The project of the two Silicon Drift Detector layers of the ALICE Inner Tracking System is reviewed. Recent results obtained from beam tests are presented

Beolè, S

Bondila, M

Bonvicini, V

Cerello, P G

Crescio, E

Giubellino, P

Idzik, M

Kolojvari, A A

Kouchpil, S

López-Torres, E

Martínez, M I

Mazza, G

Nouais, D

Piano, S

Piemonte, C

Rashevsky, A

Riccati, L

Rivetti, A

Tosello, F

Trzaska, W H

Vacchi, A

Wheadon, R

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsThe Alice Silicon Drift Detector SystemPublicationALICE reference numberALICE-PUB-2001-59 version 1.0Institute reference numberDate of last change2001-12-20Authors:D. Nouais, S. Beolè, M. Bondila, V. Bonvicini, P. Cerello, E. Crescio,P. Giubellino, M. Idzik, A. Kolozhvari, S. Kouchpil, E. Lopez Torres,M.I. Martinez, G. Mazza, S. Piano, C. Piemonte, A. Rashevsky, L. Riccati,A. Rivetti, F. Tosello, W.H. Trzaska, A. Vacchi, R. Wheadonfor the ALICE collaborationThe Alice Silicon Drift Detector SystemD. Nouaisa;1, S. Beolea, M. Bondilab;2, V. Bonvicinic,P. Cerelloa, E. Crescioa, P. Giubellinoa, M. Idzika,A. Kolozhvaria;b, S. Kouchpild, E. Lopez Torresa,M.I. Martineza;e, G. Mazzaa, S. Pianoc, C. Piemontec,A. Rashevskyc, L. Riccatia, A. Rivettia, F. Toselloa,W.H.Trzaskab, A. Vacchic, R. Wheadona,for the ALICE collaboration.aINFN Sezione di Torino, ItalybJyvaskyla University, FinlandcINFN Sezione di Trieste, ItalydRez, Czeck RepubliceCINVESTAV Mexico City, MexicoAbstractThe project of the two Silicon Drift Detector layers of the ALICE Inner TrackingSystem is reviewed. Recent results obtained from beam tests are presented.1 IntroductionThe central detector of ALICE [1,2] will consist of an Inner Tracking Sys-tem (ITS) [3,4] whose basic functions are the secondary vertex reconstructionof hyperon and charm decays, the particle identication and the tracking oflow-momentum particles, and the improvement of the overall momentum res-olution. The system, which covers the polar angular range 90  45 deg overthe full azimuth, is embedded in a large solenoidal magnet generating a eldbetween 0:2T e 0:5T. Because of the high particle density, the innermost fourlayers need to be truly two-dimensional devices. To give the inner tracking sys-tem a stand-alone particle identication capability, a minimum of four layers1Corresponding author. E-mail: nouais@to.infn.it2also at Institute of Space Sciences, BucharestPreprint submitted to Elsevier Preprint 13 December 2001need an analogue readout with a wide enough dynamic range to provide un-saturated dE=dx measurements down to the minimum momentum for whichthe tracks have a reasonable (> 20%) reconstruction probability. To achievethis performance, the ITS will consist of six barrels of high-resolution silicontracking detectors: two layers of silicon pixel detectors (SPD's) with digital readout, two layers of silicon drift detectors (SDD's)with analogue readout, two layers of double-sided silicon micro-strip detectors (SSD's) with ana-logue readout.Silicon drift detectors have been chosen to equip the third and the fourth layersof the ITS, due to their characteristics: very high-resolution two dimensionalsensors suitable for high track-density experiments with low rate, because oftheir relatively slow readout (a few microseconds). In an SDD, the electronsreleased during the passage of a particle drift under the eect of an appliedelectric eld along a direction parallel to the surface of the wafer towards anarray of collecting anodes. The drift velocity is proportional to the electriceld E and to the electron mobility e: v = eE. In this way, the distance ofthe crossing point from the anodes is determined by the measurement of thedrift time. The second coordinate is obtained from the centroid of the chargedistribution across the anodes. As a consequence of this principle the numberof front-end channels necessary to read out large area detectors is very smallcompared to other detectors with comparable spatial resolution. The reducednumber of electronic channels allows the front-end to provide the energy lossmeasurement without increasing too much the power budget.In Section 2 the ALICE SDD system will be described, including the detectordesign and an overview of the readout architecture. In Section 3 beam testresults for the spatial resolution of a nal detector prototype will be discussed.2 The Silicon Drift Detector systemThe SDD's have to provide a spatial precision of about 30m, and a two-trackseparation down to O(600)m. In addition the charge resolution should allowa dE=dx resolution dominated by Landau uctuations.2.1 The detectorThe ALICE SDD nal prototypes[5] were produced by Canberra Semiconduc-tor on 300m thick 5" Neutron Transmutation Doped (NTD) wafers with a2resistivity of 3 kcm (Fig. 1). Their active area is 7:02 7:53 cm2and the ra-tio to the total detector area (7:25 8:76 cm2) is 0.85. The active area is splitinto two adjacent 35 mm long drift regions, each equipped with 256 collectinganodes (294m pitch) and with integrated voltage dividers for the drift andthe guard regions. A careful design of the cathode strips prevents any punch-through which would deteriorate the voltage divider linearity. Since the driftvelocity is very sensitive to the detector temperature variation (v / T 2:4),the monitoring of this quantity is performed by means of three rows of 33implanted point-like MOS charge injectors[6,7].2.2 The SDD barrelThe SDD's are mounted on linear supports, called ladders, manufactured us-ing Carbon-Fibre Reinforced Plastic (CFRP) to minimize the material budget.The inner SDD layer is composed of 14 ladders, each holding six detectors. Ithas on average a radius of 14:9 cm and a 595mm length. The outer layer ismade of 22 ladders supporting eight detectors each. It has a radius of 23:8 cmand a 670mm length. 260 SDD's are needed to cover the two layers. In ev-ery ladder the detectors are mounted at dierent distances from the supportstructure and the ladders are assembled at dierent radii from the beam axisin order to allow small overlaps of the detector active areas. This is neces-sary to ensure the full coverage for all possible vertex positions and all trackswithin the acceptance. The SDD ladders are assembled onto a CFRP mechan-ical support made up of two end-cap cones connected by a thin cylinder. Theorientation of the SDD's anodes along the ladders, chosen to make the assem-bly procedure simpler, makes the drift velocity orthogonal to the beam axisand therefore to the magnetic eld. The Lorenz force acts perpendicularly tothe detector plane and is compensated by the focusing electric drift eld.2.3 The SDD readoutThe readout and data transmission of the whole SDD barrel is subdivided inhalf ladders. The total readout time of the SDD should be lower than 1ms.A block diagram of the readout of a half ladder of the third layer is shownin Fig. 2. A 256{channel front-end hybrid circuit, connected to each half de-tector, performs the preamplication and the analogue to digital conversionof the SDD signal, and transmit the data to the end ladder. The power con-sumption of the front end electronics has to be as low as possible due to thehigh sensitivity of the detector to temperature variations and to the limitedmaterial for cooling that can be located on the ladder structure. The SDDfront-end electronics is based on two 64 channel ASICs named PASCAL[8]3and AMBRA[9]. Four pairs of chips per hybrid are needed in order to readout a half SDD.PASCAL provides both the preamplication and the digitization. It has adynamic range of 32 fC which is necessary to meet the requirements for lowmomentum particle identication. To achieve the expected spatial resolutionof the detector, the preamplier has been designed to have an equivalent noisecharge of 250 e and the A/D converter to have 10-bit precision. The clockfrequency of LHC, 40 MHz, has been chosen as the sampling frequency forthe SDD signal. The preamplier peaking time (40 ns) was chosen as shortas possible so as to have a satisfactory double track resolution, within thelimit of ensuring a good sampling of the signal for all drift times. Since thepower consumption of an A/D converter operated at 40Msample=s would beunacceptable, PASCAL contains an intermediate, 256 channel, analogue ringmemory for each channel which samples the output of the preamplier every25 ns. When a trigger is received the contents of the analogue memories arefrozen and then read out by the ADC's (one ADC per two channels) with aconversion rate of 2Msample=s, for a total readout time of about 250s.The second ASIC, AMBRA, is a multi-event buer which is necessary in orderto reduce the needed bandwidth for the data transmission without increasingthe average detector dead time. The data coming from PASCAL are writtenin one of the four digital buers of AMBRA which derandomizes the events.AMBRA also implements a 10-bit to 8-bit data compression using a schemechosen to keep the percentage quantization error almost constant.The data are transferred at 40 MHz from the four AMBRA's of each half-detector to the end{ladder modules. A third ASIC (CARLOS) performs thezero-suppression and the compression of the data [10] before their transmissionto the DAQ system. CARLOS also takes care of the interface with the triggersystem via the TTCrx chip.The three ASICs, PASCAL, AMBRA and CARLOS, are implemented in acommercial 0:25m CMOS technology, using radiation-tolerant layout tech-niques developed by the CERN RD49 collaboration[11].2.4 Assembly of the SDD modulesThe main building element of the SDD subsystem (module) is based on a sin-gle drift detector connected to two hybrid circuits. The front-end hybrid car-ries four AMBRA{PASCAL pairs to equip the 256 anodes of a half{detector.Each pair PASCAL{AMBRA is rst assembled with an aluminum{Upilex mi-crocable [12] which is then TAB bonded to the hybrid circuit layer and to thedetector anodes. Finally, each hybrid circuit is glued with a thermo-conductive4compound on a 100 micron carbon bre heat bridge substrate which is thenclipped to the cooling tubes mounted on the sides of the ladder frames.The interface cards between the modules and the cables to the ITS patchpanels carrying signals and power are placed in the end-ladder region. Onelow voltage (LV) card per hybrid provide individual power supply regulation,signal interfacing to the data reduction electronics and slow control circuitry.The signal and power connection between the LV cards and the hybrids is pro-vided by Aluminum{Upilex low thickness microcables (20m Upilex + 30mAl). One high voltage (HV) card per SDD carries the lter capacitors andDC-blocking capacitors for the injector pulses. Development of microcablescapable of carrying the high voltage (up to 2.4 kV) from the HV cards to theSDD is in progress.3 Beam test results3.1 Experimental detailsDue to the complexity of the PASCAL ASIC, the rst-full chain prototypeha only recently been produced. Previous SDD tests were, instead, performedwith the OLA[13] preamplier. OLA is a 32-channels, low-noise, bipolar VLSIcircuit specically designed for SDD's. Each OLA channel features a charge-sensitive preamplier, a semi-Gaussian shaping amplier and a symmetricalline driver. The peaking time is 55 ns for a Æ-like input signal and the Equiv-alent Noise Charge (ENC) was measured to be  230 e at zero detector ca-pacitance. The preamplier was followed by a voltage amplier driving a 30mlong twisted-pair cable connected to a ash-ADC system[14] which sampledthe signal at 40 MHz. The ADC system had 8-bit resolution and a non-lineartransfer function expanding the dynamic range to 10 bits.Since 1997, several tests have been performed using CERN-PS and -SpS highmomentum (approximately minimum ionizing) particle beams. The typicalexperimental setup was built in the following way: one or more (up to three)SDD prototypes were placed on the beam line within a microstrip telescopeused to measure the impact point of the particle tracks on the SDD planes.The telescope was made up of ve pairs of single sided silicon strip detectorsalternating horizontal and vertical planes. Each plane had an area of 20 19:2mm and a strip pitch of 50m. Two pairs (upstream and downstream)of 2 cm{wide crossed scintillator were used to select only the particles passingthrough the telescope. Since the area covered by the microstrip detectors wassmaller than the SDD sensitive area, the SDD's were mounted on a mobilesupport whose position was remotely controlled and measured with a precision5of about 30m.3.2 Dopant inhomogeneity and spatial resolutionEven if the ALICE SDD's are produced on NTD wafers, which are expectedto have a particularly uniform dopant concentration, characteristic eects ofinhomogeneity, large enough to deteriorate signicantly the spatial resolutionof the detectors, have already been observed[15]. Inhomogeneity of the dopantconcentration alters the uniformity of the drift eld superimposing a localparasitic eld on the ideally constant one in the middle plane of the detec-tor. The component of the parasitic eld parallel to the drift direction locallychange the drift velocity and so causes systematic errors in the measurementof the coordinate along the drift axis. The component of the parasitic eldperpendicular to the drift direction induces deviations of the electron trajec-tories from the ideal linear path leading to systematic errors on the anodiccoordinate. During a beam test performed on an ALICE SDD prototype of thenal design, half a detector (7:5 3:5 cm2) was exposed to the beam, in orderto evaluate its performance. The local systematic deviations can be evaluatedby calculating within small areas the mean value of the residuals between thecluster centroid with respect to the impact point of the particle as measuredby the microstrip telescope. Fig. 3 shows in gray scale the systematic devia-tions X of the anodic coordinate (top) and Y along the drift coordinate(middle) assuming a constant drift velocity calculated for all the active area ofhalf an SDD. The bottom plot shows as an example the systematic deviationscalculated for both axes at X = 20mm as a function of the drift distance. De-viation of a few tens of m in average and with maximum values up to 400mare observed and must be corrected in order to reach the spatial resolution goalof about 30m. The circular structures centered on the middle of the wafer,clearly observed in this plot, can be attributed to the characteristic radial de-pendence of the dopant concentration uctuations. In addition to these sourceof electrostatic eld, vertical structures are also seen. They are produced byanomalous electronics channel with unstable gain and/or long shaping time.The horizontal step in the middle of the second plot is caused by a short cir-cuit between two cathodes which locally reduce the drift eld. Fig. 4 showsthe resolution along the anode and the drift-time directions obtained afterthe correction of these systematic deviations. The slight increase, from 30mto 35m, of the resolution along the drift direction can be attributed to thecharge diusion, which make the size of the charge cluster wider at larger drifttimes reducing in the same time the signal amplitude. The resolution alongthe anode direction has values better than 30m over more than 80% of thewhole drift path, with the best value reach 15m at 15mm from the anodes.The deterioration of the resolution at small drift distance is due to the smallsize of the electron cloud collected on the anodes. When the charge is collected6by two anodes, a correction can be applied to the anode charge distributioncentroid in order to better determine the position of the crossing point of theparticle[16]. The global improvement on the anodic resolution can be seen onthe black circle curve.4 ConclusionsIn spite of the relevant eects of the dopant inhomogeneity of the wafers onwhich the SDD's are build, it has been demonstrated that the deviations canbe measured and corrected, leading to a good resolution of the detector alongboth the anodic and the drift directions. Since it would be diÆcult to measurethe deviation maps of all the 260 SDD's with a minimum ionizing particlebeam, the possibility of a laser measurement is under study.The analysis of the rst beam tests using a single pair of the prototype versionsof PASCAL and AMBRA to read out part of an ALICE SDD is now underway. Preliminary results indicate that the signal-to-noise performance, aftercorrection for some systematic eects, is at least as good as that obtained withOLA.References[1] N. Antoniou et al., CERN/LHCC/93{16 (1993).[2] ALICE Collaboration, CERN/LHCC, 95/71[3] ALICE Collaboration, CERN/LHCC, 99/12[4] F. Tosello et al., Nucl. Instr. and Meth. A473 (2001) 210-218.[5] A. Rashevsky et al., Nucl. Instr. and Meth. A 461 (2001) 133-138[6] V. Bonvicini et al., Il Nuovo Cim. A112 (1999) 137.[7] V. Bonvicini et al., Nucl. Instr. and Meth. A439 (2000) 476.[8] A. Rivetti et al., CERN-LHCC-2000-041[9] G. Mazza et al., CERN-LHCC-2001-034[10] A. Werbrouck et al., Nucl. Instr. and Meth. A471 (2001) 281-284.[11] M. Campbell et at., Nucl. Instr. and Meth. A473 (2001) 140-145[12] A.P. De Haas et al., CERN-LHCC-99-033[13] W. Dabrowski et al., Nucl. Phys. B 44 (1995) 6377[14] F. Balestra et al., Nucl. Instr. and Meth. A323 (1992) 523.[15] D. Nouais et al., Nucl. Instr. and Meth. A 461 (2001) 133-138[16] E. Crescio et al., CERN-ALICE-PUB-2000-029, ALICE-INT-2001-09DriftDrift0.5 cm7.0 cm7.5 cmMOS injectorsanodesFig. 1. Sketch of the nal ALICE SDD prototype8CAGH TPFig. 2. Readout block diagram of a half ladder of the third layer. ( P: PASCAL(Preamplier, Analogue Storage and Conversion from Analogue to digitaL), A:AMBRA (A Multievent Buer Readout Architecture), C: CARLOS (CompressionAnd Run Length encOding Subsystem), T: TTCrx (Timing, Trigger and Controlreceiver), G: GOL (Gigabit Optical Link), H: HAL (Hardware Abstraction Layer)9-400-300-200-1000100200300400X(mm)0 10 20 30 40 50 60 70Y(mm)05101520253035-400-300-200-1000100200300400X(mm)0 10 20 30 40 50 60 70Y(mm)05101520253035Y(mm)0 5 10 15 20 25 30 35 m)µDeviation (-150-100-50050100150 X∆ Y∆Fig. 3. Systematic deviations (gray scale in m) of the anodic X(X;Y ) (top) anddrift coordinates Y (X;Y ) (middle) of the electron cloud centroid with respect tothe reference position of the crossing point of the particle coordinate, as a functionof the anodic coordinate X, and the drift distance Y. Bottom plot: X(X;Y ) andY (X;Y ) as a function of the drift distance for x = 20mm.Drift distance (mm)0 5 10 15 20 25 30 35m)µResolution (010203040506070Along anode axisAlong anode axis (corrected)Along drift axisFig. 4. Spatial resolution of the ALICE SDD along the drift-time axis and anodeaxis as function of the drift distance.10

The ALICE Silicon Drift Detector System

The ALICE Silicon Drift Detector System

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