Results from a test of a Cu-scintillator calorimeter module with photodiode readout

512

Nuclear Instruments and Methods in Physics Research A257 (1987) 512-522 North-Holland, Amsterdam

RESULTS FROM A TEST OF A Cu-SCINTILLATOR CALORIMETER MODULE WITH PHOTODIODE READOUT F. FISCHER, C. KIESLING, E. LORENZ, G. MAGERAS and S. SCHOLZ Max Planck Institute for Physics and Astrophysics, Munich, FRG

A calorimeter module of 17 radiation lengths depth has been built Wavelength shifter (WLS) bars coupled to rectangular silicon photodiodes (PDs) are used as readout. Considerations in the design of the WLS bars, with particular emphasis on optimising the efficiency for PD readout, are discussed. The energy resolution for electrons has been determined to be about 9%/rE between 2 and 50 GeV. The response to hadrons is presented and the prospects for the construction of a full-sized hadron calorimeter are discussed. 1. Introduction Calorimeters are becoming an essential element in detectors for high energy physics experiments . Various types are being pursued, depending on the prime measuring interest and the nature of the measured particles. One class of calorimeters is based on scintillators as the active medium. Fully active scintillation calorimeters have the highest achievable resolution compared to all other types for electrons and photons, but because of cost considerations it is impossible to build fully active hadron calorimeters . In this case one uses sampling calorimeters made from alternating layers of a high density absorber and active medium such as scintillator, liquid argon (LA) ionisation chambers, etc. The classical readout elements for scintillation counters are photomultipliers (PMs). Their main disadvantage is that they cannot operate in magnetic fields . Recently it has been shown that silicon photodiodes (PDs) can be used for high resolution electromagnetic calorimeters made of BGO or CsI(Tl) [1-4]. PDs work in magnetic fields, are very compact and exhibit no drift . They have a high quantum efficiency (QE) around 60% in the visible spectrum . Whereas PMs have a very high internal gain, PDs as unity gain devices need high quality charge sensitive amplifiers and filters for noise reduction. Typical noise charges are 400-800 electrons/cm' diode area and 2 fts filter time constant . For very large scintillation signals this noise is completely negligible ; however, for inefficient scintillators, or sandwich-type calorimeters with a low fraction of active material, the noise can preclude the use of PDs. Early scintillation calorimeters had complex light guides which made the construction of leaktight 47r calorimeters extremely difficult . Many of these problems were eased by the use of the wavelength shifter (WLS) bar readout [5]. WLS bars are not mechanically coupled to the scintillator and provide a high light flux 0168-9002/87/$03 .50 O Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

concentration . Recently it has been demonstrated that the PD in connection with WLS bar readout can be used for small electromagnetic sandwich calorimeters [6]. The authors report a signal of = 25 000 photoelectrons per GeV for a 4 x 3 cm cross section calorimeter made from alternate layers of 2.5 mm lead and 4 mm scintillator . Another study proposes applying this idea to the construction of hadron calorimeters in a tower structure with PD readout [7]. Here we report on the first test of such a tower module with dimensions 47 x 20 x 20 cm (length x cross section) . The paper is organized in the following way. In section 2 we discuss the factors affecting the WLS light output, and their application to the design of WLS bars optimized for PD readout. Sections 3 and 4 describe'the production of the calorimeter module and the test setup. Sections 5 through 8 describe performance of the setup in the test beam, and section 9 considers the prospects for a full sized calorimeter in light of the present results. 2. Wavelength shifter studies As already indicated, noise is an important parameter when considering PD readout. Enlarging the calorimeter element cross section or reducing the fraction of active material results in a reduction in signal per unit of deposited energy, and in turn, in a worse signal/noise (SIN) ratio. In a hadron calorimeter tower with a cross section of a hadronic absorption length squared, one measures for the PM-WLS bar readout typically 1-10 photoelectrons per 1 MeV energy deposition in the scintillator, i.e . only 1 out of 1000-10000 generated photons is converted into a detectable photoelectron . This is too low for PD readout. Therefore we have studied some of the light losses, particularly for the WLS bars . Most of the commercial WLS bars are developed for

513

F Fischer et al / Cu -scintillator calorimeter module

PM readout. Ideally the WLS bar absorbs the scintillation light over the entire emission spectrum and transmits 48% of the shifted light onto the photomultiplier, in the case of a base material polymethyl-meth-acrylate (PMMA) with n = 1.45, a perfect mirror at the opposite end of the bar and no Fresnel losses or internal losses due to surface scratches, light scatter, impurities or absorption . A good dye should have a quantum efficiency (QE) of 1, a large Stokes shift and no overlap between its own absorption and emission spectrum . As a consequence the blue scintillation light has to be shifted towards a region of low PM sensitivity. Therefore in practice compromises have to be made in the choice of dyes . For PD readout the situation is reversed . The use of WLS dyes, with extended green sensitivity to order to overlap with the tail of the scintillation light, and the subsequent shift of the light towards the red, will result in a better matching to the sensitivity of PDs. For example at a wavelength of 600 nm a QE > 70% is common for PDs, while that for PMs with bialkah photocathodes is about 1-3% . Self-absorption and re-emission processes play an important role in light losses both in scintillators and WLS materials . If the tail of the absorption spectrum overlaps with the emission spectrum, part of the photons originally trapped inside the total internal reflection cone can be absorbed and re-emitted . This process can have three undesirable consequences : (i) Some of the light quanta are lost because the dye has a QE <1 ; (ii) At increasing distances from the original light centers the visible spectrum is shifted progressively towards longer wavelengths ; (iii) The originally trapped light quanta are re-emitted isotropically and partially radiated transversely out of the bars (scintillator plates). Process (t) is generally small for common dyes and there is no cure besides the replacement of the dye by a better one. Process (ii) is not a loss of the short-wavelength fraction of the spectrum, but a true shift of the light, i.e ., after a few cm distance from the originally fluorescing dye molecules there are more long-wavelength light quanta than originally generated. Fig. 1 shows the result from a Monte Carlo simulation for a typical overlap of the emission and absorption spectra [8]. As a consequence the spectral distribution at the end of a long scintillator or WLS bar can be quite different compared to the published data and mismatching to successive WLS bars or photocathodes can occur. Process (iii) is in general the most harmful one. One can easily lose half of the originally trapped light due to transverse radiation. This process is also responsible for the short light attenuation length in many scintillators of WLS bars and it is obvious that cutoff filters can only be a partial cure for increasing the attenuation length . Process (iii) is also very harmful if

in

z r Q m CC Q

t

zw z

450

500

550

600

650mm

WAVELENGTH - D) Fig 1 . Monte Carlo calculation of the visible spectrum at different distances m a 3 mm WLS bar or scintillator, as caused by the overlap of the absorption and emission spec trum . A : absorption spectrum . B . emission spectrum, C, D, E, F: spectrum after 1, 5, 10 and 20 cm (from ref. [8]) .

one tries to widen the absorption spectrum of a WLS by increasing either the concentration or by mixing many dyes with different absorption and emission bands. The latter approach can turn the WLS bar into an efficient transverse radiator with very short longitudinal attenuation length . It should be mentioned that the problem of self-absorption and re-emission is well known for fluorescent solar panel collectors [9] and has also recently been studied for some scintillators [10] . Following the above studies we have tried to develop new WLS bars which were optimized for PD readout. We decided to use the scintillator SCNS 38 [11] because of its high light output . Many different dyes developed for fluorescent solar panel collectors have been tried [9] . Unfortunately the search was done mostly by trial and error, as there are no theoretical predictions for the exact shapes of the emission and absorption spectra in the overlap region . In order to obtain a wide absorption band which would also cover the green/yellow tail of the scintillation light, we had to work with a mixture of two dyes . The combination of the BASF dyes #084 and #241 [12] gave satisfactory results. The light output was about a factor of 2 superior to commercial WLS bars based on BBQ and about a factor 1.5 better than Y7 [11], a WLS optimized for the scintillator SCNS 38 . The attenuation length of 85 cm was within the error identical to that for Y7. In a separate measurement we tried to determine the transfer efficiency of the WLS bar, i.e. the number of photons visible at the end of the bar divided by the number of photons impinging on the bar. For a 20 cm 1. CALORIMETRY

51 4

F. Fischer et al. / Cu -scintillator calorimeter module

bar of the above quoted mixture we measured a transfer ratio of 19% compared to the theoretical limit of 46% (Fresnel losses included, difference in the diode QE for scintillator and dye emission spectrum unfolded). Additional parameters were : (i) scintillator SCNS 38, excited by a deuterium lamp : (ii) WLS bar size 200 x 30 x 3 mm, polished, teflon tape end reflector, white cardboard rear reflector; (iii) dye mixture : 6 mg of dye #084 and 7 mg of dye #241 in 180 g MMA; (iv) photodiode 30 x 3.4 mm coupled with optical cement . Although the transfer ratio is higher than for commercially available WLS bars the result indicates that significant improvements should still be possible . The WLS bar thickness and the dye concentration are also important parameters for the PD readout . In first order the SIN ratio is independent of the diode area (Groom's theorem [2]), i.e . doubling the diode area will increase both the signal and the noise. Translated to our application it means that for a given WLS bar width the noise will increase proportionally with the bar thickness . For very thin bars the amount of fluorescent light will increase proportionally to the thickness (in reality even faster because of transmission losses which are thickness dependent), while after a certain point any increase in thickness will only result in a modest gain in signal as most of the scintillation light is already absorbed . Fig. 2 shows a typical curve for the increase of signal as a function of WLS bar thickness and the resulting idealized SIN ratio when taking typical diode leakage current into account. In reality some additional effects reduce the SIN ratio. It is very difficult to produce a perfect WLS bar surface. Scratches, grease, etc., will result in losses of the trapped light. This loss is in first order inversely proportional to the thickness.

u) z z UO

zz cD ~D

in in

Fig. 3 shows a cross section of the module. The tower has a cross section of 14 x 14 cm at the front end and 20 x 20 cm at the rear end, and is assembled from 47 layers of 5 mm Cu plates and 46 layers of 4 mm scintillator SCNS 38, with 0.3 mm white cardboard between the Cu and the scintillator . The raw cut, slightly oversized plates are bolted through with 4 rods of 3 mm diameter and are then milled down to their final size and polished "en bloc" . For a tightly pressed package one can use a wet polish as the cardboard edges start to swell and to seal the gaps between the scintillator and Cu . For the positioning of the WLS bars, 4 inclined slots of 31 mm width and 3.5 mm depth are milled

3

10

WLS BAR THICKNESS - mm

Fig. 2. Increase of signal and change of SIN as a function of bar thickness for constant dye concentration. S: signal . A: SIN with constant diode leakage current. B: same as curve A but with additional light losses due to surface imperfections. C: same as curve B but with amplifier noise included . WLS

3. Production of the module

30 20

cn Q z

The preamplifier itself is an unavoidable noise source, therefore in practice deviations from Groom's theorem occur. As a rule of thumb a good preamplifier acts as an additional blind diode. Curves B and C in fig. 2 show the SIN ratio when one includes surface imperfections and preamplifier noise. In order to increase the SIN ratio one might choose a higher dye concentration and a thinner bar. The natural limit is given by the solvability of the dyes in the base material . Another limit is given by the attenuation length . A high dye concentration might shorten the attenuation length beyond acceptable values if the emission and absorption spectra overlap. In essence the optimisation of the SIN ratio is a rather complex process because of many conflicting parameters . As indicated by the curves in fig. 2 one can select easily the wrong set of parameters far away from the optimum. Detailed Monte Carlo studies are indispensable . Reflectors play also a much more important role for the PD readout than for the PM readout. For thin WLS bars with incomplete light absorption a rear reflector can as much as double the SIN ratio.

__ _4_4 Cm yers of 5mm Cu and 46 Payers of 4mm SCNS 38

__

47

-cc

PHOTO DIODE, 3Cx3 4 mm

Fig. 3. The calorimeter module

F Fischer et al / Cu -sctntillator calorimeter module

along all for sides of the tower. The slots were not polished ; the rough surfaces act as light diffusors and improve the light collection uniformity with negligible losses compared to a polished surface. There are two reasons for the inclined arrangement . The inclined grooves allow a densely packed tower configuration without gaps filled only with low density material . Also the inclined WLS bars smoothen the nonuniformity in light collection across the tower. The WLS bars of size 470 x 30 x 3 mm are produced according to a technique used by the Fraunhofer Institute [9]. The base material is MMA. The commercially available liquid contains an inhibitor in order to prevent polymerisation . Therefore the MMA has to be purified by distillation . Due to the high inflammability of MMA the distillation has to be done in a nitrogen atmosphere, which also keeps dissolved oxygen at a low level. The boiling point of MMA is 98'C and the combustion point only 12'C . Finally the dyes #084 and #241 and a polymerisation starter (a, a' Azo iso-butyl nitril, NC(CH 3 ) 2 =NC(CH 3)2CN) are added. It is important to filter the mixture a few times under low pressure in order to remove undissolved dye particles and traces of oxygen . The liquid is then poured into a mold made from two glass plates separated by polypropylene or teflon tubing of a diameter slightly larger than the desired bar thickness. The tubing is flexible enough to follow the shrinkage of the MMA during polymerisation . For the polymerisation the mold is heated to 50 ° C in an oven or waterbath for about 15 h. Finally the sermhard PMMA has to be tempered for a few hours at 125 ° C for completion of the polymerisation . A rectangular Hamamatsu PD S 2575 of 30 x 3.4 mm size was glued with Epotek 302-3 onto one end of each WLS bar while the other end was covered with a white teflon tape reflector. Four such bars were positioned in the inclined groves of the module .

z Q

m

r

LIGHT EMISSION OF SCINTILLATOR

20

Q 10 -SIGNAL AT END OF WLS BAR

áz

m

0

20

40

TOWER HEIGHT-cm

Fig. 4. Variation of the light attenuation m the scintillator and the WLS bar and the combined response as a function of tower depth.

51 5

Due to the tapered geometry and the light absorption process both in the scintillator and WLS bars the light collection is nearly uniform along the tower except for the last few cm . Fig. 4 shows the depth variation for the scintillator light output due to the changing cross section, the attenuation of the WLS bar and the resulting longitudinal response . The completed tower was covered with a white cardboard reflector and mounted in a thin Cu can acting both as a light seal and electric shield . Each pair of PDs were connected to a Canberra 2003BT charge sensitive preamplifier . The output signals were added in an adjustable linear mixer and fed to a Tennelec shaping amplifier. Finally the shaped signal was analysed with a LeCroy qVt multichannel analyser (MCA). The PDs were reverse biassed to 22 V. 4. The test setup The calorimeter module was installed in the X3 test beam in the west hall of the CERN SPS. Two small scintillation counters of 1 x 1 cm area in front of the module defined the incident beam position and generated the gating signal for the MCA. The calorimeter was exposed to beams between 2 and 50 GeV/c momentum and a typical flux of 1000-6000 particles/ burst with a 2 .5 s flat top. The electron content of the beam varied between 90% at 2 GeV/c and = 3% at 50 GeV/c. 5. Calibration and conversion yield The module was exposed to muons of 10 GeV/c momentum . The most probable energy loss in the calorimeter is 359 MeV (34.7 MeV in the scintillator and 324 MeV to the inactive material). This is calculated by applying the Bethe-Bloch formula, including relativistic rise and density-effect corrections [13] . We like to note that the Bethe-Bloch formula has been confirmed to an accuracy of = 1%, by measurements of muons up to 10 GeV/c [14] and of electrons, peons and protons in silicon [151 and nuclear emulsions [16] . Fig. 5 shows the measured pulse height spectrum for muons together with the pedestal distribution for a shaping time constant of 3 ps . Cross calibration of the muon peak against a precision charge pulse injected at the preamplifier input yielded a signal conversion factor of 8200 ± 350 photoelectrons which corresponds to 23000 ± 1000 photoelectrons/GeV deposited energy . From the SIN ratio of = 15 : 1 (see fig. 5) we determined the noise an-, = 24 MeV (--- 550 photoelectrons) . A 10 GeV/c muon generated a signal of = 170 photoelectrons (most probable energy loss) when passing through one layer of 4 mm scintillator . We observed 1 . CALORIMETRY

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F. Fischer et al / Cu -scintillator calorimeter module 3000

2000

Q

1000

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0

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20

30

40

50

ENERGY [GeV]

Fig. 5 . Pedestal and energy loss distribution for 10 GeV muons . no change of the most probable energy loss for incident muons of 6 GeV/c and less than a 5% lower value at 2 GeV/c momentum . This variation is consistent with the theoretical calculation . 6. Linearity As already mentioned the module is only a subelement of a calorimeter and the primary intention of this test was to evaluate if the PD readout concept is also suitable for modules with a cross section of roughly a hadronic absorption length squared, i.e . a factor 30 increase in area over the test calorimeter of ref. [6]. Our module has only a depth of 17 radiation lengths (X,) and is too short to contain electromagnetic showers fully. Substantial rear leakage already occurs for electrons above 1 GeV energy . The rear leakage increases slowly with incident energy and causes some nonlinearity between incident and observed energy . The correlation between observed and incident energy from 2 to 50 GeV is tabulated in table 1 and shown in fig. 6. The leakage losses have been calculated with the Monte Carlo program SCHEME [171 and are also tabulated in table 1. The program SCHEME is a generalized shower

Fig. 6. Correlation between incident and measured energy . o raw data . ": after correction of the leakage losses. calculation program combining the EGS code [18] for the treatment of electromagnetic showers, the HETC code [19] for the hadronic showers, the MORSE code [201 for low energy neutron transport and the LEANT code [21] for geometrical parametrisations . The Monte Carlo program showed that the predicted leakage, partition of energy between the active and passive material and the resolution depend quite critically on the cutoff parameters for electrons and y's. For the calculation of the leakage losses and energy partition we used the following cutoff parameters of Ec to , = 0.1 MeV and EEcorr = 0 .6 MeV. According to the Monte Carlo calculations the side leakage is well below 1% while the rear leakage varies between 2.5 and 6% in the tested energy range. The calculations predict an energy partition between active and passive medium of 0.107 . After correction of the leakage and longitudinal response, as shown in fig. 4, linearity between incident and observed electron energy is observed . For comparison also the corrected energy values for a flat response are quoted in table 1 . For the ratio Rpe of the muon to electron energy we obtained a value of Rie = 1 .02 ± 0.05 . This number is based on the assumption that the beam momentum was precisely as calculated but we were unable to verify the absolute beam momentum by a direct measurement. We want to point out that the knowledge of the absolute

Table 1 Correlation between incident and observed energy for electrons E,e Eohs Mc Eexpecb ed Ecorr ) Ecorr c ~

2 118+1 1.95 121±1 122+1

6 352+2 5.8 364±2 370+2

10 590+2 9.62 613±3 628+3

20 1172+88 19 .1 1227+10 1261+10

Most probable energy loss of muons of 10 GeV/c momentum . Leakage corrected, flat response . `) Leakage corrected, response as shown m fig. 4. a)

50 2830+_ 15 47 .0 3010±20 3101+20

0.359 23+11 -

a)

GeV ADC counts GeV ADC counts ADC counts

F. Fischer et al / Cu -scmtillator calorimeter module

momentum for electrons is essential for the determination of R.e. 7. Energy resolution for electrons The resolution has been studied at 2, 6, 10, 20 and 50 GeV. Fig. 7 shows examples of the pulse height distribution for 6, 10 and 50 GeV beams containing a mix-

51 7

b Z O

0

w r w Z w

Fig. 8. Energy resolution for electrons as a function of incident momentum, data corrected for beam momentum spread . lure of electrons, peons and muons. Table 2 lists the measured resolution for electrons. As both the observed and expected distributions are always slightly asymmetric due to rare large leakage fluctuations we define the rms error to be a = fwhm/2 .34. The measured resolution has to be also corrected for the beam momentum spread . The beam was equipped with a beam spectrometer but due to the analysis method used no correction of the individual particle momentum could be applied. The beam momentum spread was determined from separate runs and is tabulated in table 2. Fig. 8 shows the energy resolution after unfolding the beam momentum spread . Fitting the expression a/E = c/FE to the data yields a value of c = (8 .9 ± 0.3)% . The data are very close to the theoretical limit of 8 .33%/F [22] for the sampling fluctuations of an infinite sampling calorimeter . We observe reasonable agreement between the data and the resolution as calculated with SCHEME . We calculated the resolution for three different sets of cutoff parameters and extrapolated then to a cutoff at zero energy . Fitting also the Monte Carlo prediction with a/E = c/VE yields c = (9 .4 ± 0.3)% . The calculations predict also a slightly better resolution for the actual response (fig. 4) as compared with a flat response . This is consistent with the recent calculation that the enhancement of the signal is the last few cm can improve the resolution of a calorimeter that is not long enough to absorb the electromagnetic shower fully [23] . Because of lack of beam time and only one module we could not study edge effects or transverse nonuniformities across the tower cross section. 8. Response to hadrons Fig. 7. Examples of the pulse height spectra at 6, 10 and 50 GeV/c incident momentum . The sample at 10 GeV was collected with a beam momentum spread of 3% .

The module is 1.7X0 (Xo is the hadronic absorption length) long and about 1X o wide . This is much too small to obtain a useful measurement of the energy of hadrons. I . CALORIMETRY

51 8

F Fischer et al / Cu -scintillator calorimeter module Since more than 50% of the primary energy leaks out of the rear end, the expected leakage fluctuations are very large

and dominate completely over the sampling fluctuations . Fig. 9 shows an example of the energy distribution for 10 GeV peons (together with electrons) .

A clear peak around 43% of the electron energy is observed . When defining

the rms resolution as a = = 45%. The expected energy deposition in the scintillator by ionisation has fwhm/2 .34 we obtain a value of

also been calculated by SCHEME . Fig. 9c shows the spectrum for the Birks parameter kb = 0, i.e . no saturation for highly ionising particles. The predicted peak position coincides with the experimental one but the predicted average is somewhat lower than the data . Also the MC distribution is less peaked than the observed

one. Very likely the disagreement is caused by subtle differences in the true cross sections and the theoretical input into the HETC code. For example a slightly higher fraction of 7r ° production of a higher multiplicity in the secondary particles can alter the predicted

energy distribution for a short calorimeter but would not be noticeable in a long one. The MC calculation predicts the following energy loss balance. On average

5.43 GeV energy leaks out of the calorimeter for pions

of 10 GeV incident energy . About 700 MeV of the

tn z

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MONTE CARLO PREDICTION

_

C

remaining energy is lost

due to

nuclear binding or

creation of masses of heavy particles (7T, k, p .). 3.87 GeV are deposited in the module by ionisation . The partition of energy deposition in the active and passive material has been predicted to be 0.117 which is significantly higher than the value of 0.107 for electromagnetic showers. The difference is caused mainly by slow

a

neutrons which are dominantly stopped in the scintillator. Therefore about ; of the energy that went into

) co

Q

nuclear binding should be compensated by slow neu-

trons. We want to point out another effect that can

affect the resolution . The long shaping time constant of 3 is reduces somewhat saturation effects in the scintil-

w 4

lator by highly ionising particles, also some decay products from long-lived excited nuclei might be detected .

z

C

Nevertheless one has to be rather careful to extrapolate, without further tests, from the relatively good resolution

E/E inc -

0'

of the small tower to the performance of a large hadron calorimeter. We would like to point out that the pro-

gram SCHEME has been tested against the calorimeter of ref. [24] . Good agreement between the predictions

Fig. 9 (a) Pulse height distribution for pions of 10 GeV/c momentum together with pedestal, muon and electron signal . (b) Same distribution with expanded vertical scale. Due to the scale expansion the pedestal, muon and electron peaks are blurred. (c) Monte Carlo prediction of the energy loss by ionisation in the scintillator (Birks parameter k,=0) . The horizontal scale has been normalized to 10 GeV deposited electromagnetic energy . It should be noted that electrons of 10 GeV incident energy would peak at 0.964 on this scale due to the leakage losses . The response of fig. 4 is used

and data has been observed .

9. Auxiliary tests For subsequent tests a second module has been built. In addition the readout scheme has been modified . For

each WLS strip a separate hybrid

preamplifier and shaping amplifier was used . The preamplifiers had about the same noise performance as the commercial ones but

F Fischer et al. l Cu-scintillator calorimeter module

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Table 2 Energy resolution for electrons E-c

.b' °

~P/P beam a MC

2 7.5+0.5 1 .8+00.2 .2 7.3+0.5 0.5 5.8 0.9

6 3.6 _+ 0.3 0.9+0 0.1 .1 3.5 +_ 0.3 3.5 0.8

10 2.3+02 _ 0.6+00.1 .1 22+0.2 _ 2.8 0.9

the relatively simple shaping amplifiers, set to T = 1 p s, were substantially worse. The signals were recorded via Camac ADCs . The subsequent tests were carried out in the T7 testbeam at the CERN PS . The uniformity of the modules has been studied with a scan at half height across the two modules. The gap between the modules was aligned to better than 10 mrad with respect to the beam axis . Fig. 10 shows the pulse height variation as a function of the transverse position . A spread of ±1 .5% is observed in the region where side leakage is negligible . Nevertheless we observe the same but reduced pattern as in a scan with an UV light beam across individual scintillator plates, i.e . slight enhancements in the tower centre and close to the WLS. From the similarity of the optical and particle beam scan we conclude that there will be a 5% signal drop close to the 90' corners. We are confident that one can reduce the nonumformity for showers to = 1% by varying the reflectivity of the wrapping material . In another test the effect of particles passing the WLS or diode were studied. No signal (<< 200 photoelectrons) could be seen from particles passing either 3 400

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BEAM DISPLACEMENT - (cm)

Fig. 10 . Variation of the average energy deposition for a scan at half height across two modules. Incident energy was = 7 GeV.

GeV % % % %

or 15 mm of the WLS. When the the WLS was placed behind a preshowering material of 5 rl depth a signal of = 3300 photoelectrons has been observed . We conclude that the effect from particles passing the WLS bar can be in general completely neglected. Particles passing the depletion layer of the photodiode can produce a sizable signal . Fig. 11 shows the typical Landau distribution when the beam is sent directly through a separate diode. The most probable signal corresponds to 13 000 photoelectrons which in turn would simulate an additional energy deposition of = 0.6 GeV in our calorimeter configuration . The signal of 13 000 electrons is consistent with a diode depletion layer of 140 Ium as calculated from the diode capacitance of 75 pF . Noise reduction of photodiode readout is normally achieved with shaping amplifiers which limit high rate application to much less than the limit set by the diode risetime. In order to get some estimate of the time resolution some simple measurements have been carried out. The signals of the preamplifiers were added and shaped by a Canberra shaping amplifier or an EGG fast filter amplifier. The shaped signal was connected to either a simple discriminator consisting essentially of an AMD 685 fast comparator or a NIM discriminator. Various measurements were done with either leading edge discrimination on the unipolar shaped pulse or crossover point discrimination on the bipolar shaped pulse. Table 3 summarizes most of the results. As expected the time resolution improves generally with shorter shaping time constants although the energy resolution degrades . Fig. 12 shows the time spread for 7 GeV electrons and for all beam particles when trigger-

W

0

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DESTAL PARTICLES PASSING DIODE 33900 e (FROM C0 57 )

Z 0 0

MEMO

CHANNEL

256

Fig. 11 . Energy deposition of minimum ionising particles passing the diode depletion layer. 1. CALORIMETRY

520

F Fischer et al. / Cu -scintillator calorimeter module

Table 3 Measurement of the time resolution Shaping time

a (electrons) (ns)

Measurement with the Canberra shaping amplifier 2 Fi s 94 2 [is 44 1 is 23 0.5 gs 16 Measurements with EGG fast filter amplifier 7.6 T,nl = 200 ns Td,t = 500 ns 6.5 T,ni =100 ns Td,f = 200 ns

a (hadrons) (ns)

Comments

103 46 26

leading edge discrimination on unipolar pulse zero crossing discrimination on bipolar pulse zero crossing discrimination on bipolar pulse zero crossing discrimination on bipolar pulse

16 12

leading edge discrimination on unipolar pulse leading edge discrimination on unipolar pulse

ing on the zero crossing of the bipolar pulse set to T = 0.5 ,its . As most of the hadrons deposit only a fraction of their energy in the calorimeter the time resolution for the entire sample of beam particles is worse than for 7 GeV electrons. We extrapolate that with constant fraction discriminators and not too small energy deposition in the module one might achieve a time resolution close to 1 ns . Als already mentioned the conditions for best time resolution and energy resolution are somewhat contradictory . A possible (but expensive) solution for high

rate applications, where one wants to achieve maximum energy resolution, might be to record the structure of the shaped pulse (or even the preamplifier output signal) with a CCD analog storage of large dynamic range and perform a subsequent software pulse height analysis, taking pileup etc into account. Fig. 13 shows examples of a clean pulse and pileup recorded with a LeCroy Camac ICA model 2261 . Finally we tested the electron to hadron (e/h) separation capabilities of this type of calorimeter module . The electrons in the beam were vetoed by two beam

u N N r-

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ln

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TIME -

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TIME - (nsec)

Fig. 12. Distribution of the time spread of the photodiode readout . The timing signal was generated from the zerocrossing of the bipolar pulse with T = 0.5 us (a) for 7 GeV electrons, (b) for 7 GeV hadrons.

Fig. 13. Examples of shaped energy signals as recorded with a CCD transient recorder. Clock frequency was 10 MHz.

F Fischer et al. / Cu -scintillator calorimeter module

Cherenkov counters . About 0.6% of the hadrons of 7 GeV momentum generated a signal within ±2a of the electron peak . Part of the signal is caused by pileup and by a small fraction of electrons breaking through the Cherenkov counters . As we were unable to determine the exact amount of pileup and breakthrough we quote only a lower limit for the e/h separation of 130 :1 . Additionally there are three reasons for this rather poor separation : (i) the module is not subdivided in length ; (ii) the module has a rather large cross section in terms of radiation length ; (iii) copper is not the optimal material for good e/h separation due to its unfavorable ratio of radiation length to by nuclear absorption length, which is about a factor 3 worse than for lead . While studies for improvements related to (ii) and (iii) would require an entire new construction we were able to partially imitate the longitudinal subdivision by blinding one of the WLS bars over the last 20 cm . From the analysis of the data we determined an e/h separation of 280 : 1 . 10 . Discussion of the results From the above studies and tests we conclude that a large calorimeter of a modular tower structure with PD readout can be built. The measurements indicate that one could achieve: (1) a noise level well below 100 MeV per tower : (ii) calibration with muons; (iii) an energy resolution for electrons close to the theoretical limit; (iv) a good hadron energy resolution, if the extrapolation from the test calorimeter is valid; (v) a simple and compact construction ; (vi) a safe design, as no explosive liquids, cryogenics or high voltage are involved . Most of the conclusions are linked to the high conversion yield of = 23000 photoelectrons/GeV . Although this number is much higher than in most other calorimeters the measurements indicate that substantial improvements should still be possible. The essential measurements were done with a shaping time constant of 3 ps. This limits very much the rate capabilities of such a calorimeter. The shaping time constant r can in principle be shortened to a few ns but the noise will increase proportional to 1/V-,7 (in reality slower as the contribution from the leakage current to the noise will diminish). Also a factor of = 1 .3 can be regained by using four separate readout chains . This is in any case advisable for redundancy in case of failures, corrections of local inhomogeneities and the nuclear counter effect in the PDs (see discussion below) . The progress in fabrication of low capacitance PDs and

52 1

thinner WLS bars should allow one a shaping time of = 100 ns with the same noise level as the measurements presented here . In the future, PDs working on the drift principle should result in another substantial reduction in noise. For trigger purposes, where utmost resolution is not required, it should be possible to operate with 10-20 ns shaping time constants. For a large system with many towers the preamplifier signals can be summed by a linear adder and then discriminated . Noise suppression from nonhit towers can be achieved by biassed amplifiers . PDs act also nuclear counters . In a calorimeter with the previously described features a charged relativistic particle will create an additional signal corresponding to = 0.6 GeV when traversing the depletion layer. This is not distinguishable from a normal hadronic shower signal as the resolution is poor anyway . If the diode is not located close to the shower maximum the effect will also hardly be noticable in electromagnetic calorimeters . For a sample of 50 GeV electromagnetic showers randomly distributed across the tower, only in about 1% of all events will an electron pass a diode. Very rarely large signals can occur if a hadronic interaction takes place in the depleted layer of the PD . In principle most of the signals induced in the diodes by passing particles can be detected when reading the 4 WLS bars separately . Radiation damage is a severe problem for scintillation calorimeters in high particle flux environments . No tests of the radiation resistance of the newly made WLS bars have been performed, but in the recent development program for dyes for solar energy collectors much attention has been paid to high stability of dyes against prolonged and intense solar light [9]. For example solar collectors containing the dye #241 faded by only = 4%/year when exposed to sunlight. The fading even showed some recovery when the collectors were kept in the dark or at low light levels as in winter time . Hopefully the dyes used in this test will show a similar stability when exposed to particles. The following tests have still to be done : (i) long term stability studies; (ii) radiation damage of the WLS and PD ; (iii) study of hadrons in a longer calorimeter build from similar modules. Acknowledgement We would like to thank the L3 collaboration for their help and assistance in using the X3 test beam and J.P . Martin for providing the data on the beam momentum spread . References [1] G. Blanar et al ., Nucl Instr. and Meth . 203 (1982) 33 [2] D Groom, Nucl . Instr. and Meth . 219 (1984) 141 . 1. CALORIMETRY

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