Abstract   The operation and performance of the silicon strip Multiplicity Detector in the heavy-ion experiment NA50 at CERN are presented together with a selection of physics results. The main features of the detector are high speed (50 MHz sampling frequency), high granularity (more than 13,000 strips), and good radiation resistance. The detector provided a measurement of event centrality in Pb–Pb collisions, as well as target identification and the measurement of charged particle pseudorapidity distributions as a function of centrality

A. Marzari-Chiesa

Bruno Alessandro

F Prino

G. Bonazzola

L. Riccati

Luciano Ramello

M. Masera

Marco Monteno

Marek Idzik

Mario Sitta

P Deremigis

P. Rato Mendes

Paolo Giubellino

Pawel Grybos

S. Beole

W. L. Prado Da Silva

Wladyslaw Dabrowski

English

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ARTICLE IN PRESSNuclear Instruments and Methods in Physics Research A 513 (2003) 107–111*CorrespTorino, via7372; fax: +E-mail a0168-9002/$doi:10.1016The silicon Multiplicity Detector for the NA50 experimentB. Alessandroa, S. Beoleb, G. Bonazzolab, W. Dabrowskic, P. Deremigisa,P. Giubellinoa, P. Grybosc, M. Idzika,c,*, A. Marzari-Chiesab, M. Maserab,M. Montenoa, W.L. Prado da Silvad, F. Prinoe, L. Ramelloe, P. Rato Mendesb,L. Riccatia, M. Sittaea INFN, Torino, via P. Giuria 1, Torino 10125, ItalybDipartimento di Fisica Sperimentale dell’Universita, Torino, ItalycFaculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, Cracow, PolanddUERJ, Rio de Janeiro, BrazileDISTA, Universita del Piemonte Orientale, Alessandria, ItalyAbstractThe operation and performance of the silicon strip Multiplicity Detector in the heavy-ion experiment NA50 at CERNare presented together with a selection of physics results. The main features of the detector are high speed (50MHzsampling frequency), high granularity (more than 13,000 strips), and good radiation resistance. The detector provided ameasurement of event centrality in Pb–Pb collisions, as well as target identification and the measurement of chargedparticle pseudorapidity distributions as a function of centrality.r 2003 Elsevier B.V. All rights reserved.PACS: 29.40Keywords: Position sensitive detector; Silicon strip detector; Front-end electronics; Experimental techniques; Data analysis1. IntroductionThe NA50 experiment [1] at the CERN SPSinvestigates nuclear matter under extreme condi-tions of energy density with the ultimate goal ofdetecting signals of a phase transition fromordinary matter to a plasma of deconfined quarksand gluons (QGP). In particular the experiment islooking for dimuon production from charmoniumonding author. Address for correspondence: INFN,P. Giuria 1, Torino 10125, Italy. Tel.: +39-011-670-39-011-669-9579.ddress: idzik@to.infn.it (M. Idzik).- see front matter r 2003 Elsevier B.V. All rights reserve/j.nima.2003.08.012resonances and its suppression due to Debyescreening in a deconfined medium [2] in fixed-target Pb–Pb interactions at 158AGeV/c beammomentum. To collect enough Journal logostatistics on the production of J=c and c0 mesons,the experiment worked at the maximum beamintensity which was compatible with a correctoperation of the various detectors, namely (1–1.4) 107 beam particles/s, and with a targetthickness up to 32% of an interaction length.The main detector components are a high resolu-tion muon spectrometer and three centralitydetectors, namely a zero-degree calorimeter(ZDC) for forward energy (EZDC) measurements,d.ARTICLE IN PRESSB. Alessandro et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 107–111108an electromagnetic calorimeter (EMC) for trans-verse energy (ET) measurements and a siliconmicrostrip Multiplicity Detector (MD) for chargedparticle multiplicity measurements [3]. The MDwas also used to identify whether the interactiontook place in the target.A very fast binary readout electronics schemewas chosen to provide the excellent double pulseresolution required by the high beam rate. Itconsists of an analog and a digital chip. Theformer (FABRIC) [4] includes preamplifier(Tpeak ¼ 15 ns), shaper and discriminator, whilethe latter (CDP) [5] is a digital buffer working withsampling frequency of 50MHz. The need tosuppress the background due to the decays of pand K mesons imposed an absorber very close tothe target, and therefore a very small spaceavailable for the silicon detectors. Two discs(MD1, MD2) of radial silicon microstrip detec-tors, 10 cm apart, were built. The inner radius ofthe detectors in each disc is 0.5 cm while the outerone is 8.5 cm. The detector granularity was mainlydetermined by particle occupancy, keeping itbelow 30%. Taking into account also multiplescattering in the target and size of the beam(0.5mm RMS) a constant segmentation of thedetector DZ ¼ 0:02 in pseudorapidity and Df ¼100 in azimuth angle was chosen. So each discconsists of 36 azimuthally segmented sectors, 192Fig. 1. Picture of MD1.strips in each sector. Estimated fluences and dosesfor the innermost part of the detector were,respectively, 1014 particle/cm2 and 10Mrad, whilefor the front-end electronics region 1012 particle/cm2 and 200 krad, respectively. Standard p–on–ndetectors and a full-custom rad-hard front-endelectronics were chosen. A single detector disc isshown in Fig. 1.2. Setting up and operationIn order to establish the initial working condi-tions of the MD, and to maximize the detectionefficiency, it was necessary to set and periodicallyverify some control parameters: the trigger delay,the threshold voltage of the FABRIC discrimina-tors, and the biasing voltage of the silicondetectors [6]. In addition, an initial geometricalalignment on the beam line was made at thebeginning of each data-taking period, which lateron, was refined using experimental data. Some ofthese controls are discussed in this section.2.1. Trigger delayThe trigger latency period was about 1 ms, whilethe time interval necessary to have the CDPpointer on the RAM column where the triggeredevent was stored was 1.6 ms, corresponding to 80clock cycles of 20 ns duration (at 50MHz fre-quency). So it was necessary both to align thephase of the trigger signal with the CDP clock, andto add the right amount of delay. The optimizationof the delay value was done during a series of shortacquisition runs (trigger delay scan). Fig. 2 showsthe results of such a scan for a typical detector,both for low beam intensity (about 106 Pb/s) withminimum bias (ZDC) trigger, and for high beamintensity (about 107 Pb/s) with dimuon trigger. Thepeak occupancy with low intensity beam and ZDCtrigger corresponds essentially to non-interactingPb ions giving d-rays plus a small fraction of Pb–Pb interactions. The peak occupancy for dimuontrigger is of course higher, since it corresponds tothe inelastic Pb–Pb interactions, and is found forthe same delay value as for the ZDC trigger. Sincethe actual sampling instant of the FABRICARTICLE IN PRESSVth(V)0 0.1 0.2 0.3 0.4 0.5 0.6Occupancy00.050.10.150.20.250.3Vth(V)0 0.1 0.2 0.3 0.4 0.5 0.6Diff.Occupancy00.10.20.30.40.50.6Fig. 3. Occupancy per strip vs. threshold voltage with protonbeam (top: integral curve, bottom: differential curve).00.020.040.060.080.10.120.140.160.186 8 10 12 14 16 18 20 22 24 26 28 30trigger delay (20 ns units)average occupancy ZDC TRIGGERDIMUON TRIGGERFig. 2. Average occupancy per strip vs. trigger delay.B. Alessandro et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 107–111 109discriminator output must be in phase with thefree-running CDP clock, the sampling efficiencymay depend on the time difference betweensampling and trigger (the latter is strictly relatedto particles crossing the detector). This differencewas recorded by a TDC installed specially for thispurpose. Usually there was a wide efficiencyplateau and only a small TDC-based offlinecorrection was necessary when evaluating the totalmultiplicity.2.2. The discriminator threshold voltageIn order to set the correct discriminator thresh-old for the analog chip and to measure the signalto noise ratio (S/N) of our system, a few thresholdscans with the SPS proton beam impinging directlyon the detector were made. Fig. 3 shows the notnormalized integral occupancy curve and itsderivative. In order to estimate the MIP peakposition a simple gaussian fit is performed only inthe region around the peak of the differentialcurve. From these curves we estimate the MIPpeak to be at about 470mV. The gaussian noiseevaluation from a separate study with pulser gavesnoise ¼ 30mV, what would result in a S/N ratio ofabout 15. In fact during standard Pb–Pb runsoperation of the whole system we observed higherrate of noise counts than expected from themeasured snoise: This could be attributed to pick-up from the digital part of the front-end electro-nics. One should remember that a binary systemworking at a strip occupancy of about 10–30% hasa very high rate of switching between high and lowvoltage states, which could give rise to some non-gaussian noise component. Taking into accountthe system performance in standard conditions wefixed the FABRIC threshold at the relatively highlevel of about 200mV. This value was adjustedseveral times during different years of operationseparately for MD1 and MD2 detector planes.Threshold scans taken with secondary particlesproduced in Pb–Pb interactions were recordedfrom time to time for this purpose.2.3. The geometrical alignmentThe position of each detector with respect togeometrical references in the supporting stesalitedisc was measured with an accuracy of about50 mm. These geometrical data were used in asimple geometrical Monte-Carlo program toARTICLE IN PRESSStrip number0 20 40 60 80 100 120 140 160 180Occupancy00.050.10.150.20.250.30.350.4Fig. 4. Correlation between chosen strip of MD1 andcorresponding sector of MD2.Estimator value0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Number of events0100200300400500600Fig. 5. Estimator distribution for target 4.B. Alessandro et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 107–111110compute, for each target, the distribution of stripson MD2 corresponding to a given strip on MD1for each sector. This was done under the assump-tion that the corresponding strips are defined fromthe position of MD1 and MD2 hits by a particleoriginating from a given target.The MD1–MD2 strip correspondences werealso obtained from the measured data where, forevents with a particular MD1 strip being hit, apeak position and width in the MD2 occupancymap was found (see Fig. 4). The correspondencesobtained from the geometrical Monte-Carlo werecompared with the ones extracted from experi-mental data. The beam offsets in X and Y adoptedin the Monte-Carlo were tuned until a goodagreement with real data was obtained: the beamoffsets found in this way were about 1mm in Yand 0mm in X ; with a precision of about 0.2mm.3. Results3.1. Target identificationAn important result from the MD has been theidentification of interactions coming from thetarget, which must be separated from those dueto Pb–air interactions or upstream/downstreaminteractions. The identification is based on thecomparison of estimators for different targetpositions, using the MD1–MD2 correlations dis-cussed previously. The estimators were defined asthe ratio of found MD1–MD2 strip correspon-dences to the number of possible correspondences,under different hypotheses on the interactionposition (namely target, nearby surrounding airand target box windows). In Fig. 5 the estimatordistribution for semi-central interactions is shown.The peak at high estimator values identifies Pb–Pb interactions while the smaller peak at lowerestimator values corresponds to other competitiveprocesses like Pb–air interactions. Setting theappropriate threshold for the estimator value onecan select only events coming from target interac-tions. The threshold value will depend somewhaton the event centrality since the number of randomcorrespondences between multiplicity planesgrows with particle occupancy. The study ofestimator distributions in different centralityclasses was done in order to set the right thresholdsand to select minimum bias event samples forfurther analysis.3.2. Centrality estimationThe MD provided the estimation of centralityvia the global measurement of the charged multi-plicity. As an example, the correlation betweenmultiplicity (MUL1) and ET is shown in Fig. 6.3.3. Charged particle pseudorapidity distributionsThe MD provided also detailed pseudorapiditydistributions as a function of centrality, fortwo-beam energies [7]. Two analyses were per-formed using two independent centrality-relatedARTICLE IN PRESSFig. 6. Correlation between charged multiplicity and ET.B. Alessandro et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 107–111 111observables: the energy of the projectile spectatornucleons measured by the ZDC and the neutraltransverse energy measured by the EMC. In bothcases, the centrality selection was made usingobservables, which are independent of the MDitself, in order to avoid autocorrelations. Specialruns taken with the minimum bias trigger at lowbeam intensity were used for the sample taken at158GeV/nucleon and the second at 40GeV/nucleon energy. The charged particle pseudora-pidity distributions dNch=dZ in Pb–Pb collisionswere measured in six centrality classes defined interms of fractions (between 0% and 35%) of thetotal inelastic cross-section.4. ConclusionsWe have operated and analyzed data from asilicon strip Multiplicity Detector (MD) in thechallenging environment of the NA50 experiment.Very severe timing spatial and radiation con-straints have been satisfied with a design based onstandard p–on–n detectors and full-custom radia-tion-hard very fast front-end electronics. The MDhas provided centrality measurement and targetidentification in Pb–Pb collisions for the NA50data taken between the years 1995 and 2000. Thecharged particle pesudorapidity distributions as afunction of centrality have also been measured.AcknowledgementsWe are grateful to the members of the NA50collaboration who provided a stimulating andfriendly working environment during the construc-tion and operation of the detector. Financialsupport from the Istituto Nazionale di FisicaNucleare of Italy is acknowledged. This work wassupported in part by the Polish State Committeefor Scientific Research, Project no. 2P03B03319.References[1] M.C. Abreu, et al., Study of muon pairs and vector mesonsproduced in high energy Pb–Pb interactions, ProposalCERN/SPSLC 91-05, SPSLC/P265, October 1991.[2] T. Matsui, H. Satz, Phys. Lett. B 178 (1986) 416.[3] B. Alessandro, et al., Nucl. Instr. and Meth. A 409 (1998)167.[4] W. Dabrowski, et al., Nucl. Instr. and Meth. A 350 (1994)548.[5] J. DeWitt, IEEE Nuclear Science Symposium, San Fran-cisco, October–November 1993.[6] B. Alessandro, et al., Nucl. Instr. and Meth. A 432 (1999)342.[7] M.C. Abreu, et al., Phys. Lett. B 530 (2002) 33.

The silicon Multiplicity Detector for the NA50 experiment

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The silicon Multiplicity Detector for the NA50 experiment

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