We report on the performance of a doublet of 3" cylindrical silicon drift detectors installed as an upgrade of the CERES/NA45 electron pair spectrometer for the Pb-beam at the CERN SPS. The silicon detectors provide external particle tracking and background rejection of conversions and close Dalitz pairs. Results on vertex reconstruction and rejection from Pb test-run in 1994 are presented

Agakichiev, G

Baur, R

Ceretto, F

Drees, A

Faschingbauer, U

Fraenkel, Zeev

Fuchs, C

Gatti, E

Glässel, P

Hemberger, M

Hess, F

Holl, P

Irmscher, D

Jacob, C

Kemmer, J

Minaev, Yu I

Panebratsev, Yu A

Pfeiffer, A

Pérez de los Heros, C

Ravinovich, I

Razin, S V

Rehak, P

Sampietro, M

Schükraft, Jürgen

Shimansky, S S

Socol, E

Specht, H J

Tel-Zur, G

Tserruya, Itzhak

Ullrich, T S

Voigt, C A

Wurm, J P

Yurevich, V I

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN{PPE/95-13223 August 1995A DOUBLET OF 3"CYLINDRICAL SILICON DRIFT DETECTORSIN THE CERES/NA45 EXPERIMENTU. Faschingbauer1), G. Agakichiev2), R. Baur3), F. Ceretto1), A. Drees3), Z. Fraenkel4),Ch. Fuchs1), E. Gatti5), P. Glassel3), M. Hemberger1), C. P. de los Heros4), F. Hess1),P. Holl6), D. Irmscher3), C. Jacob1), J. Kemmer6), Y. Minaev2), Y. Panebrattsev2),A. Pfeier3), I. Ravinovich4), S. Razin2), P. Rehak7), M. Sampietro5), J. Schukraft8),S. Shimanskiy2), E. Socol4), H. J. Specht3), G. Tel-Zur4), I. Tserruya4), Th. Ullrich3),C. Voigt3), J. P. Wurm1), V. Yurevich2)AbstractWe report on the performance of a doublet of 3" cylindrical silicon drift detectors in-stalled as an upgrade of the CERES/NA45 electron pair spectrometer for the Pb-beamat the CERN SPS. The silicon detectors provide external particle tracking and back-ground rejection of conversions and close Dalitz pairs. Results on vertex reconstructionand rejection from a Pb test-run in 1994 are presented.(Talk given at the 7th European Symposium on Semiconductor Detectors, SchlossElmau, Germany, May 1995, submitted to Nuclear Instruments and Methods A)1)Max-Planck-Institut fur Kernphysik, 69117 Heidelberg, Germany2)JINR, RU-141980 Dubna, Moscow Region, Russia3)Physikalisches Institut der Universitat Heidelberg, 69120 Heidelberg, Germany4)Weizmann Institute, Rehovot 76100, Israel5)Politecnico di Milano, 20133 Milano, Italy6)KETEK GmbH, D-85764 Oberschleissheim, Germany7)Brookhaven National Laboratory, Upton, NY 11973, USA8)CERN, 1211 Geneva 23, SwitzerlandIntroductionElectron pairs (e+e ) are considered a unique probe of the hot and dense matterproduced in ultra-relativistic A-A collisions. Of electromagnetic origin, they obey theStefan-Boltzmann law for the emission rate and leave the interaction zone freely. However,they are also extremely rare compared to the enormous number of hadrons produced inthe reball (e+e = 10 5). Moreover, there is a huge background of pairs only slightlyopened, originating from late decays of free hadrons, from Dalitz decays as well as fromconversions of ubiquitous photons (! 2) in the target and detector material.The CERES/NA45 experiment has succeeded to measure low-mass electron pairsin proton and sulphur induced collisions at the CERN SPS [1] with a double ring-imagingCherenkov (RICH) spectrometer. For S-Au collisions, with a charged particle density ofdnch=dy  160 in central collisions, the signal-to-background ratio was marginal. Elaboratealgorithms for nding the Cherenkov rings and for rejection of close pairs had to besupplemented by the powerful ability of a silicon-drift detector (SSD) to reject those pairsnot recognized in the RICHes as well as ctitious pairs of electron tracks not originatingfrom a common vertex.For our future experiments at the 160 GeV/u Pb beam, characterized by centralcharged particle densities of dnch=dy  500, the addition of an external particle tracking isabsolutely mandatory to reduce the much higher background arising from combinations ofelectron tracks belonging to close but unrecognized pairs. Furthermore the SDD-doubletincreases the ring nding eciency by pointing at ring center candidates in the rst RICHdetector.As a major upgrade of the CERES spectrometer, a doublet of improved SDD's hasbeen implemented to serve as a stand-alone vertex detector. Here, we report on a rstsuccessful test with Pb beam in fall 1994 [2].Silicon Drift Detectors in the CERES ExperimentThe detector principle is known since long [3]. As truly two-dimensional deviceswith excellent spatial resolution, SDDs are free of ambiguities in hit location even whenexposed to the extreme particle densities encountered in ultra-relativistic heavy-ion col-lisions. Compared to pixel devices, their high-end competitors, they allow fast readoutwith modest electronics.CERES by now has accumulated ve years of pioneering experience in the designand production of cylindrical drift detectors [4] and in operating them in the environmentof a large-scale experiment [5]. Cylindrical symmetry was chosen to comply with that ofthe RICH detectors; it brings the benet to apply the high voltages for the drift eldwithout a guard structure. The rst generation of detectors, which was used during theproton and sulphur runs, was highly successful. It's main design features were 360 n+-implanted charge collecting anodes on the outer periphery of a 3"wafer, intrinsic voltagedividers p+-implanted on each side of the wafer, and the collection of surface leakagecurrents in sink anodes.However, in the rst generation of detectors the azimuthal resolution was deteri-orated by deviations from a strictly radial geometry of the 'eld cage'. Due to layoutconstraints the eld rings had to be approximated by concentric regular polygons on bothsurfaces of the detector with sides that span 3. This 120-fold symmetry of the eld re-sulted in a higher charge collection to one third of the anodes, those in the center of a 3segment, the remaining two thirds, those at the segment edges, received a substantially1smaller portion [6]. For the second generation of 3"-detectors1, which were installed forthe lead beam in 1994, this was overcome by rotating the front lithography masks by 1.5with respect to the back lithographies, resulting now in a well approximated radial drifteld inside the depleted silicon, where the generated charge moves towards the anodes.A sensitive measure of the deviations from radial symmetry is the transverse electriceld, deecting the electron cloud sideways. Using the program of Castoldi and Gatti, ap-plying a closed-form solution of the 3-D Poisson equation presented at this Conference [7],we have calculated the tangential elds for one sector, as shown in Fig. 1, for the detectorswith and without rotation. The maximum transverse electrical eld of 610 2V/m (1% of the longitudinal drift eld) is reduced by a factor of 6 due to the rotation (Fig. 1).Laser tests indeed show uniform charge collection on all anodes.-0.8-0.6-0.4-0.200.20.40.60.8x 10-3-400 -300 -200 -100 0 100 200 300 400r * phi (um)E transversal (V/um)with rotationAAKwithout rotationFigure 1: Transverse electrical eld plotted vs the transverse coordinate (400m  1.5,zero corresponds to the border of two neighbouring 3-sectors) at radius r = 15.4 mm. Due tothe 1.5rotation of front vs back lithographies, the transverse eld is reduced from 610 2to110 2V/m.Experiment at the Pb BeamThe target used for the Pb beam is segmented into 8 individual disks of 600 mdiameter and 25m thickness, separated along the beam direction by 2.9 mm. This way,a high interaction length is maintained, while the radiation length for photons enteringthe spectrometer acceptance is kept low (X/X= 0.37%). The SDD doublet is situated inthe inner part of the spectrometer, about 9 cm behind the segmented target, and coversthe full spectrometer acceptance of 8to 15for all target disks. Both SDDs are traversedby about 600 minimum ionizing particles in central Pb-Pb collisions. The detector setupwithin the spectrometer is shown in Fig. 2. The spacing between the two detectors waschosen to compromise between a suciently long lever arm to point to the vertex and asuciently low confusion of tracks due to multiple scattering in the rst detector.Electrical pickup was a severe problem of the old setup, which required a high de-tection threshold resulting in a loss of eciency. We have employed the 32-channel bipolar1)produced by SINTEF, 0134 Oslo 3, Norway2Au)85 mm15 mmSDD -1   SDD - 2rr Armaxminoo815   20 mmBEAMTARGET(8 x 25 µmFigure 2: Schematic drawing of the cylindrical SDD doublet covering the full acceptance of theRing Imaging CHerenkov (RICH) counters (to the right, not shown), setup 1994.preamplier/shaper (OLA chip) developed for future silicon drift detector applications inthe ALICE experiment [8]. The readout is fully based on complementary signals, whichcompletely removes electrical pickup and allows to set a low detection threshold. Thepower consumption of this new front end chip is only about 2.3 mW/channel, a factorof 10 less compared to the former hybrid electronics. However, the operation of the fullsystem with 1.8 m of 60 Ohm terminated at cables was accompanied by a lack of sta-bility. In order to suppress oscillations, we had to choose a mode, which caused somedeterioration of the dynamic range as well as the pulse shape.The time constant of the semi-Gaussian shaper was measured to be telec= 42 ns,about twice the design value foreseen as an optimal compromise between the competingrequirements of good double-pulse resolution and low ballistic decit. By the latter wedescribe the loss of signal amplitude at the output of the shaper with increasing drifttime, due to the diusive broadening of the charge cloud.01002003004005006007008009000 100 200 300 400 500 600 700 800integral amplitude (mV)nr. of entries0501001502002503003500.5 1 1.5 2 2.5 3A0   150.3   1.737A1   26.21   .8195radius (cm)integral amplitude (mV)Figure 3: Left: Amplitude (Landau) distribution of hits in SDD-1 with radius > 1.5 cm. Right:Amplitude of hits vs. the radial position (ballistic decit).3An amplitude distribution of minimum-ionizing particles reconstructed in SDD-1 {mostly pions { is depicted in Fig. 3 (left). Hits of low amplitude are due to soft X rays.The ballistic decit with the new amplier was reduced to less than 30%, over the fullrange of radial drift (Fig. 3 right). The small ballistic decit guarantees high detectioneciency also for hits in the inner part of the detector.Figure 4 demonstrates the quality in the reconstruction of interaction vertices us-ing all charged particle tracks without recourse to any sophisticated hit algorithms: thepositions of the 8 disks of the segmented Au target are perfectly resolved along the beamline (Fig. 4 left). The lateral xy-position distribution of the reconstructed vertices (rightside) reects the folding of the beam prole with the extension of the target disks (600m diameter), indicated by the circle. The vertex reconstruction eciency is very high;more than 98% of the target interactions have a reconstructed vertex.0100200300-1 0 1z (cm)eventsx (cm)y (cm)-0.0500.05-0.05 0 0.05Figure 4: Determination of the interaction vertices with the doublet of silicon drift chambers:distribution of reconstructed vertex positions along the beam axis showing the resolved 8 Autarget disks of 25 m thickness each (left), and the distribution of the lateral xy-position of thereconstructed vertices (right), reecting the beam prole folded with the target disks of 600 mdiameter (marked as a circle) and the reconstruction resolution.The position resolution measured with charged particle tracks is displayed in Fig.5. The values quoted in the gure represent the dispersion in SDD-1 for tracks dened bythe interaction vertex and a hit in SDD-2. They result from the combined eect of theintrinsic resolution of the detectors, the accuracy in vertex reconstruction and multiplescattering. We notice the very good azimuthal resolution, considering the 17 mrad pitchof the anode structure, which results from charge sharing of single hits among severalanodes due to diusion. Laser tests showed excellent position resolution of better than 2mrad in the azimuthal direction, and of 20 m in the radial direction.Rejection of close e+e pairs from photon conversions and Dalitz decays is themajor task for the silicon drift detector telescope. Their signature is a double-dE/dxsignal or a close double hit. For the rejection the correlated information of both detectorsis used. Figure 6 illustrates some rst results. The left side shows the correlation of theanalog signals measured in the two silicon drift detectors from identied tracks; theseare mainly single tracks with measured signal amplitudes that correspond to minimumionizing particles; they must not be rejected. The right-hand side of Fig. 6 shows the same40200400600800-400 -200 0 200 400∆r (µm)events85 µm Pt disk targetσ = 44 µm∆φ (mrad)eventsσ = 2.6 mrad0200400600800-20 0 20Figure 5: Position resolution of the silicon drift chambers: radial and azimuthal residuals inSDD-1 for tracks dened by the interaction vertex and a hit in SDD-2 for a 85m single diskPlatinum target.020040060080010000 500 1000amplitude SDD-2amplitude SDD-1tracks from vertexamplitude SDD-2reconstructed conversions020040060080010000 500 1000Figure 6: Distributions of dE/dx in SDD-1 vs. SDD-2 for all tracks pointing to the vertex (left),and for a sample of reconstructed conversions (right), exhibiting double-dE/dx.correlation for a sample of conversions identied in the RICHes.A large number of hits with double-dE/dx signals in both silicon detectors areclearly visible. One also observes a second class of events with no signal in SDD-1 and alarge amplitude spread in SDD-2, corresponding to conversions in material between theSDDs and within the second detector. Late conversions behind SDD-2 (not shown) canbe rejected by the absence of hits in both detectors. These gures demonstrate only partof the rejection power; an additional double-hit recognition will further improve it.5ConclusionCERES expanded its use of Cylindrical Silicon Drift Detectors (SDD) in order tomaintain its ability to measure rare electron pairs in the hadron-dense environment of Pb-Au collisions. A doublet of 3" SDDs replaced the single 3" SDD used in the proton/sulphurera and acts as a vertex detector for external tracking at the entrance of the CERESspectrometer. New integrated readout chips were implemented. Clean signals free of pickupand a low detection threshold together with moderate ballistic decit assure high detectioneciency. The observed instabilities of the chips will be cured by a new design of thepreamp/shaper unit with enhanced driving capability at the expense of lower gain.The strategy at production stage, to engineer a better radial symmetry of the driftpotential by rotating front vs. back lithographies, succeeded in uniform charge collection,and improved the azimuthal hit resolution considerably. First results from a 160 GeV/uPb-Au test run in 1994 prove an excellent vertex nding eciency with the expectedlongitudinal and lateral resolution and the capability to reject close electron pairs.A new generation of 4"detectors uses a novel interlaced anode structure, whichforces charge division among several anodes also for short drift times in order to improvethe azimuthal resolution for the outer part of the detector. The design of the new SDDuses 360-fold symmetry for the eld structure; front lithographies are rotated by 0.5withrespect to the back lithographies. Both measures will reduce the transverse electrical eldby two order of magnitudes compared to the original 3" design. The novel 4"-detectors,presently in a new production, are the subject of a contribution by P. Rehak to thisconference [9].AcknowledgementsThe CERES collaboration acknowledges the good performance of the CERN PSand SPS accelerators. We are grateful for support by the German Bundesministeriumfur Forschung und Technologie under grant BMFT 06HD525I, the U.S. Departementof Energy under Contract No. DE-AC02-76CH00016, the italian INFN, the MINERVAFoundation, Munich/Germany, the German-Israeli-Foundation for Scientic Research andDevelopement and the H. Gutwirth Fund.We thank D.A. Pinelli at Brookhaven Nat. Lab. for his artist work in design andconstruction of the multi-layer motherboards and for the mounting of the detectors.We appreciate the possibility to use the OLA chip in this experiment and acknowl-edge the generous support by V. Bonvicini, W. Dabrowski, P. Giubellino and A. Vacchi.References[1] G. Agakichiev et al., preprint CERN-PPE/95-26 and Phys. Rev. Lett., in print[2] CERES Report to Cogne 1995, CERN M542/SPSLC 95-20[3] E. Gatti and P. Rehak, Nucl. Instr. Meth. A225 (1984) 608[4] W. Chen et al., IEEE Trans. Nucl. Sci. NS-39 (1992) 619[5] U. Faschingbauer, PhD Thesis Heidelberg 1993[6] W. Chen et al., Nucl. Instr. Meth. A326 (1993) 273[7] A. Castoldi and E. Gatti, these Proceedings[8] W. Dabrowski, Univ. of Cracow and Univ. of Torino, R&D for the ALICE experiment[9] P. Rehak, these Proceedings6

A doublet of 3" cylindrical silicon drift detectors in the CERES/NA45 experiment

A doublet of 3" cylindrical silicon drift detectors in the CERES/NA45 experiment

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