344 Nuclear Instruments and Methods in Physics Research A245 (1986) 344-350 North-Holland, Amsterdam

E V A L U A T I O N O F T H E P E R F O R M A N C E O F S I L I C O N P H O T O D I O D E S

I N C O M B I N A T I O N W I T H P L A S T I C S C I N T I L L A T O R S

G. H A L L , D. R O B I N S O N and I. S I O T I S

The Blackett Laboratory Imperial College, bmdon S W7 2BZ, UK

Received 14 October 1985 and in revised form 10 January 1986

Tests have been performed with photodiodes of conventional and ultraviolet enhanced response type in combination with several different plastic scintillators to establish their ability to detect minimum ionising charged particles. Results are reported and assessments made of likely improvements in the near future.

1. Introduction

For many years there has been interest in the use of silicon photodiodes as photodetectors in high energy physics experiments [1]. Some of the desirable qualities of photodiodes are their compactness and ruggedness, lack of sensitivity to magnetic fields and low voltage, room temperature operation and particularly their high quantum efficiency over a wide spectral range. Their stability is expected to be good because of the lack of intrinsic amplification in the device but this also means that the signal observed is quite small and dictates the use of high quality, low noise amplifiers. Another possi- ble disadvantage is the uncertain ability of the detectors to withstand radiation damage for long periods. How- ever developments in recent years in the use of solid state charged particle vertex detectors have both ad- vanced the technology for manufacturing silicon devices and led to greater knowledge of the effects of radiation [2]. These trends have renewed interest in the use of photodiodes for applications in particle physics. It is now possible to obtain photodiodes of up to several cm 2 in area and with leakage currents of only a few nanoamperes manufactured using oxide-passivated, ion implantation technology pioneered by Kemmer [3]. Such diodes are sufficiently inexpensive to be considered in some cases as alternatives to photomultiplier tubes and for large scale applications to be a very attractive, cost-effective solution. Inexpensive amplifiers are also readily available so the essential limitation remains only that of the achievable signal to noise ratio. This depends on a number of factors but chiefly on the available light signal. In many situations in high energy physics use is made of plastic scintillators and the light yield from single minimum ionising charged particles is expected to be of the order of a few thousand photons leading to a

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signal which is not very large relative to the expected noise from the detector-amplif ier combination. In these cases photomultipliers are normally used but in some cases, especially those where the speed of the signal is not crucial, a photodiode might be appropriate. We present here measurements made using a number of different commercially available plastic scintillators in conjunction with photodiodes which enable us to estab- lish some of the limits of applicability of currently available photodiodes and scintillators. We also discuss possible future developments of the technique.

The important characteristics of a photodiode in most applications in high energy physics are the quan- tum efficiency, the leakage (dark) current under reverse bias and the capacitance. The speed of response, i.e. the rise time, is of secondary importance since most diodes have rise times of only a few nanoseconds which is usually much less than the preamplifier rise time with a capacitive load. However a slow diode response can indicate the presence of a resistance in series between the diode and the amplifier, most likely at the surface of the diode, which is undesirable as it leads to increased noise. The contributions of leakage current and capaci- tance to the measured noise at an amplifier output are discussed in detail in several places [7,8] and one can show that, essentially, as the shaping time constant is increased the leakage current noise increases while capacitance associated noise decreases. Therefore there is an opt imum choice of shaping time which minimises the equivalent noise charge, usually in practice a few microseconds for applications involving photodiodes; shorter time constants are often preferred for practical reasons in fixed target high energy physics experiments but colliding beam experiments allow delays of that order.

G. Hall et al. / Performance of silicon photodiodes used with plastic scintillators 345

2. Experimental setup

The diodes used in these tests were obtained from Micron Semiconductor Ltd., [4] and from Hansen [5]. They were all made of n-type silicon with a p-type implant of 1 cm 2 active area and were operated at voltages up to about 50 V. The Hansen diodes had a spectral response which extended into the UV and was thus better matched to the emission spectrum of blue scintillators [9]. Fig. 1 shows the typical spectral re- sponse of the Micron diodes and that of the Hansen diode [9]. They were instrumented with a conventional charge sensitive preamplifier with a field effect tran- sistor at its input and an Ortec 575 semi-Gaussian pulse-shaping amplifier with variable time constant (0.5, 1.5, 3.0 #s). We measured an equivalent noise charge of 370 electrons + 3.2 e lec t rons /pF for the preamplifier using this setup with a time constant of 0.5 /zs. The preamplifier was a prototype developed for use with photodiodes for a LEP experiment at C E R N and was of inexpensive hybrid construction [6]. Generally we mea- sured leakage currents at the same time as establishing a suitable operating bias voltage and shaping time con- stant to minimise noise. Typical noise measurements are shown in fig. 2. These were obtained by measuring the rms voltage using a R a c a l - D a n a 9310A True RMS voltmeter at the output of the main amplifier. The calibration for the ordinate was obtained by exposing the photodiode to 59.5 keV photons from an americium-241 source; electrons from photoionisation in the silicon have a range of only about 25 /zm so are detected with high efficiency. A conversion constant of

3.6 eV per e lect ron-hole pair was used to obtain the electron yield. A test capacitor was also available on the preamplifier which gave consistent results. We generally chose to operate with a shaping time constant of 0.5 ffs because of variations between the leakage currents of different diodes but also, because of heating from elec- tronics in our laboratory, we expected, and observed, increases in the leakage currents as the temperature rose. Separate measurements of the detector capacitance and leakage current as a function of bias voltage are shown in fig. 3. They are consistent with interpreting the noise as entirely generated by leakage current and capacitance with little series resistance contribution. This was not surprising since the manufacturers have attempted to minimise any surface resistance during the production process for other applications where re- sponse time is important. For the purpose of the tests reported here we focussed on the signal from minimum ionising /3's going through various 1 cm thick scintilla- to r -photodiode combinations. We did not attempt to optimise the noise performance of each photodiode- preamplif ier-shaping amplifier chain. The noise figure of o = 750 to 900 electrons for typical reverse bias voltages of 20 to 50 V can be improved with low leakage current diodes [10].

All the scintillator material tested was in the form of strips 1 x 1 cm 2 cross-section and 20 cm lengths. They were loosely wrapped in reflective aluminium foil and black PVC tape with one end left open. The scintillator strip was mounted on a movable support so that the light emitting end could be brought up close to the surface of the photodiode under test without actually

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346 G. Hall et al. / Performance of silicon photodiodes used with plastic scintillators

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making contact. It was established that without full optical coupling the distance between the scintillator face and the diode surface was not critical and even for separations greater than about 1 mm no diminution of the signal was observed. In a second series of measure- ments we introduced optical grease between the scintil- lator surface and the photodiode.

Measurements of light output were made using minimum ionising fl electrons from a ruthenium-106 source. A trigger on electrons completely traversing the

scintillator was provided by placing 1 cm 2 silicon charged particle detectors of 250 /~m thickness as close as possible to opposite sides of the scintillator being tested. Lead collimators 2.5 mm thick were inserted to reduce the effective entrance and exit apertures to less than 1 cm diameter and to avoid triggers where elec- trons had only partially traversed the scintillator. Lead was also placed around the photodiode so that electrons could not scatter into the detector. The trigger detectors were read out using LeCroy HQV810 preamplifiers and

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G. Hall et al. / Performance of silicon photodiodes used with plastic scintillators

Table 1 Properties of scintillators used in tests

Peak emission Decay time Density Refractive index (nm) (ns) (g/cm 3 )

347

NE104 406 1.9 1.032 1.581 NE108 535 1.5 1.032 - NEl l0 434 3.3 1.032 1.58

Ortec 474 C R - R C shaping amplifiers with equal shap- ing time constants of 50 ns. The output signals were then discriminated and brought into coincidence. Sin- gles rates were low enough to make accidental coinci- dences very rare while typical trigger rates were 10-15 Hz. The trigger generated a 1 ~s gate which was delayed so that the peak of the output pulse from the photodi- ode electronics was contained within it (timing was in fact done using test pulses because of the small signals from the photodiode-scint i l lator) and the signal read out using a LeCroy 2259A peak sensing ADC, and a minicomputer. The zero of the pulse height spectrum could be established by adding a large extra delay before the gate and observing the pedestal from "ou t of t ime" triggers.

3. Results

Tests were made using three different plastic scintil- lators produced by Nuclear Enterprises Ltd., NE104,

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NE108 and N E l l 0 , whose principal characteristics are listed in table 1 [11]. Some results obtained with N E l l 0 are displayed in fig. 4 which shows pulse height spectra obtained using two of the photodiodes, and a spectrum taken with a delayed gate representing zero signal broadened by electronic noise. A delayed spectrum was taken for each diode but only the one taken with the Micron diode is shown, for clarity. However both de- layed spectra are similar in shape and width to in-time spectra taken with the same diode, and the signal was estimated by fitting the distributions to Gaussian shapes and taking the difference between the fitted means of the in-time and delayed spectra. The results are given in table 2, where A D C channels have been converted to photoelectrons using an 241Am calibration, and it can be seen that the Gaussian shape is an excellent representa- tion of the data. The noise was estimated from the fitted standard deviations. (The small increase observed with the Micron diode compared to the RMS voltmeter measurements (fig. 2) is attributable to the increase in

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Fig. 4. Pulse height spectra obtained with minimum ionising l°6Ru fl's in a 1 cm thick sample of NEll0. The spectrum labelled 'Noise' was obtained with a delayed trigger and represents zero signal broadened by electronic noise. The two other spectra were obtained from in-time triggers using diodes Micron 1 and Hansen PIV 1, 1. The solid curves are the results of Gaussian fits to the data.

348 G. Hall et al. / Performance of silicon photodiodes used with plastic scintillators

Table 2 Measured signal for scintillator-photodiode combinations tested

Device Scintillator No optical coupling Optical grease coupling signal ") signal a~ (electrons) (electrons)

Micron 1 NE110 1103 ± 21 - Micron I a NE110 1300 ± 60 2390 ± 60 Micron 2a N E110 1250 ± 60 2390 _+ 60

Hansen NE110 1545 ± 40

Micron la NE108 1650 ± 60 3030 ± 60 Micron 2a NE108 1730 ± 60 2950 + 60 Micron 5a NE108 2020 + 60 3400 + 60

Micron 2a N E104 2240 + 60

a) Quoted errors are from fits to pulse height spectra. No allowance has been made for uncertainties in the calibration.

the leakage current due to heating.) We conclude that, as expected, fluctuations in the observed signal are dominated by electronic noise and by variations in the number of incident photons either from statistics or electron path fluctuations. We also note a 40% improve- ment in the signal observed with the Hansen diode which has an enhanced ultraviolet response.

Further tests were carried out using N E l l 0 in com- bination with other diodes supplied by Micron Semi- conductor and some of the results are given in table 2.

An increase in the signal size compared to the first diode was observed in each case accompanied by a

small increase in the noise. All of the Micron diodes tested gave smaller signals in combinat ion with N E l l 0 than the Hansen device, though only by about 20-30%.

The second scintillator to be evaluated was NE108 which was specially developed for applications involv- ing photodiodes [1] and which has its peak emission at = 540 nm. This is still not at the wavelength of maxi- mum photodiode sensitivity but it is already close

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Fig. 5. Pulse height spectra obtained with minimum ion±sing I°6Ru fl's in a 1 cm thick sample of NEI08. Spectrum (a) is the out of time pedestal distribution. Spectrum (b) was obtained with no optical coupling between the diode and the scintillator, whereas (b) was obtained with an optical grease contact between scintillator and diode as described in the text. The low pulse height tail of spectrum (c) is partially caused by an inefficiency in the trigger not visible with lower scintillation signals. The solid curves are from fits of single Gauss±an distributions to the data.

G. Hall et al. / Performance of silicon photodiodes used with plastic scintillators 349

enough to expect some improvement in signal size pro- vided the light output of NE108 scintillator is not greatly less than N E l l 0 . The manufacturers literature makes no mention of the expected light output. Our tests demonstrated a substantial improvement in signal when using the NE108 samples and we were able to obtain signals in excess of 2000 electrons for the first time (table 2). In addition, all the Micron diodes tested gave larger signals with NE108 than the Hansen diode (UV enhanced) with N E l l 0 .

We then went on to examine the effect of improving the optical coupling between scintillator and photodi- ode. This was only done for the diodes supplied by Micron as they were coated with a silicon resin to provide extra protection for the surface and the aluminium bond. (This also explains some of the loss of sensitivity at short wavelengths). Therefore we chose to improve optical contact in a nonpermanent way by means of optical grease (Dow-Corn ing 20-057 Optical Coupling Compound), so that we could compare differ- ent combinations of diode and scintillator. (We inciden- tally wished to find out what tolerance the diodes would have to treatment of this kind and, in fact, decided to make no similar tests with the Hansen diodes because of danger of damage to the gold bonds.) The grease was gently daubed onto the scintillator surface using the clean, flat blade of a screwdriver until a uniform layer of about 0.5 mm thick was built up. The scintillator was then brought slowly into contact with the photodiode using the motion of the support until grease began to squeeze out of the gap. This method was found to work well and led to no apparent degradation in the perfor- mance of the diodes, even after several mountings.

Substantial improvements were obtained with all diodes and scintillators when using optical grease. Sig- nals were observed to increase by -- 70-90% with little if any increase in the noise (table 2 and fig. 5). Signals of = 3000 photoelectrons from NE108 were typical compared to = 2400 photoelectrons from N E l l 0 . The third scintillator, NE104, was found to give very similar results to N E l l 0 (table 2).

Various attempts were made to increase the scintilla- tor signals above the levels observed here by rewrap- ping, or by applying titanium dioxide paint. Either no improvement resulted, with rewrapping, or there was a large degradation, factor -- 4, with white diffusing paint. All the scintillator material had been cut to size using a diamond saw without further polishing. No significant differences were observed between scintillator strips of excellent external appearance and those with surface defects or saw marks.

4. Conclusions

With the best scintillator (NElO8)-photodiode com- bination tested we have observed signals from minimum

ionising fl electrons in 1 cm thick layers of scintillator of -- 3400 electrons. Allowing for multiple scattering of the electrons in the scintillator we might expect to observe = 2500 photoelectrons from high energy charged particles in a similar set up. The measured noise in the case of the largest signal was 900 electrons. however since this depends very much on the exact conditions of operation this limit is not fundamental and we could expect to achieve noise levels of - -600 electrons without too much difficulty. If one were able to tolerate shaping time constants of several microsec- onds it seems quite reasonable to expect noise levels to be reduced below about 400 electrons with low leakage diodes of similar area to those tested [12].

Clearly the greatest scope for immediate improve- ment lies in improvements to the diodes, particularly in increasing the quantum efficiency. With the existence of a well matched scintillator there is great incentive to extend the short wavelength response of the diodes only a little further (similar improvements are also desirable for applications involving bismuth germanate). If the efficiency at 540 nm can be raised to the peak sensitiv- ity of about 85% rather than being on the rising part of the response curve a further gain of up to -- 60% could be obtained, leading to signals of about 4000 electrons for a high energy charged particle through a 1 cm scintillator. In that case and with an rms noise of 600 electrons a minimum ionising signal to noise ratio of about 6 -7 could be obtained which would allow unam- biquous identification of single particles, for example in a hodoscope. With a preamplifier similar in perfor- mance to the one used here this could be achieved with a relatively short shaping time and thus without placing too great a demand on the dark current performance of the diode. With longer shaping times (few/~s) signal to noise ratios of 10 seem realistic.

As well as detector sensitivity both the leakage cur- rent and the capacitance are under the control of the manufacturer. The leakage current is minimised by the use of oxide-passivated techniques originally borrowed from the semiconductor industry; currents as low as fractions of nanoamperes are possible under opt imum operating conditions though great care is needed at all stages of the production process and after delivery to maintain this current level. The detector capacitance is a far more reliable parameter in that it only depends on the bias voltage and the resistivity of the silicon. It therefore seems desirable to operate the diodes at low voltage to minimise leakage currents but to achieve low capacitance by using higher resistivity material. Since raw material costs are not the major contribution to the price of a photodiode a higher yield and lower cost could be expected if constraints on leakage currents are able to be relaxed. A different issue related to the use of photodiodes as scintillation light detectors is that of charged particles traversing the photodiodes. The result-

350 G. Hall et al. / Performance of silicon photodiodes used with plastic scintillators

ing signals and the way they affect the per formance depends on the appl icat ion and is discussed elsewhere in the context of a sc in t i l l a to r -pho tod iode electromag- netic shower posi t ion detector [13].

Acknowledgement

We should like to thank T.E. Hansen and Micron Semiconductor Ltd., for providing the diodes used in these tests, and for helpful discussions. We thank M. Goyo t for providing the preamplif ier used here.

References

[1] J.E. Bateman, Nucl. Instr. and Meth. 71 (1969) 269. [2] R. Klanner, Nucl. Instr. and Meth. A235 (1985) 209. [3] J. Kemmer, Nucl. Instr. and Meth. 169 (1980) 499.

[4] Micron Semiconductor Ltd., 1 Royal Buildings, Churchill Industrial Estate, Lancing, Sussex, England.

[5] T.E. Hansen, Central Institute for Industrial Research, Oslo 3, Norway.

[6] M. Goyot, Institut de Physique Nucleaire, Universit6 de Lyon, private communication.

[7] P.W. Nicholson, Nuclear Electronics (Wiley, New York, 1982).

[8] D.E. Groom, Nucl. Instr. and Meth. 219 (1984) 141. [9] T.E. Hansen, Nucl. Instr. and Meth. A235 (1985) 249.

[10] The first diodes used were given to us by the manufac- turers at short notice for these tests and were regarded by them as poor quality examples in that they had leakage currents of more than a few nA at all operating voltages. As this did not represent the major contribution to the noise when using a shaping time constant of 0.5 /~s we proceeded to use them.

[11] Nuclear Enterprises catalogue. [12] E. Lorenz, Nucl. Instr. and Meth. 225 (1984) 500. [13] G. Hall, D. Robinson, C. Seez, I. Siotis, Imperial College

preprint, in preparation.