Nuclear Instruments and Methods in Physics Research A253 (1987) 491-499 North-Holland, Amsterdam

491

ELECTROMAGNETIC SHOWER POSITION DETECTION WITH A SCINTILLATOR-PHOTODIODE HODOSCOPE

G. HALL, D. ROBINSON, C. SEEZ and 1. SIOTIS

The Blackett Laboratory, Imperial College, London

A position detector comprised of two orthogonal planes of scintillator fingers read out by silicon photodiodes was evaluated in a test beam as part of an electromagnetic calorimeter prototype.

Measurements of the position resolution were made over a large energy range with particular emphasis on the effects of charged particle leakage from the showers into the photodiodes.

1. Introduction

In large calorimeters in colliding beam experiments in particle physics it is often required to have, in addition to an energy measurement of an electromag- netic shower, a good spatial measurement to disting- uish multiple showers in a single calorimeter cell. Not only is the information on overlapping showers impor- tant, but the improved effective mass measurements obtained are valuable in establishing an overall mass calibration of the detector. To be maximally effective the calorimeter should have as few inactive regions as possible. We have considered silicon photodiodes in combination with scintillator as a method of achieving this goal; a shower position detector could be compact and relatively inexpensive and the photodetectors would have high immunity to a magnetic field within the apparatus. We have shown in laboratory tests [l], that with an optimal choice of scintillator of reason- able dimensions, currently available photodiodes are capable of measuring minimum ionising charged parti- cle signals of 2000-2500 electrons. However an impor- tant problem which has not been studied is how well such a detector would perform in the presence of a charged particle background since the diodes are also excellent charged particle detectors and particles leak- ing from showers might be expected to obscure or distort the genuine scintillator signal. To evaluate this problem we have built two planes of scintillator- photodiode hodoscopes each comprised of 18 strips 20 X 1 X 1 cm3 which we have tested in a beam. The signal size and position resolution have been studied as a function of beam energy and position using active and passive absorbers to generate showers.

2. Experimental apparatus

The tests were made in the X5 and ‘I7 beam lines at

0168-9002/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Phvsics Publishing Division1

CERN. The momentum of the X5 beam, where most of the data were taken, was variable from -5 to lOOGeV/c with a momentum spread Aplp of 1.5% rms [2]. The beam is produced by protons and is of mixed composition; muons, hadrons or electrons could be selected and tagged. A delay line wire chamber upstream of the apparatus under test allowed us to measure the position of the incident particle with a precision of -0.2 mm.

Measurements in the T7 beam allowed us to extend the momentum range of our data down to 2 GeV/c. The T7 beam is also produced by protons and contains mostly electrons and pions. During our tests beam particles were not individually identified but separate measurements [3] established that at 2 GeV/c the beam was ~100% electrons and at 5 GeV/c -82% electrons.

A schematic diagram of the apparatus is shown in fig. 1. Each scintillator plane comprised 18 strips of orange emitting NE108; a photodiode was mounted at one end of each strip and optically coupled to it using optical grease so that the diodes were placed alternate- ly at opposite ends of adjacent scintillators. The two planes were mounted orthogonally to each other in an aluminium box with 3 mm thick walls.

The photodiodes were supplied by Micron Semicon- ductor Ltd. They were all fabricated from n-type silicon implanted with a 1 cm’ p-type area and moun- ted on a small piece of ceramic. The p-type region was ultrasonically bonded to a gold plated contact with an aluminium or gold wire, and the device was coated in a silicon resin to protect the surface and the bond. The quantum efficiency at 540 nm was measured to be -50% by comparison with a calibrated diode and was found not to vary much from sample to sample. The diodes were designed to operate at bias voltages up to 40 V and had capacitances of =80 pF at 20 V, typical leakage currents measured by the manufacturer were l-2 nA at 20 V. We generally observed currents higher

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492 G. Hall et al. I Electromagnetic shower position detection

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:a1 se

E;

4

< 3 20cm

Fig. 1. Schematic diagram of a section of the hodoscope. Each scintillator element had dimensions 20 cm x 1 cm x

1 cm.

than these by about a factor 2 in laboratory measure- ments, which we attributed to higher temperature operation.

Each diode was biased through a 47 MR resistor using a positive supply and the signals read out by Laben 5254 hybrid charge-sensitive preamplifiers which drove differential outputs over 30m of twisted pair cable to a simple shaping amplifier. The shaping characteristics of the amplifier were approximately equivalent to a high pass filter followed by two low pass filters with time constants =l ps. The amplifier had a variable gain and was found to have good linearity up to output levels exceeding 1 V The mea- sured noise performance of the preamplifier-amplifier combination was found to be ~600 electrons at zero input capacitance rising to ~900 electrons at 85 pF. Finally the amplifier output was digitised using a 100 ns gate by a LeCroy 2282 charge sensitive ADC.

The preamplifiers were mounted in groups of nine on printed circuit boards placed inside the detector box so that each board read out one side of the hodo- scope. We observed a small coupling between one channel and another on the same side which took the form of ringing on other channels when a signal was injected into a single preamplifier, at the level of l-2% of the output from the pulsed channel. We verified that this cross-talk was caused by the lay- out of the preamplifiers but we did not have time to eliminate it.

3. Performance of the diodes

After assembling the complete detector we ex- perienced a few diode failures where the ultrasonic

bond was later found to have lifted away from its contact. All of the failures had gold bonds and after discussions with the manufacturer it was found that the silicon resin protection layer did not adhere to the gold wire and was not holding the bond firmly in place.

The tests were carried out at bias voltages of 2 and 20V. The leakage currents throughout were very stable at 2V whereas at 20V the average current increased from =2 nA/diode to lo-20 nA/diode over a period of days. Some part of this was simply due to heating as the area was very warm during the tests and our preamplifiers were not well ventilated. Since leak- age currents were measured in groups of nine diodes we could not observe changes in individual diodes directly, but it was possible to measure the rms voltage at the output of the main amplifier and estimate the current indirectly from a knowledge of the shaping characteristics. It appeared that though most diodes were stable a small number had begun to draw larger than normal currents. We initially attributed problems with aluminium bonded diodes to damage during handling or from vibration transmitted during move- ment of the detector even though great care was taken during the mounting of the diodes. However, apart from initial failures almost all photodiodes performed well at 2V bias. During the tests the detector was moved a great deal and the whole detector was trans- ported twice between London and Geneva without further diode failures. It appears therefore that the leakage current behaviour may be similar to that observed by the authors of ref. [4] (who have tested a large sample of photodiodes) where runaway leakage currents occurred in a small proportion of cases.

4. Results

To establish a calibration for the observed shower energies we took data with muons and hadrons with no absorber present to observe the signals of minimum ionising particles. The diodes were all biased at 20 V to minimise electronic noise. A typical pulse height spectrum is shown in fig. 2 where the data consist of events in which the wire chamber predicted a hit in that finger. The relative transverse positions of the wire chamber and the scintillator hodoscope were not well measured so we relied on establishing the offset by maximising the signal in each strip which could be done with a precision of a few mm only because of beam divergence. This implies that the beam was not sufficiently well defined to be certain of having a hit in the predicted finger and it is clear that the spectrum of fig. 2 contains a fraction of zero pulse heights, broadened by electronic noise.

The noise distributions for each channel were mea-

G. Hall et al. I Electromagnetic shower position detection 493

80

t

Hadrons 40 GeV

ADC channel

Fig. 2. Pulse height spectrum for 40 GeV hadrons observed in one of the scintillator fingers with no absorber present. The

spectrum was fitted to two Gaussian distributions of identical width to represent the noise and the scintillation signal. The solid

line shows the noise distribution from this detector element normalised to the data by the fit.

sured separately by taking data with a random trigger between bursts; they were found to be stable and were well fitted by Gaussian shapes. To obtain the minimum ionising signal two Gaussians with the width of the noise were fitted to each distribution triggered on single particles. We obtained signal to noise ratios of 2.0-2.8 with an average of 2.4kO.2, which com- bined with our estimate of electronic noise above yields pulse heights of ~2200 electrons. This agrees quite well with expectations based on laboratory mea- surements [l]. The minimum ionising signal corres- ponded to 17.3 * 1.7 ADC channels and all our sub- sequent results are expressed in units of energy depo- sited by a single high energy particle.

We performed three series of tests. In the first the scintillator hodoscope was positioned immediately downstream of a 3.8 radiation length passive absorber consisting of 2 mm thick depleted uranium plates sepa- rated by 2.5 mm thick plastic sheets. Though passive this converter had a similar structure to the prototype calorimeter [5] with which we performed the second series of tests. In these the scintillator hodoscope was placed inside the calorimeter after 7.6 radiation lengths. Finally data was taken at 2 and 5 GeV/c using a passive absorber of 7 radiation lengths. Data were taken with electron beams of different energies and the beam was scanned over the face of the counter at one energy (40 GeV). Showers were generally clearly visible, and in fig. 3 we plot typical event profiles in the scintillator fingers at 20 GeV.

To identify showers and their component strips a realistic algorithm was used, based on shower finding

in a hodoscope used in another experiment [6]. Horizontal and vertical elements were considered separately. A minimum pulse height was demanded for a finger to be considered a candidate for inclusion in the shower which was placed at 1.25 + 0.02 C PH,, where PH, was the pulse height of strip i in minimum ionising units, and the summation runs over the 18 strips in a plane. The second term was inserted to allow for the effect of the cross-talk described above. Having identified the strips making up the shower the total shower energy and the centre of gravity could be calculated. Some of the results of this procedure are shown in fig. 4. The small fraction of events where a shower was not observed do not necessarily represent an inefficiency in the detector but can be explained as the result of fluctuations in the size of the showers. Fig. 5 shows the observed energy as a function of the beam energy.

The resolution of the position measurement of the shower was obtained by fitting the difference in pos- ition measured by the scintillator hodoscope and the wire chamber to a Gaussian shape (fig. 6). This was then studied as a function of beam energy and position relative to a diode array. Results are shown in fig. 7 where the energy behaviour of the resolution is plot- ted and parameterised as (Y + /3/e for data taken with the beam near the centre of the array. Such behaviour was observed [7] in a scintillator-photomul- tiplier hodoscope over an energy range from 2 to 40 GeV. We do not have an estimate for the position resolution at 2 GeV because at this energy showers were identified with an efficiency of -30% and statis-

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494 G. Hall et al. I Electromagnetic shower position detection

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Fig. 3. The two projections of the shower profiles for two typical electromagnetic showers initiated by 20 GeV electrons and measured after 3.8 radiation lengths. Strips l-18 measure the horizontal projection of the shower and strips 19-36 measure the

vertical projection. The incident electrons crossed the detector roughly in the centre.

tics were very limited. Fig. 8 shows the behaviour of close to the diodes. We also observed an increase in the position resolution in the vertical plane as the the apparent energy of a shower (fig. 9) as it ap- 40 GeV/c beam was scanned horizontally across the proached the edge of the detector. Both observations face of the detector. We observed a worsening of the are consistent with charged particles from the shower resolution at the edges of the detector but not by a entering the diodes. large factor, and ~only when the beam approached We have studied charged particle leakage in more

1000

800

n 20 GeV e-

Total shower signal (Min I particles)

Fig. 4. The total shower signal from 20 GeV electrons measured after 3.8 radiation lengths of uranium-plastic absorber. The peak below about 2 minimum ionising units corresponding to the absence of a shower is seen in both hodoscope projections and is

attributed to fluctuations in the size of showers. It is absent in data taken at 20 GeV after 7.6 radiation lengths.

G. Hall et al. I Electromagnetic shower position detection 495

103c

0 7.6 X, 0 3.8 X, A 7.0 X0

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Fig. 5. Incident energy dependence of the shower observed after 3.8-7.6 radiation lengths.

signal

(a) 20 GeV 7.6 X0

detail by attempting to isolate events where a charged particle appears to have crossed a diode. Fig. 10 shows some events where this appears to have occurred. The large spikes seen in the vertical fingers but not visible in the horizontal profile are typical of such events. Because of the scintillator readout at alternate ends we do not expect these pulses to simulate other show- ers but they could distort the position measurement. The fact that they do not suggests that they are quite infrequent until the shower is very close to a diode. In principle measurements of the same shower in both projections could provide an extra mechanism for identifying occurrences of this kind. However we have ignored the strong correlation between horizontal and vertical hodoscopes because the showers of most inter- est are so close to the edge of the detector that energy is lost in one plane.

An algorithm was developed to locate charged parti- cle spikes in two regions, in the scintillator strips not included in the shower when a straightforward pulse height cut could be applied, and in the shower strips where a more sophisticated approach was required. Briefly, this involved finding the most significant dip in the shower profile and identifying its neighbour as a spike if its pulse height was greater (by 10 minimum ionising units) than a prediction calculated from near- by pulse heights. To demonstrate that the method gave reasonable results figs. lla and llb show distri- butions of spike containing scintillator fingers inside and outside the shower. The histogram shows the expected odd-even behaviour indicating that spikes

400

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Fig. 6. Difference in position measured by the hodoscope and a delay line wire chamber upstream of the detector. The solid lines are the results of Gaussian fits to the data. (a) Beam energy = 20 GeV measured after 7.6 radiation lengths, (T = 2.48 mm. (b)

Beam energy = 5 GeV measured after 3.8 radiation lengths, cr = 2.88 mm.

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G. Hall et al. I Electromagnetic shower position detection

Position resolution

Cl 7.6 rad.lengths 0 3.8 rad.lenaths

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Fig. 7. Incident energy dependence of the rms position resolution measured at 3.8 and 7.6 radiation lengths

been parameterised in the form o + /3~% with a = 1.0 mm and p = 6.3 mm/m

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Horizontal scan 280 Horizontal scan

O 0 8

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Fig. 8. The rms position resolution as a function of distance

along the hodoscope element. The data were taken at 40 GeV electron beam energy after 7.6 radiation lengths.

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.- ; 160 V

w” 120

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Fig. 9. The average energy of showers observed in the

vertical projection (i.e. horizontal scintillator fingers) using a

40GeV electron beam and measured after 7.6 radiation

lengths. The beam was scanned horizontally across the face of the detector. The error bars represent the rms variations in

the size of the showers, not the error’on the mean observed energy.

G. Hall et al. I Electromagnetic shower position detection 497

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Fig. 10. The two projections of the shower profiles for two typical electromagnetic showers initiated by 20 GeV electrons and measured after 3.8 radiation lengths when the beam was close to a detector edge. The incident electrons crossed the detector about 1 cm from the nearest end of the horizontal hodoscope elements. The vertical fingers (strips l-18) show evidence of

charged particles crossing the diodes in the form of spikes in the profile.

(a) Inside shower 50

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10 I Oo 4

-I I I I I I

8 12 16 Strip number

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Strip number

Fig. 11. The distribution of vertical scintillator strips found to contain spikes by the algorithm described in the text. The beam was incident close to the top of the detector so that charged particles from the shower were expected to be seen only in the fingers read out at that end. (a) Spikes identified in the fingers containing a shower. (b) Spikes identified in the fingers not containing a

shower. The data were taken using 20 GeV electrons after 3.8 radiation lengths.

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498 G. Hall et al. I Electromagnetic shower position detection

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Fig. 12. The frequency of identified spikes greater than 10 minimum ionising units as the beam was scanned horizontally across

the detector. The squares represent spikes observed at x = 12 cm while the circles represent spikes observed at x = 8 cm. (a) Spikes

identified in the fingers containing a shower. (b) Spikes identified in the fingers not containing a shower.

are found mainly at the end of the hodoscope near to the beam.

Using this algorithm we were able to study the frequency of charged particle induced spikes as a function of beam position. Fig. 12 shows the fraction of 40 GeV events where a spike was identified inside

or outside the fingers containing the shower. A large increase is seen when the beam is close to the diodes, but only when the centre of the shower is less than 2cm from the diodes are more than S-10% of the events affected. Our statistics were too limited to study in detail only events containing spikes but some esti-

t

(0) 20 volts bias

Apparent size (Min I)

t (1 b 2 volts bias

’ oII,, 0 Apparent size (Min I)

0

Fig. 13. The distribution of spikes as a function of their apparent size in units of minimum ionising particles observed in the

scintillator. The data were taken at 20GeV after 3.8 radiation lengths with the beam close to the top of the detector.

G. Hall et al. / Electromagnetic shower position detection 499

mate of the effect on the position measurement for these events can be made by looking at the apparent size of the spikes. Fig. 13 shows the distribution of spikes as a function of their size, and although statistics are limited it is clear that large charged particle signals are rare.

These results may be understood by considering the composition of an electromagnetic shower after sever- al radiation lengths of converter. Low energy electrons which are likely to be Coulomb scattered the most and appear some distance from the shower core are also the most easily stopped by the scintillator. Higher energy electrons are mainly moving forward with in- sufficient transverse momentum frequently to enter a photodiode far from the shower centre. Photons with sufficient range to penetrate the scintillator have a small probability of converting in the silicon. Overall the outcome is to detect particles in the diodes close to the shower centre and not destroy the position mea- surement. In the very worst case where the core of a shower passes close to or through a diode it would still identify the shower position quite accurately, though not its energy.

term stability, are expected. This should allow better matching of the electronics and thus lower noise per- formance leading to enhanced efficiency for identify- ing minimum ionizing particles and possibly to a re- duction in the thickness of scintillator required. How- ever it is clear that a plastic scintillator-photodiode device does represent a workable solution to the problem of position measurement of showers in a calorimeter and that charged particles crossing the diodes do not give rise to a major degradation of the spatial resolution.

Acknowledgements

We should like to thank our technical staff for their work on the assembly of the detector, in particular A. Rochester, T. Barnes and G. Barber. We should also like to acknowledge the members of the UAl collaboration concerned with the evaluation of the prototype calorimeter module. We also thank Piera Brambilla for the preparation of this document.

References 5. Conclusions

In these tests we have demonstrated the viability of an electromagnetic shower detector using readily available plastic scintillator and photodiodes. We were concerned to prove a working detector rather than achieve “state of the art” performance and clearly further optimisation is possible. Improvements in the quality of photodiodes in terms of leakage current behaviour and quantum efficiency, as well as long

[l] G. Hall, D. Robinson and I. Siotis, IC/HENP/85/4, Nucl. Instr. and Meth. A245 (1986) 344.

[2] D.E. Plane, CERN/SPS/83-22 (1983). [3] B. Denby, private communication. [4] Z. Bian, J. Dobbins and N. Mistry, Nucl. Instr. and

Meth. A239 (1985) 518. [5] M. Albrow et al., UAl Technical Note UAl/TN/85/85. [6] C. Seez, Ph.D. thesis, unpublished. [7] R. Rameika et al., Nucl. Instr. and Meth. A236 (1985)

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