The silicon detectors of the CDF experiment at the Tevatron collider are operated in a harsh radiation environment. The lifetime of the silicon detectors is limited by radiation damage, and beam-related incidents are an additional risk. This article describes the impact of beam-related incidents on detector operation and the effects of radiation damage on electronics noise and the silicon sensors. From measurements of the depletion voltage as a function of the integrated luminosity, estimates of the silicon detector lifetime are derived

Husemann, Ulrich

UNT Digital Library

CDF/PUB/CDF/PUBLIC/7974FERMILAB-CONF-05-606-ERadiation Experience with theCDF Silicon DetectorsUlrich Husemannon behalf of the CDF Silicon and Radiation Monitoring GroupsUniversity of Rochester, Rochester, New York 14627AbstractThe silicon detectors of the CDF experiment at the Tevatron collider are operated in a harsh radiation environment.The lifetime of the silicon detectors is limited by radiation damage, and beam-related incidents are an additionalrisk. This article describes the impact of beam-related incidents on detector operation and the effects of radiationdamage on electronics noise and the silicon sensors. From measurements of the depletion voltage as a function ofthe integrated luminosity, estimates of the silicon detector lifetime are derived.Key words: CDF, silicon detectors, radiation damagePACS: 29.40.Gx1. IntroductionThe CDF experiment [1] is a multi-purpose par-ticle physics experiment at the Tevatron proton-antiproton collider at Fermi National AcceleratorLaboratory. At the Tevatron, protons and an-tiprotons are collided at a center-of-mass energyof 1.96 TeV. The CDF apparatus includes detec-tors for for calorimetry, particle identification, andtracking of charged particles in a 1.4 Tesla mag-netic field. Tracking close to the interaction pointand vertexing are performed by silicon micro-stripdetectors. The CDF silicon detector system iscomprised of three different detectors: Layer 00,Email address: husemann@fnal.gov (Ulrich Husemann).SVX II, and the Intermediate Silicon Layers (ISL).Layer 00 [2] consists of LHC-style single-sidedsilicon sensors mounted directly on the Tevatronbeam-pipe to provide precision tracking close tothe interaction point. The core detector for three-dimensional tracking and vertexing is SVX II [3],built of five layers of double-sided silicon sen-sors. SVX II is used in the Silicon Vertex Trigger(SVT) [4], a trigger system sensitive to displacedvertices from the decays of long-lived particles.The ISL [5] link silicon tracks to the main driftchamber and extend the acceptance of the silicondetectors from |z| = 44cm at a radius of r . 10cm(SVX II) to |z| = 90 cm at r . 30 cm. A summaryof the detector properties and naming conventionfor Layer 00 and SVX II is given in Table 1.Preprint submitted to Elsevier Science 24 May 2006Table 1Naming convention and detector properties for Layer 00and SVX II.Layer Sensor Strip Radius Max.Manufacturer Orientation (cm) Bias (V)Layer 00 SGS-Thompson axial 1.35 500Layer 00 Hamamatsu axial 1.62 500SVX L0 Hamamatsu axial/90◦ 2.54 160SVX L1 Hamamatsu axial/90◦ 4.12 160SVX L2 Micron axial/1.2◦ 6.52 60SVX L3 Hamamatsu axial/90◦ 8.22 160SVX L4 Micron axial/−1.2◦ 10.10 60At the time of writing this article, the CDF sil-icon detectors have been irradiated with an inte-grated luminosity of 1.4fb−1, equivalent to a dose ofapproximately 3 kGy from proton-antiproton col-lisions at the radius of SVX L0 [6]. The silicon de-tectors were originally designed to tolerate 2 fb−1to 3 fb−1 before being replaced by a “Run IIb” up-grade. This upgrade was canceled, thus the silicondetectors must now operate to the end of TevatronRun II in 2009 with an expected integrated lumi-nosity of 4fb−1 to 8fb−1. Therefore, enhancing thedetector longevity is a prime goal of the CDF sili-con group.2. Beam-related IncidentsHigh radiation levels due to uncontrolled beamlosses of the Tevatron collider put the silicon detec-tors at risk. For example, 18 incidents in which thekicker magnets used to abort the Tevatron beamramped spontaneously caused permanent damagein at least 26 of the 5644 readout chips. The CDFshift crew regularly evaluates potential risks for thesilicon detectors based on systems to monitor thebeam losses and the radiation dose close to the sil-icon detectors. In addition, an automated systemtriggers a beam abort if the dose rate is greaterthan 0.12 Gy/s. A prototype of a novel systemto monitor and eventually abort the beam basedon polycrystalline CVD diamond sensors has beensuccessfully installed and tested in the CDF track-ing volume. Fig. 1 shows a measurement of the dia-mond current during beam injection, acceleration,and collimation at the Tevatron.The silicon detectors are powered by custompower supplies in CAEN SY527 mainframes. The09:30 09:45 10:00 10:15time10:30current (nA)110210Fig. 1. Measured diamond current during Tevatron “shotsetup”. Beginning at 9:30, protons are injected, followed bynine antiproton injections after 9:45, and beam collimatorpositioning after 10:10.power supplies are installed inside the CDF col-lision hall, at locations where no thick shieldingcan be installed. They show three common beam-related non-permanent failure modes: spontaneousswitch-offs, loss of communication, and corruptedread-back of currents and voltages. In 2005, theaverage rate of such failures was nine per month,and the failure rate of power supplies in areas ofhigher radiation was significantly larger than forpower supplies in low-radiation areas [7].3. Radiation DamageThe efforts to enhance the longevity of the CDFsilicon detectors include measures to mitigate theeffects of radiation damage and close monitoringof changes of the detector parameters under irra-diation.Radiation damage is mitigated by reducing thenumber of thermo-cycles of the system to a mini-mum and operating the silicon detectors at a lowertemperature. The detector volume was thermallyisolated in October 2004, and the detector temper-ature was lowered from −6◦C to −10◦C in early2005.A measure of the usable lifetime of the CDFsilicon detectors is the lowest possible signal-to-2noise ratio (S/N) acceptable for the SVT and forthe ability to perform efficient tagging of b quarkjets. Based on Monte Carlo simulations from CDFRun I, the lowest possible value is S/N ≈ 6 [8].The signal-to-noise ratio is determined by the noiselevel of sensors and readout electronics and by thecharge collection efficiency of the sensors. Under ir-radiation, the noise level increases, and the chargecollection efficiency decreases. A prediction of S/Nas a function of integrated luminosity can be foundin [8].Detailed tests have been performed to study theradiation hardness of the SVX3D readout chipsand the optical transmitters used in the system.After 8 fb−1 of integrated luminosity, an increaseof 17% in the chip noise is expected for SVX L0 [9].For the optical transmitters, a 10% degradation ofthe light level is anticipated after 8 fb−1; however,the waveform remains unchanged [10]. Therefore,neither the SVX3D chip nor the optical transmit-ters are expected to be the limiting factor for thesilicon detector lifetime.The dominant source of noise is the shot noise ofthe silicon sensors. The shot noise is increased bycrystal defects in the sensors due to radiation dam-age. For SVX II, the shot noise can be parametrizedas Q = 900 e ×√Ileak, where Ileak is the leakagecurrent in µA [8]. Changes in the leakage currentcan be approximated by changes in the bias cur-rent Ibias, assuming that all other contributions tothe bias current are constant: ∆Ileak ≈ ∆Ibias. Anexample of the evolution of Ibias with integratedluminosity is shown in Fig. 2. The change in biascurrent is related to the fluence Φ by ∆Ibias =αΦV , with the damage constant α and the knowndetector volume V . An effective damage constantof αeff = (3.98 ± 0.15) × 10−17 A/cm is derivedby comparing the radial dependence of ∆Ibias withthe radial dependence of the radiation field. Theradiation field in the CDF tracking volume hasbeen measured precisely by approximately 1000thermal luminescent dosimeters (TLD) inside theCDF tracking volume [6]. The TLD measurementsagree with measurements of ∆Ibias within 10% [11].Hence, the fluence can be inferred from bias cur-rent measurements. The main systematic uncer-tainty of this method (13%) is due to the impre-cise knowledge of the sensor temperatures. From )−1integrated luminosity (pb0 50 100A)µbias current (20304050Fig. 2. Bias current Ibias as a function of the integratedluminosity∫Ldt for a single silicon detector module inSVX II. The data points are compatible with a constantslope dIbias/d∫Ldt.the measurement of the fluence, an update of theS/N predictions as a function of luminosity in [8]will be derived.Sufficient charge collection efficiency is achievedby applying a bias voltage between the two sides ofa silicon sensor. Under irradiation and annealing,the effective number of charge carriers changes, andthe bulk of the sensor undergoes “type inversion”from net n-type to p-type silicon. The bias volt-age required to fully deplete the sensor decreasesuntil type inversion occurs and increases again,eventually surpassing the maximum safe bias volt-age. Breakdown of the coupling capacitors on theSVX II sensors limits the maximum safe bias volt-age to approximately 160 V for the Hamamatsusensors and to 60 V for the Micron sensors. TheLHC-style sensors in Layer 00 can be biased up to500 V. In [8], a model of the depletion voltage ispresented, and the lifetime of the silicon detectorsis predicted, albeit with large uncertainties. To testthe predictions of the lifetime model with data, thedepletion voltage is measured on a regular basisin specialized “bias scan” runs [12]. For the single-sided sensors in Layer 00, the depletion voltage isdetermined from the charge collected from siliconhits on well-measured tracks as a function of thebias voltage. The double-sided sensors in SVX IIallow for a measurement of the depletion voltagebased on the reduction of inter-strip noise on theohmic side of the sensor, without beam in the Teva-tron. During the first 1.3 fb−1 of integrated lumi-3data60 2 4 8depletion voltage (V)−1integrated luminosity (fb   )050100150200250300350−1   prediction+1   predictionσσcentral predictionFig. 3. Comparison of the depletion voltage Vdep as a func-tion of the integrated luminosity measured in bias scan runs(solid line) with a lifetime prediction for SVX L0 for Vdepderived from the capacitance–voltage characteristic of thesensor [8] (central prediction: dashed line, +1σ prediction:dot-dashed line, −1σ prediction: dot-dot-dashed line).nosity, 8 bias scans for Layer 00 and 13 bias scansfor SVX II have been performed. The depletionvoltage Vdep as a function of integrated luminosity,is fitted with a straight line for every detector mod-ule and extrapolated to the point of type inversion.The results are consistent with the lifetime predic-tions presented in [8], as shown in Fig. 3. Layer 00and SVX L0 are expected to undergo type inver-sion after 2 fb−1 to 3 fb−1. Assuming the same ab-solute value of the rate of change of Vdep after typeinversion, an upper limit of detector lifetime is esti-mated from the maximum safe bias voltage. Fig. 4shows the predictions of the inversion point and thedetector lifetime for SVX and Layer 00. Based onthis estimate, the silicon detectors are expected tooutlast Tevatron Run II; however, only a measure-ment of the rate of change of the depletion voltageafter type inversion will lead to definite conclusionsabout the lifetime of the CDF silicon detectors.This material is based upon work supported by the U.S.Department of Energy under Award Number DE-FG02-91ER40685.References[1] A. Abulencia, et al., Measurements of inclusive W andZ cross sections in pp collisions at√s = 1.96 TeV,submitted to Phys. Rev. D.[2] C. S. Hill, et al., Initial experience with the CDFLayer 00 silicon detector, Nucl. Instrum. Meth. A511L00 (Ham.)L00 (Thom.)SVX L0SVX L1SVX L2SVX L3SVX L4integrated luminosity (fb0 5 10)−10 20 40Inversion Point LifetimeFig. 4. Expected integrated luminosity for the point of typeinversion (left) and lifetime (right) for the layers of SVX IIand Layer 00 based on depletion voltage measurements [12].(2003) 118–120.[3] A. Sill, et al., CDF Run II silicon tracking projects,Nucl. Instrum. Meth. A447 (2000) 1–8.[4] W. Ashmanskas, et al., The CDF silicon vertex trigger,Nucl. Instrum. Meth. A518 (2004) 532–536.[5] A. Affolder, et al., Intermediate silicon layers detectorfor the CDF experiment, Nucl. Instrum. Meth. A453(2000) 84–88.[6] R. J. Tesarek, et al., A measurement of the radiationenvironment in the CDF tracking volume, Nucl.Instrum. Meth. A514 (2003) 188–193.[7] M. Stanitzki, et al., Operational experience with theCDF Run II silicon detectors, Proc. 11th Workshopon Electronics for LHC and Future Experiments,Heidelberg, Germany, September 12–16, 2005.[8] S. Worm, et al., Life expectancy of the CDF Run IIsilicon, Nucl. Instrum. Meth. A549 (2005) 126–129.[9] M. Garcia-Sciveres, et al., The SVX3D integratedcircuit for dead-timeless silicon strip readout, Nucl.Instrum. Meth. A435 (1999) 58–64.[10] M. L. Chu, et al., Radiation hardness of the 1550-nmedge emitting laser for the optical links of the CDFsilicon tracker, Nucl. Instrum. Meth. A541 (2005) 208–212.[11] S. D’Auria, et al., Measurement of the radiation fieldin CDF, Nucl. Instrum. Meth. A513 (2003) 89.[12] A. Boveia, Status and performance of the CDF Run IIsilicon detector, PoS HEP2005 (2005) 377.4

Radiation experience with the CDF silicon detectors

https://digital.library.unt.edu/ark:/67531/metadc881925/m2/1/high_res_d/892261.pdf

Radiation experience with the CDF silicon detectors

Abstract

Similar works

Full text

Available Versions

UNT Digital Library