A silicon pad shower maximum detector for a shashlik calorimeter

UCLEAR PHYSIC.c

EI.SEVIER

Nuclear Physics B (Proc. Suppl.) 44 (1995) 74-78

A Silicon Pad Shower Maximum Calorimeter

PROCEEDINGS SUPPLEMENTS

D e t e c t o r for a S h a s h l i k

Presented by V.A. Cassio, Univ. a n d Sezione INFN, Torino

S.J. Alvsvaag, O.A. Maeland, A. Klovning [~1 A.C. Benvenuti, V. Giordano, M. Guerzoni, F.L. Navarria, M.G. Verardi [bl T. Camporesi, E. Vallazza [~1 M. Bozzo, R. Cereseto [dl G. Barreira, M.C. Espirito Santo, A. Maio, A. Onofre, L. Peralta, M. Pimenta, B. Tome [~1 H. Carling, E. Falk, V. Hedberg, G. Jarlskog, I. Kronkvist UI M. Bonesini, F. Chignoli, P. Ferrari, , S. Gumenyuk, R. Leoni, R. Mazza, P. Negri, M. Paganoni,L. Petrovykh, F. Terranova M D.R. Dharmasiri, B. Nossum, A. L. Read, B. Skaali [hi L. Castellani, M.Pegoraro F] A. Fenyuk. I. Gouz, I. Ivaniouchenkov, A. Karioukhine, V. Obraztsov, E. Vlasov, A. Zaitsev U] M. Bigi, V. Cassio, D. Gamba, E. Migliore, A. Romero, L. Simonetti, E. Torassa, P.P. Trapani [kl M. Bari, G. Della Ricca, L. Lanceri. P. Poropat. M. Prest [q Bergen %Bologna b-CERN %Genova d-Lisbon ~- Lurid f-Milano a-Oslo h-Padova i-Serpukov J- Torino k-Trieste The new luminosity monitor of the DELPHI detector, STIC (Small angle Tile Calorimeter), was built using a Shashlik technique. This technique does not provide longitudinal sampling of the showers, which limits the measurement of the direction of the incident particles and the e - r~ separation. For these reasons STIC was equipped with a Silicon Pad Shower Maximum Detector (SPSMD). In order to match the silicon detectors to the Shashlik read out by wavelength shifter (WLS) fibers, the silicon wafers had to be drilled with a precision better than 10 pm without damaging the active area of the detectors. This paper describes the SPSMD with emphasis on the fabrication techniques and on the components used. Some preliminary results of the detector performance fiom data taken with a 45 GeV electron beam at CERN are presented.

1. I n t r o d u c t i o n During the 1993-94 s h u t d o w n a new luminosity monitor, S T I C (Small angle Tile Calorimeter)[1][2], was installed in the D E L P H I detector at LEP. It consists of two cylindrical sampling calorimeters m a d e of 47 lead-scintillator sandwiches. B o t h the lead and the scintillator layers have a thickness of 3 m m for a total depth of about 27 radiation lengths. Each scintillator plane consists of tiles, m a d e of injection molded polystyrene d o p e d with P T P and P O P O P , arranged in 8 azimuthal sectors 2 2 . 5 ° wide and 10 radial sectors per half cylinder module. The scintillators tiles are optically isolated from each other by 120 p m thick Tyveck diffuser paper, and they are m o u n t e d on the lead plates with alignement pins, with a mechanical accuracy which is better t h a n 50 tim. T h e lead plates are continuous to achieve a good spatial uniformity in the energy response and a hermetic detector. 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(95)00511-0

The light readout is done by W L S fibers running perpendicularly to the sampling planes. The two calorimeters are located symmetrically on the b e a m axis at 4-2.2 m from the interaction point and they cover the angular region 29 m r a d < (9 <180 m r a d (where (9 is the angle with respect to the b e a m line). T h e tower structure defined by the scintillators tiles points to the interaction region. There are two reasons why the collaboration decided to insert a S P S M D in the sampling structure of the calorimeter in view of the LEP200 running at energies above the Z peak: -

the rejection of background by requiring a 10 m r a d accuracy in the determination of the 7 direction in the e+e - --+ ? + Z events;

- the improvement in e-rr separation which is limited in Shashlik calorimeters due to the absence of longitudinal samplings.

S.J Alosvaag et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 74 78

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2. Mechanical design The SPSMD[3] consists of two planes of silicon detectors inserted in the sampling structure at a depth of 4 and 7.4 radiation lengths and covering the angular region 29 m r a d < O <80 mrad. A SPSMD half plane is shown in figure 1. Each plane consist of a 3.5 m m thick aluminum support on which about 1 m m thick ceramic tiles are fixed by 2 screws and aligned by means of 3 dowels. Silicon detectors are glued with a conductive glue onto a metallized area deposited on the ceramic tiles in order to apply the bias voltage to the backplane of the diodes. The most critical mechanical constraint is due to the holes which have to be drilled in the silicon wafers, without damaging the active area of the detectors. Three different techniques have been tried: laser cutting, ultrasonic grinding and chemical etching and all gave satisfactory mechanical results. The laser drilling technique 1 was chosen since it gave the best quality to price ratio. Each 45°sector is made out of 3 silicon wafers (see figure 2) with the inner one covering the radial region from 7.15 cm up to 11.3 cln. This wafer is split into two separate 22.5°sector with 24 strips each. The other two wafers each covers a 22. 5 ° sector and they extend out to 18.03 cm in radius and consist of 36 strips each. The pitch of the strip is about 1.7 m m and it was determined from a Monte Carlo study[4] in order to optimize the spatial resolution. The holes are preferentially positioned between two neighbouring strips and only two out of six strips include holes. 3. T h e S i l i c o n d e t e c t o r s The silicon detectors are single-sided ACcoupled FOXFET[6] devices fabricated using planar techniques, on 300/ml thick high resistivity 7~ type silicon bulk. The p+ strips were implanted through the SiO_~ layer using a high-energy boron ion beam. An arsenic implant provided the detector back n + ohmic connection. The readout strips, the b i a s and guard ring connections, the backplane and the F O X F E T gate were produced l MICRON Semiconductor Ltd., UI<..

Figure 1. A SPSMD half-plane. The region covered by the silicon detectors is shaded in black. The outline describes the K a p t o n circuit used to carry the signal to the outer edge of the detectors (about 35 am from the diodes). through metallization with aluminum. The detectors are covered with a polymide protective layer. An unbiased guard ring surrounds every hole. A cross section of the structure surrounding the holes is shown in figure 3. The depletion characteristics and the leakage currents were measured for strips with and without holes, to evaluate possible degradation of the performance of the devices, caused by the laser drilling. In figure 4 (a) the depletion characteristic is shown for two neighboring strips, one with and one without holes. The small difference is due to geometrical effects. In figure 4 (b) the full depletion voltage for the strips of one detector can be seen. The strips with holes do not shown breakdown or anomalous values. The full depletion voltage of the detectors was found to be vary between 15 V and 40 V. Typical leakage currents of less than 1 nA were measured. In order to match the characteristics of the charge

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S.J. Alvsvaag et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 74 78

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amplifier chip, the detectors are operated with a gate-drain voltage o f - 5 V and a drain voltage of 5 V. Taking into account the operational voltages of the FOXFET, the detectors are operated with bias voltages of 30 V or 50 V depending on their full depletion voltage. 4. T h e e l e c t r o n i c s To allow access to the front-end electronics the preamplifiers are installed on the external surface of the STIC and the signals from the detectors are carried out a distance of about 35 em (see

fig. 1) by fiat kapton cables. They are made of 50 p m substrate of kapton material on which 35 # m thick copper tracks are deposited. Tracks are then covered with a 50 #m layer of protective coating. On the detector side the flat kapton cables are fixed on the silicon with a non conductive glue and the electrical continuity between the strips and the copper tracks is obtained by means of two 33 # m aluminum bonding wires per pad. The bonding wires are protected by an epoxy coating. On the other and of the flat kapton cables a connector with 500 pm pitch is mounted. The signals from the strips of a 450 sector of a silicon plane (corresponding to 120 channels) are amplified by an MX4 Microplex chip[7,8] which consists of a 128 channels charge amplifier array with multiplexed output 2. Each channel of the MX4 includes a charge-sensitive preamplifier followed by two storage capacitors and which are used to sample and hold the signals. The first sampling is done just before a LEP beam crossing and the second one after the charge collection has taken place. The signals are stored and subsequently read out under the control of the 128channel shift register. To match the 44 p m pitch of the MX4 inputs with the 500 #m pitch of the flat kapton cables a ceramic board was designed, manufactured and 2manufactured by Rutherford Appleton Laboratories

(UK)

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S.J. Alvsuaag et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 74-78

tested at CER.N. A 1 p m thick aluminum layer was evaporated under vacuum on a ceramic substrate and the circuit was printed by chemical etching. The ceramic board was cut to final dimension by laser. MX4 chips were glued on the ceramic board and the bonding between the inputs of the chips and the aluminum tracks was carried out using a ultrasonic wedge-bounding machine. The connectors of 500 Inn pitch were soldered onto the boards. Due to the difficulties of soldering directly on aluminum, a layer of eromium and then a layer of gold (both less then 1 # m thick) were deposited on the aluminum pad. Because of the weak adhesion of the gold layer to the substrate, 10-15% of the connections came off. Some alterna.tives to this techinque are presently under study in order to provide a more reliable soldering. For each MX4 a so called "slave card" further amplifies the output of the chip. General distribution, reshaping of control signals and multiplexing of 8 slave-card outputs are accomplished by the so called "master card". Only standard techniques and components are used in these cards. Control signals for the MX4 chips are generated by a special unit located in the control room, where the multiplexed analogue signals from the detector are processed by two FASTBUS SIROCCO modules[9].

ii) evaluation of the coherent noise from the

neighbouring strips, readout by the same preamplifier; iii) subtraction of the c o m m o n noise using the

correlation coefficients calculated in the pedestal runs. After the common noise subtraction a signal to noise ratio of about 40 was obtained for a 45 GeV electromagnetic shower in the strip with the maxi m u m energy deposition. The radial position of the incoming particle was estimated, in both silicon planes, using the center of gravity method applied to the five strips around the shower m a x i m u m , without correcting for possible variation in the gain of different strips. In figure 5 the transverse shower profile is 1

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A spare module of the STIC detector with the same read out chain as used m D E L P H I was exposed to a 45 GeV electron beam in the CERN West Area in July 1994. Pedestal runs were taken to monitor the noise in a systematic way. Both data and pedestal runs showed the presence of a coherent noise source which produces a conlnlon shift of the baseline for the strips read out by the salne MX4 chip. C o m m o n noise was subtracted offline on an event by event basis by adopting the following procedure: i) determination of the region with the maxi-

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depicted for the two silicon planes and compared with the prediction of a simulation based on the G E A N T code[5]. The agreement between tile two distributions is good. An estimate of the angular resolution of the shower axis was done by correlating the position

XJ Alvsvaag et al./Nuclear PhysicsB (Proc. Suppl.) 44 (1995) 74-78

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We gladly acknowledge the help of A. Bream (CERN, PPE division) and from C. Millerin (CERN, ECP-SMD section) for the the fanin cards production. We also gladly acknowledge the help from A. Gandi and the PC workshop team at. CERN for the flat kapton cables production and O. Runolfsson for his support for the MX4 bonding.

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REFERENCES

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of the m a x i m u m of the shower in the two planes. In figure 6 the distribution of the difference between the radial position reconstructed by the two silicon planes in shown (taken into account the projectivity of the detector structure). The FWHM of the distribution is 630 p m which corresponds to an angular resolution of 13 mrad. The non gaussian tails of the distribution are probably due to systematics effects e.g. the biases caused by the centre of gravity estimator, the presence of the holes in the strips and inaccuracies in the calibration. The angular resolution obtained in this way compares well with the goal of a 10 mrad resolution as stated in the proposal[10].

6. C o n c l u s i o n s A SPSMD matching a Shashlik calorimeter was built by the STIC collaboration. It provides a measurement of the direction of the incoming particle with an accuracy of about 13 mrad. DELPHI may thus, in the LEP200 phase., reject backgrounds from beam halo and correctly identities events with only one photon in the final state.

1. G.Atojan et al.,"Lead-Scintdlator Electromagnetic Calorimeter with 14"L5,'Fiber readour', Nucl. Instr. Meth. A320 (1992), 144. 2. S.J.Alvsvaag et al.,"The DELPHI Small Angle Tile CalorimeteF', these Proceedings. 3. S.J.Alvsvaag et al.," The Silicon. 5'bower Ma.rimum Detector for the STIC', Contibution to the "6th Pisa Meeeting on Advanced Detectors". Elba,Italy, May 22-28, 1994. 4. G.Della Ricca, Diploma Thesis, University of Trieste, 1993. 5. CERN Program Library Long Writeup W5013. 6. P.P.Allport et al.."FOXFET biased microstrip detectors", Nucl. Instr. Meth. A310 (1991), 1,5,5. 7. J.Santon et. N.Kurtz, "An introduction to the MX Chip", RAL-89-028. 8. J.T. Walker et al., "Development of High Densitg Readout for Silicon Strip Detectors". Nucl. Instr. Meth. A226 (1990), 243. 9. M.Burns et al., "Progress in the Co~,structiot~ of the DELPHI Mwrovertex Detecto/', Nucl. Instr. Meth. A277 (1989), 154. 10. The DELPHI Collaboration, "'Proposal for the replacement of the Small Angle Calorimeter of DELPHr'. CERN-LEPC/92-6