Silicon drift detectors with an active area of 7.0 × 7.5 cm2 will equip the two middle layers of the Inner Tracking System of the ALICE experiment. The performance of several prototypes was studied during beam tests carried out at the CERN SPS facility. The results of the beam test data analysis are discussed in this paper

Bondila, M

Bonvicini, V

Cerello, P

Crescio, E

Giubellino, P

Kolojvari, A A

Martínez, M I

Mazza, G

Montaño-Zetina, L M

Nouais, D

Piano, S

Piemonte, C

Rashevsky, A

Rivetti, A

Tosello, F

Vacchi, A

Wheadon, R

CERN Document Server

PublicationALICE reference numberALICE-PUB-2000-29 version 1.0Institute reference numberDate of last change2001-03-14Authors:E. Crescio, M. Bondila, V. Bonvicini, P. Cerello, P. Giubellino, A. Kolojvari,M. I. Martinez, G. Mazza, L. M. Montano, D. Nouais,  S. Piano, C. Piemonte,A. Rashevsky, A. Rivetti, F. Tosello, A. Vacchi, R. Wheadonfor the ALICE CollaborationAbstract:Silicon drift detectors with an active area of  7.0 × 7.5 cm2 will equip the two middlelayers of the Inner Tracking System of the ALICE experiment.  The performance ofseveral prototypes was studied during beam tests carried out at the CERN SPS facility.The results of the beam test data analysis are discussed in this paper.Recent results from beam tests of large areasilicon drift detectorsRecent results from beam tests of large areasilicon drift detectorsE. Crescio a,1, M. Bondila b,4, V. Bonvicini c, P. Cerello a,P. Giubellino a, A. Kolojvari a,b,3, M. I. Martinez a,2, G. Mazza a,L.M. Montan˜o a,2, D. Nouais a, S. Piano c, C. Piemonte c,A. Rashevsky c, A. Rivetti a, F. Tosello a, A. Vacchi c,R. Wheadon a for the ALICE CollaborationaTorino University and INFN-Sez.Torino, ItalybJyva¨skyla¨ University, FinlandcINFN Sezione di Trieste, ItalyAbstractSilicon drift detectors with an active area of 7.0 × 7.5 cm2 will equip the two middlelayers of the Inner Tracking System of the ALICE experiment. The performance ofseveral prototypes was studied during beam tests carried out at the CERN SPSfacility. The results of the beam test data analysis are discussed in this paper.1 IntroductionSilicon Drift Detectors (SDDs) [1–3] are particle detectors well suited for highmultiplicity experiments, since they can provide unambiguous two-dimensionalposition information and dE/dx measurement with a reduced number of read-out channels. In a SDD, under the eﬀect of an applied electric ﬁeld, the elec-trons generated by the crossing particle drift along a direction parallel to thesurface of the wafer to an array of anodes perpendicular to the drift direction.The coordinate along the drift direction is determined by the measurement ofthe drift time, the second coordinate is obtained from the charge distribution1 INFN, Via P. Giuria 1, 10125 Torino, Italy – E-mail: crescio@to.infn.it2 also at CINVESTAV Mexico City, Mexico3 also at St.Petersburg University, St. Petersburg, Russia4 on leave from Institute of Spatial Sciences, Bucharest, RomaniaPreprint submitted to Elsevier Science 14 March 2001across the anodes.SDDs have been chosen to equip the two middle layers of the Inner TrackingSystem (ITS) of the ALICE experiment at the LHC [4]. The detectors will beproduced on 3kΩ cm 5” n-type neutron transmutation doped silicon wafers,with an active area of 7.0 × 7.5 cm2. The active region of each detector isdivided centrally in two separate zones each with a 35 mm maximum driftpath read out at the edge by 256 anodes with a pitch of 294 µm.The results discussed in this paper were obtained from prototypes with char-acteristics close to those from the ﬁnal design for ALICE [5], read out by theOLA preampliﬁer [6] and a Flash ADC system [7]. A brief description of theSDD front-end electronics designed for ALICE, which will be used for the ﬁrsttime in beam tests during this year, is also given.2 Beam test resultsThe SDDs were exposed to the 375 GeV/c pion beam of the CERN SPS.They were placed on the beam trajectory between two microstrip telescopeswhich allow the measurement of the particle impact point on the SDD with aprecision of 5 µm r.m.s..Particular attention was devoted to the analysis of the detector spatial reso-lution, deﬁned as the r.m.s. of the distribution of the residuals between thecoordinates measured by the SDD and by the microstrip telescope.In order to make this analysis, it is necessary to take into account the eﬀectswhich lead to systematic errors in the determination of the coordinates alongboth directions, thus aﬀecting the resolution.For the resolution along the time direction, it is necessary to correct for thedrift velocity variations due to temperature instability during time and to thetemperature gradient present in the detector, caused by the integrated resis-tive voltage divider heating.The position measurement is also aﬀected by the presence of ﬂuctuations inthe doping concentration within the wafer [8]. These ﬂuctuations cause a non-uniformity of the drift ﬁeld, which alters the proportionality between the drifttime and the distance of the crossing point of the particle from the anodes,and a parasitic electric ﬁeld component along the anode direction, which in-duces deviations of the electron trajectories from the ideal linear path. In ﬁg. 1the values of the resolution along the drift direction as a function of the driftdistance, before correction for the doping ﬂuctuations, are shown. The testeddetector presented a systematic non-linearity along the drift time axis, due tothe punch-through current between cathodes. The systematic non-linearitieswere corrected with a polynomial ﬁt. The black circles represent the resultobtained studying a large area (40 anodes) of the detector. These values canbe considered an upper limit of the drift time resolution. This preliminary20510152025303540450 5 10 15 20 25 30 35Drift distance mmResolution µmFig. 1. Resolution along the drift direction as a function of the drift distance. Theblack circles represent the resolution obtained studying an area corresponding to 40anodes, the light circles represent the values for an area corresponding to 3 anodes.result was obtained taking into account the temperature eﬀects but assuminga uniformly doped wafer. As shown in the ﬁgure (light circles), a selectionof any small region (1 mm) along the anode direction, in which the dopingconcentration can be considered independent of the anodic coordinate, allowsthe improvement the resolution. This suggests that a global improvement canbe obtained after the correction for the doping ﬂuctuations.In ﬁg. 2 the curve with triangles shows the anodic resolution corrected forthe doping ﬂuctuations. In order to optimize the anodic resolution, it is alsopossible to correct the anodic coordinate (obtained from the centroid of thecharge distribution across the anodes) by using a technique based on the goodknowledge of the electron cloud evolution during the drift [9].Usually, the anodic coordinate is determined by the center of gravity (xCoG)of the charge measured by the anodes. This value can be signiﬁcantly diﬀer-ent from the centroid of the electron cloud gaussian distribution. A correc-tion function ∆x(xCoG, σ) can be applied, where σ represents the r.m.s. ofthe gaussian distribution and depends on the drift time and on the releasedcharge. Neglecting in ﬁrst approximation the Coulomb repulsion eﬀect duringthe drift, the time dependence of the cluster size is σ(t) =√2Dt+ σ20 , whereD is the diﬀusion coeﬃcient and σ0 represents the initial size of the cloud.The black curve in ﬁg. 2 illustrates the improvement obtained for the anodicresolution by applying such a CoG correction. The correction is applied onlyfor the two-anode clusters. A global improvement of the resolution can be ob-served. The degradation in the region close to the anodes is due to the intrinsicposition uncertainty of the single-anode clusters.3Fig. 2. Anodic resolution already corrected for the doping fluctuations along theanode direction as a function of the drift distance before and after the centroidcorrection.3 The ALICE front-end readout unitA schematic representation of the front-end readout unit designed for ALICEis shown in ﬁg. 3. It is based on two ASICs, implemented with a 0.25 µmFig. 3. The front-end readout unit for the ALICE silicon drift detectors.CMOS radiation tolerant technology. The ﬁrst one, PASCAL, performs lownoise preampliﬁcation, analog storage and analog to digital conversion . Thesecond one, AMBRA, is a multi-event digital buﬀer. Since the SDDs are verysensitive to temperature variation, the average power consumption should bekept below 5mW/anode. The required sampling frequency, in order to improvethe separation of close tracks, is 40 MHz. At this speed the design of an ADCwith this low power consumption is very diﬃcult. The adopted solution is tostore continuously the data coming from the detector in a ring analog mem-ory. The data are digitized and transferred to the DAQ only when the triggersystem validates them. In this way the most power demanding componentswork for a reduced percentage of the time strongly reducing the power con-4sumption. Prototypes of both ASICs have been produced: test results showthat the circuits fulﬁl the ALICE requirements [10,11]. During the next beamtests, a 32-channel prototype of PASCAL will be used.L. M. M and M. I. M wish to thank Conacyt for ﬁnancial support.References[1] E. Gatti and P. Rehak, Nucl. Instr. and Meth. A225 (1984) 608-614.[2] S. Beole´ et al., Nucl. Instr. and Meth. A377 (1996) 393-396.[3] S. Beole´ et al., Il Nuovo Cimento Vol. 109A, N. 9 (1996).[4] ALICE Collaboration, CERN/LHCC 99-12,http://alice.web.cern.ch/Alice/documents.html.[5] A. Rashevsky et al., proceedings at 8th Pisa Meeting on advanced detector, LaBiodola, Isola d’Elba, Italy, 21-27 May 2000.[6] W. Dabrowski et al., Nucl. Phys. B 44 (1995) 637.[7] F. Balestra et al., Nucl. Instr. and Meth. A323 (1992) 523.[8] D. Nouais et al., proceedings at 8th Pisa Meeting on advanced detector, LaBiodola, Isola d’Elba, Italy, 21-27 May 2000.[9] E. Crescio, D. Nouais and P. Cerello, ALICE-INT-2001-09,http://alice.web.cern.ch/Alice/documents.html.[10] A. Rivetti et al., accepted for publication in the TNS.[11] G. Mazza et al., proceedings at LEB 2000, CERN 2000-010,CERN/LHCC/2000-041.5

Recent results from beam tests of large area silicon drift detectors

Silicon drift detectors with an active area of 7.0x7.5 cm2 will equip the two middle layers ofthe Inner Tracking
System ofthe ALICE experiment. The performance of several prototypes was studied during beam tests carried out at
the CERN SPS facility. The results of the beam test data analysis are discussed in this paper

Recent results from beam tests of large area silicon drift detectors

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