and the optoisolator. The optoisolator could easily be replaced by a system which transmittedthe frequency over an optical fibre giving considerably higher isolation. Powering the circuit at much highervoltages is more problematic since transformers capable of much greater isolation are not widely available.5. AcknowledgmentsThis work was developed as part of a Particle Physics and Astronomy Research Council PIPPS scheme. Iwould like to thank the staff of Electron Tubes Ltd. and Chris Grigson, Phil Scholey, John Watson and RichardWebb for helpful discussions.References[1] S. Argiro` et al., Nucl. Instrum.& Meth., A435, 484 (1999)[2] D. Frishman and C. Akerlof, Nucl. Instrum.& Meth., A311, 306 (1992).[3] M. Kleifges et al., IEEE Trans. Nucl. Sci., NS-50, 1204 (2003).

A high voltage current monitor for measuring photomultiplier anode current

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