The BABAR Silicon Vertex Tracker (SVT) has been efficiently operated for five years since the start of data taking in 1999. It has met design requirements and no degradation in its performance has been observed thus far. However, because of higher than expected background levels, and anticipated further increases in luminosity and dose rates, we have done a thorough study to assess the viability of operating the SVT until the end of the decade

/INFN, Pavia /Bergamo U. /UC, Irvine /UC, Santa Barbara /UC, Santa Cruz /INFN, Ferrara /Ferrara U. /NIKHEF, Amsterdam /LBL, Berkeley /Maryland U. /INFN, Milan /Milan U. /Pavia U. /INFN, Pisa /Pisa U. /Princeton U. /SLAC /Stanford U., Phys. Dept. /INFN, Turin /Turin U. /INFN, Trieste

Beck, T.

Berryhill, J.

Bruinsma, M.

Burke, S.

Callahan, D.

Campagnari, C.

Curry, S.

Dahmes, B.

Eisner, A.M.

Hale, D.

Hart, P.

Kirkby, D.

Kroseberg, J.

Kyre, S.

Levy, S.

Long, O.

Mazur, M.

Re, V.

Richman, J.

Stoner, J.

Verkerke, W.

Eisner, A. M.

UNT Digital Library

ARTICLE IN PRESSSLAC-PUB-11867CorrespondE-mail addrContributStatus and prospects of the BaBar SVTV. Rea, M. Bruinsmab,, S. Curryb, D. Kirkbyb, J. Berryhillc, S. Burkec, D. Callahanc,C. Campagnaric, B. Dahmesc, D. Halec, P. Hartc, S. Kyrec, S. Levyc, O. Longc, M. Mazurc,J. Richmanc, J. Stonerc, W. Verkerkec, T. Beckd, A.M. Eisnerd, J. Krosebergd,W.S. Lockmand, G. Nesomd, A. Seidend, P. Spradlind, W. Walkowiakd, M. Wilsond,C. Bozzie, G. Cibinettoe, L. Piemontesee, H.L. Snoekf, D. Browng, E. Charlesg, S. Darding,F. Goozeng, L.T. Kerthg, A. Gritsang, G. Lynchg, N.A. Roeg, C. Chenh, W. Hulsbergenh,C.K. Laeh, V. Lillardh, D. Robertsh, A. Lazzaroi, F. Palomboi, L. Rattij, P.F. Manfredij,E. Mandellij, C. Angelinik, G. Batignanik, S. Bettarinik, M. Bondiolik, F. Bosik, F. Buccik,G. Calderinik, M. Carpinellik, M. Ceccantik, F. Fortik, M.A. Giorgik, A. Lusianik,P. Mamminik, G. Marchiorik, M. Morgantik, F. Morsanik, N. Nerik, E. Paolonik, A. Profetik,M. Ramak, G. Rizzok, G. Simik,l, J. Walshk, P. Elmerm, A. Perazzol, P. Burchatn,A.J. Edwardsn, S. Majewskin, B.A. Petersenn, C. Roatn, M. Bonao, F. Bianchio, D. Gambao,P. Trapanio, L. Bosisiop, C. Cartarop, F. Cossuttip, G. Della Riccap, S. Dittongop,S. Grancagnolop, L. Lancerip, L. Vitalep, M. Dattaq, A. MihalyiqaINFN-Pavia and Universita` di Bergamo, ItalybUniversity of California, Irvine, CA, USAcUniversity of California, Santa Barbara, CA, USAdUniversity of California, Santa Cruz, USAeINFN-Ferrara and Universita` di Ferrara, ItalyfNIKHEF, Amsterdam, NetherlandgLawrence Berkeley National Laboratory, Berkeley, CA, USAhUniversity of Maryland, MD, USAiINFN-Milano and Universita` di Milano, ItalyjINFN-Pavia and Universita` di Pavia, ItalykINFN-Pisa and Universita` di Pisa, ItalylStanford Linear Accelerator Center, Stanford, CA, USAmPrinceton University, USAnStanford University, Stanford, CA, USAoINFN-Torino and Universita` di Torino, ItalypINFN-Trieste and Universita` di Trieste, ItalyqUniversity of Wisconsin, Madison, USAAvailable online 19 December 2005AbstractThe BABAR Silicon Vertex Tracker (SVT) has been efﬁciently operated for ﬁve years since the start of data taking in 1999. It has metdesign requirements and no degradation in its performance has been observed thus far. However, because of higher than expectedWork supported in part by Department of Energy contract DE-AC02-76SF00515ing author.ess: bruinsma@slac.stanford.edu (M. Bruinsma).ed to 13th International Workshop on Vertex Detectors for High Energy Physics, 13-18 Sept. 2004, Menaggio-Como, ItalyARTICLE IN PRESSbackground levels, and anticipated further increases in luminosity and dose rates, we have done a thorough study to assess the viability ofoperating the SVT until the end of the decade.1. The BABAR silicon vertex trackerThe BABAR experiment [1] is dedicated to a systematicstudy of CP asymmetries in B decays and to precisionmeasurements of the CKM quark mixing matrix. It hasthus far recorded over 200 million BB pairs produced incollisions of a 3.1GeV positron beam and a 9.0GeVelectron beam. The beams provide a center-of-mass energyof 10.58GeV, near the Uð4SÞ resonance, with a net boostthat gives a measurable separation of the two B decays.The BABAR SVT is the sub-detector that is nearest to theeþe interaction point. Its primary purpose is a precisedetermination of track parameters of charged particles toallow the reconstruction of vertices with a resolution that issufﬁcient to disentangle the two B decays. They have anaverage separation along the beam axis (z) of 280 mm andthe achieved vertex resolution in z is 80 mm. The SVT alsoenables the reconstruction of low-momentum ðpTo120MeVÞ charged particles, in particular slow pions fromD decays, that do not fully traverse the main BABAR -tracker.The SVT, described in greater detail elsewhere [2], is asilicon micro-strip detector comprising over 140 000channels on 340 double-sided, AC-coupled silicon wafersarranged in ﬁve layers. The ﬁrst layer is as close as 3.2 cmto the beam axis, while the last layer is 14.0 cm away. Eachwafer has strips oriented perpendicular (‘z-strips’) andparallel (‘f-strips’) to the beam axis on either side, and thereadout pitch ranges from 50 mm for the f-strips in the ﬁrstlayer to 210 mm for the z-strips in the outermost layers. Theentire detector is immersed in a 1.5 T magnetic ﬁeld.The strip signals are digitized using custom built front-end chips called AToM [3], each of which serves 128channels in parallel. The digitization pipeline of a singlechannel on the AToM has an analog front end (pre-ampliﬁer, shaper and a comparator with programmablethreshold) and a 193-wide circular buffer, which is clockedat 15MHz and provides a maximum trigger latency of12:7ms. Upon the reception of a L1-trigger, the AToMsearches for a hit within a 1ms window of the buffer, andsums up the number of consecutive hits. This numbermeasures the ‘Time-Over-Threshold’, which is propor-tional to the logarithm of the collected charge.2. PerformanceOf the 208 readout sections of the SVT, nine wereinoperable from the start of data taking in 1999. Five ofthese failures were caused by bad connections andrecovered during an access to the detector in 2002. Inaddition, approximately 2% of the individual channelswere unbonded or otherwise unresponsive. The 5% missingchannels do not signiﬁcantly affect the overall performanceof the SVT, thanks to a sufﬁcient redundancy in thedetector design.The hit efﬁciency, measured using reference trackspassing the active area, averages at 97%, when the fourdead readout sections are discounted. The efﬁciency forreconstructing low momentum particles is 75% formomenta above 100MeV and 90% for momentum above200MeV. The hit resolution is measured using high-momentum particles in two-prong events. For particlesincident on the wafer under a straight angle, the hitresolution is between 10 and 15 mm for the three innermostlayers, and between 30 and 40 mm for the two outermostlayers. This gives a vertex resolution in for fully recon-structed decays of better than 80 mm in z. The resolution inthe separation of the vertices of a fully reconstructed decayand an inclusively reconstructed decay, relevant for most ofthe time-dependent CP analyses in BABAR, is 190 mm,dominated by the uncertainty in the inclusively recon-structed (tag side) vertex. No loss of efﬁciency or decreasein resolution has been observed in the ﬁrst ﬁve years ofoperation.3. Radiation monitoringTo prevent excessive radiation damage to the SVT, aradiation monitoring and protection system (SVTRAD [4])triggers beam aborts whenever background radiation levelsare anomalously high. An abort decision is issued within1ms when dose rates higher than 1Rad/ms are observed inorder to prevent acute damage. Furthermore, a long-termradiation budget is enforced by triggering a beam abortwhen the dose rate persistently (10 min) exceeds 50mrad/s,more than twice the level observed during normal runningconditions. Much of the background radiation is concen-trated in the horizontal plane, where off-momentum beamparticles are swept into the detector by the ﬁnal-focusingdipole magnet.Radiation levels have been higher than anticipated: someparts of the detector have already accumulated over2MRad, a level not expected to be reached over the fulllifetime of the experiment. While the PEP-II acceleratorhas already delivered instantaneous luminosities of over9 1033 cm2s1 (over three times the design value),ARTICLE IN PRESSDate [M/Y]01/00 12/00 12/01 12/02 01/04 12/04 12/05 12/06 01/08 12/08Dose (Mrad)024681012 FE:MIDBW:MIDFW:MIDBE:MIDFig. 1. Measured (solid) and predicted (dashed) radiation doses in fourlocations in the horizontal (MID) plane. The labels refer to Forward(F)/Backward(B) and East(E)/West(W), where Forward(Backward) is ap-proximately 10 cm upstream(downstream) the interaction point andEast(West) is 3 cm inside(outside) the circular beam line.Table 1Potential sources of damage to the SVT due to acute or chronic radiationlevelsWafers Readout electronicsAcute c p-stop shortsc coupling capacitor breakdownChronic c Leakage current increase c noise increasec shift in depletion voltage c gain decreasec loss of charge coll. efﬁciency c pedestal shiftc digital failuresfurther increases are planned. Dose rates are expected toincrease correspondingly, posing a potentially criticalproblem for the SVT in the future. Fig. 1 shows theaccumulated dose and the dose expected until the end ofrunning in 2009.4. Radiation damage studiesThe SVT group has performed extensive tests to assessthe viability of running the SVT until 2009. The potentialdegradation due to any of the mechanisms listed in Table 1has been evaluated in a series of dedicated radiation tests atElettra [5], LBNL and SLAC.4.1. Damage to the wafersBulk damage to the silicon wafer causes a loss of chargecollection efﬁciency (CCE), an increase in leakage currentand corresponding shot noise and a shift in depletionvoltage. To determine the magnitude of these effects, Siwafers equipped with readout electronics were irradiatedwith the 0.9GeV electron beam of Elettra in six steps up toa total dose of 9MRad [6]. After each irradiation step, theloss in CCE was measured using a LED with l ¼ 1060 nm.The depletion voltage was determined by measuring theCCE and the inter-strip capacitance as a function of thebias voltage. A loss in CCE of about 1%/MRad wasobserved. The depletion voltage steadily dropped until typeinversion at 3MRad, in agreement with the NIELhypothesis, and rose to 70V at 9MRad. This indicatesthat we will be able to fully deplete the detector until theend of the decade. The increase in leakage current of0.5–0:7mAMRad1cm2 at 17 C is in agreement withobservations of the installed SVT and shows shot noise tobe a negligible problem.4.2. Damage to the front-end electronicsA set of AToM chips connected to a half module wasirradiated up to 5MRad with 60Co sources at SLAC andLBNL and up to 9MRad with the 0.9GeV electron beamof Elettra [7]. The chips were powered and clocked duringirradiation and showed no digital failures. An increase ofnoise of 15–20%/MRad and a decrease in gain of 4%/MRad was observed, consistent with observations on theinstalled modules.The installed modules showed an unexpected rise in thecomparator pedestals after an accumulated dose of1MRad. After rising to approximately half of the dynamicrange at 1.5MRad, the pedestals slowly decreased again totheir nominal levels at 3MRad. This effect has beenconﬁrmed in the lab tests and is attributed to an asymmetryin the comparator circuitry of the AToM. The affectedmodules can still be operated by regularly adjusting theprogrammable threshold values.4.3. Expected impact on performanceAs shown in Fig. 2, the noise increase on the AToMchips is the dominant effect in the reduction of the signal-to-noise (S=N) ratio due to radiation damage. We expect asigniﬁcant impact on SVT performance at S=N ratioslower than 10, reached after an accumulated dose ofapproximately 6MRad. We expect this dose within thenext two years for some of the chips in horizontal plane(see Fig. 1) of the ﬁrst two layers.In view of our expectation that some parts of the SVTwill become inoperable, detailed studies have been per-formed to quantify the possible impact on physics analyses.In a set of scenarios ranging from realistic to pessimistic,we ﬁnd a loss in the reconstruction efﬁciency of 2–4% forB0 ! J=cK0S decays and 2–10% for slow pions. Nosigniﬁcant impact on the mass or vertex resolution hasbeen observed. We judge the loss in efﬁciency to beacceptable and therefore do no plan to replace affectedmodules.The foreseen increase in background levels will alsoincrease the hit occupancy during normal running. Thiscan jeopardize the event reconstruction with a loss in hitARTICLE IN PRESS01000200030004000500060000 10 15 20 250510152025S/N ratiototal noiseAToM noiseshot noiseDose (MRad)Noise (electrons)Signal/Noise5Fig. 2. Predicted noise levels from wafer (shot noise) and readout chip(AToM) and signal-to-noise ratio as a function of accumulated dose.efﬁciency, when background hits precede real hits in thecircular buffer, and a loss in resolution due to thedeformation of clusters. Studies show that a signiﬁcantimpact is expected at occupancies above 20%, with a lossof hit efﬁciency of 4% and a reduction in hit resolution of afactor two. Such occupancies will, however, only bereached in 10% of the layer 1 and 2 (midplane) channels,overlapping those most affected by radiation damage.In summary, the SVT of BaBar has performed excel-lently in the ﬁrst ﬁve years of operation, meeting designspeciﬁcations in efﬁciency and resolution. We expect that inthe next few years some small parts of the detector will beaffected by an increase of noise on the readout chips due tohigher than expected radiation doses. Due to sufﬁcientredundancy in the design, this will only moderately affectthe physics performance, and we therefore expect tosuccessfully operate the SVT until the end of data taking.References[1] B. Aubert, et al., Nucl. Instr. and Meth. A 479 (2002) 1.[2] V. Re, et al., IEEE Trans. Nucl. Sci. NS-49 (2002) 3284.[3] V. Re, et al., Nucl. Instr. and Meth. A 409 (1998) 354.[4] T.I. Meyer, Int. J. Mod. Phys. A 16S1C (2001) 1084.[5] V. Re, et al., Nucl. Instr. and Meth. A 530 (2004) 7.[6] S. Bettarini, et al., IEEE Trans. Nucl. Sci. 52 (2005) 1054.[7] V. Re, et al., IEEE Trans. Nucl. Sci. 52 (2005) 787.

Status And Prospects of the BaBar SVT

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Status And Prospects of the BaBar SVT

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