Bertini, O.

Chatterji, S.

Frankenfeld, U.

Heuser, J.

Larionov, P.

Singla, M.

Sorokin, I.

GSI Repository

Performance of neutron irradiated prototype sensorsfor the CBM Silicon Tracking System ∗J. Heuser1, U. Frankenfeld1, S. Chatterji1, O. Bertini1, P. Larionov2,1, M. Singla2,1, and I. Sorokin2,1,31GSI, Darmstadt, Germany; 2Goethe University, Frankfurt, Germany; 3KINR, Kiev, UkraineWe report on the performance of prototype microstripsensors for the CBM Silicon Tracking System (STS) aftertheir exposure to neutron equivalent fluences as they areexpected for the running scenario in the CBM experiment.CBM radiation environmentThe neutron fluence at the STS detector is expectedto reach about 2×1013 neqcm−2 per year, depending onthe detailed physics programme performed and accumu-lating over several years of running. The maximum inte-grated fluence in some areas of the tracker will amount to1014 neqcm−2 beyond wich the affected detector moduleswill be replaced [1]. During the CBM experimental runs,periods of two or three months per year, the sensors will beoperated at -5◦C to limit radiation-induced leakage currentsand to prevent from thermal runaway. During the periods ofshutdown, the STS detector system may have to be warmedup to allow for maintenance of its components. The radi-ation damaged sensors will undergo annealing processes,with properties like leakage current, full depletion voltage(Vfd) and breakdown voltages changing with temperatureand time.Irradiation of prototype sensorsWe have irradiated small test sensors from the mostrecent production of CBM05 prototypes, produced byCiS, Erfurt, Germany. The exposure to neutrons wasperformed at Institute Jozef Stefan, Ljubljana, Slovenia,within the EU-FP7 project AIDA. The sensors were irra-diated in sets of four to the fluences 1×1013, 5×1013 and1×1014 neqcm−2. During transport and storage after irra-diation, a log of temperature and humidity was kept.Full depletion voltage, charge collectionThe sensors were installed in printed circuit boards al-lowing to apply bias voltage and to read out a number ofstrips with fast self-triggering front-end electronics. Thesensors were operated in a nitrogen conditioned freezer at-5◦C. Through the bias cable, scans of the bulk current andcapacitance were made as a function of the applied reversevoltage. The determined depletion voltages as a function ofthe neutron equivalent fluence are summarized in Table 1.Applying significantly higher bias, the sensors’ charge col-lection performance was determined with reading out sig-nals induced with a 241Am source and its 59 keV and lower∗ supported by EU-FP7 AIDAenergy gamma lines. Spectra of collected charge are shownin Fig. 1, applying a simple clustering algorithm to com-bine charge simultaneously seen on neighbouring strips.The results are reported in the table, assuming gain cali-bration factors of 117±3 (p) and 113±3 (n) ADC units forthe full signal to yield the charge collection efficiencies.ClustAmp0pEntries  12005Mean    82.08RMS     25.45ADC (baseline subtracted)0 20 40 60 80 100 120 140 160Entries05001000150020002500300035002/cmeqpside, 1e14 n2/cmeqpside, 5e13 n2/cmeqpside, 1e13 nCluster amplitude on STS station 0, p-side ClustAmp0pEntries  36276Mean    36.93RMS     34.78ADC (baseline subtracted)0 20 40 60 80 100 120 140Entries05001000150020002500300035002/cmeq n14 10×nside, 1 2/cmeq n13 10×nside, 5 2/cmeq n13 10×nside, 1 Cluster amplitude on STS station 0, p-sideFigure 1: Charge distributions from the sensors’ p (left) andn (right) sides as a function of neutron equivalent fluence.Table 1: Charge collection performance of the sensors.fluence Vfd Vbias peak ADC eff. (%)(neqcm−2) (V) (V) p ±3 n p ±4 n0 80±2 160±1 117 102 100 901×1013 35±5 130±1 105 100 90 885×1013 45±5 180±1 95 95 81 841×1014 110±2 300±1 95 81 81 71Annealing studiesThe leakage currents increase linearly with neutron flu-ence. The value of the damage constant, α, is initially ataround 4.0×10−17 Acm−1. After annealing at differenttemperatures, α can be used to yield the evolution of darkcurrents and Vfd. According to [2], radiation damage insilicon has several contibutions: a constant, a “beneficialannealing” and a “reverse annealing” component. Their de-termination is in progress.References[1] J. Heuser et al., Technical Design Report for theCBM Silicon Tracking System, GSI Report 2013-4,http://repository.gsi.de/record/54798[2] M. Moll et al., Nucl. Instrum. Methods. A 439 (2000) 282.GSI SCIENTIFIC REPORT 2013 NQM-CBM-08doi:10.15120/GR-2014-1-NQM-CBM-08 23

Performance of neutron irradiated prototype sensors for the CBM Silicon Tracking System

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Performance of neutron irradiated prototype sensors for the CBM Silicon Tracking System

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