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Radiation Tolerance of Silicon Detectors The Challenge for Applications

Radiation Tolerance of Silicon Detectors The Challenge for Applications in Future High Energy Physics Experiments. Gunnar Lindstroem, University of Hamburg. Reliable detection of mips S/N ≈ 10 reachable with. Experimental request Detector property. Proton-proton collider

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Radiation Tolerance of Silicon Detectors The Challenge for Applications

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  1. Radiation Tolerance of Silicon Detectors The Challenge for Applications in Future High Energy Physics Experiments Gunnar Lindstroem, University of Hamburg Hamburg workshop 24-Aug-06

  2. Reliable detection of mips S/N ≈ 10 reachable with Experimental requestDetector property Proton-proton collider Energy: 2 x 7 TeV Luminosity: 1034 Bunch crossing: every 25 nsec Rate: 40 MHz pp-collision event rate: 109/sec (23 interactions per bunch crossing) Annual operational period: 107 sec Expected total op. period: 10 years LHC properties period of 10 years low dissipation power atmoderate cooling Silicon pixel and microstrip detectors meet all requirements for LHC How about future developments? LHC-Challenge for Tracking Detectors employing minimum minimum detectorthickness material budget High event rate excellent time- (~10 ns) & high track accuracy & position resolution (~10 µm) Silicon Detectors: Favorite Choice for Particle Tracking Example: Large Hadron Collider LHC, start 2007 • Proton-proton collider, 2 x 7 TeV • Luminosity: 1034 • Bunch crossing: every 25 nsec, Rate: 40 MHz • event rate: 109/sec (23 interactions per bunch crossing) • Annual operational period: 107 sec • Expected total op. period: 10 years LHC properties Experimental requests Detector properties Reliable detection of mips S/N ≈ 10 High event rate time + positionresolution: high track accuracy~10 ns and ~10 µm Complex detector design low voltage operation in normal ambients Intense radiation field Radiation tolerance up to during 10 years 1015 hadrons/cm² Feasibility, e.g.large scale availability 200 m² for CMSknown technology, low cost ! Silicon Detectors meet all Requirements ! Hamburg workshop 24-Aug-06 Intense radiation field Radiation tolerance up to throughout operational 1015 1MeV eq. n/cm²

  3. LHC ATLAS Detector – a Future HEP Experiment Overall length: 46m, diameter: 22m, total weight: 7000t, magnetic field: 2T ATLAS collaboration: 1500 members principle of a silicon detector: solid state ionization chamber micro-strip detectorfor particle tracking 2nd general purpose experiment: CMS, with all silicon tracker! For innermost layers: pixel detectors Hamburg workshop 24-Aug-06

  4. 5 years 10 years 2500 fb-1 500 fb-1 Main motivations for R&D on Radiation Tolerant Detectors: Super - LHC • LHC upgradeLHC (2007), L = 1034cm-2s-1f(r=4cm) ~3·1015cm-2 • Super-LHC (2015 ?), L = 1035cm-2s-1f(r=4cm) ~1.6·1016cm-2 • LHC (Replacement of components)e.g. - LHCb Velo detectors (~2010) - ATLAS Pixel B-layer (~2012) • Linear collider experiments (generic R&D)Deep understanding of radiation damage will be fruitful for linear collider experiments where high doses of e, g will play a significant role.  5 CERN-RD48 CERN-RD50 Hamburg workshop 24-Aug-06

  5. Radiation Damage in Silicon Sensors Two types of radiation damage in detector materials: Bulk (Crystal) damagedue to Non Ionizing Energy Loss (NIEL - displacement damage, built up of crystal defects – I. Increase of leakage current (increase of shot noise, thermal runaway) II. Change of effective doping concentration(higher depletion voltage, under- depletion) III. Increase of charge carrier trapping(loss of charge) • Surface damagedue to Ionizing Energy Loss (IEL)- accumulation of charge in the oxide (SiO2) and Si/SiO2 interface – affects: interstrip capacitance (noise factor), breakdown behavior, … ! Signal/noise ratio = most important quantity ! Hamburg workshop 24-Aug-06

  6. Deterioration of Detector Properties by displacement damage NIEL Point defects + clusters Dominated by clusters Damage effects generally ~ NIEL, however differences betweenproton&neutron damage important for defect generation in silicon bulk Hamburg workshop 24-Aug-06

  7. Radiation Damage – Leakage current with time (annealing): 80 min 60C Increase of Leakage Current …. with particle fluence: • Leakage current decreasing in time (depending on temperature) • Strong temperature dependence: • Damage parameter  (slope in figure)Leakage current per unit volume and particle fluence •  is constant over several orders of fluenceand independent of impurity concentration in Si can be used forfluence measurement Consequence:Cool detectors during operation! Example: I(-10°C) ~1/16 I(20°C) Hamburg workshop 24-Aug-06

  8. …. with time (annealing): Short term: “Beneficial annealing”Long term: “Reverse annealing” - time constant depends on temperature:~ 500 years (-10°C)~ 500 days ( 20°C)~ 21 hours ( 60°C) Radiation Damage – Effective doping concentration Change of Depletion Voltage Vdep (Neff) …. with particle fluence: „Hamburg model“ “Type inversion”: Neff changes from positive to negative (Space Charge Sign Inversion) before inversion afterinversion p+ n+ n+ p+ Consequence:CoolDetectors even during beam off (250 d/y)alternative: acceptor/donor compensation by defect enginrg.,e.g. see developm. with epi-devices (Hamburg group) Hamburg workshop 24-Aug-06

  9. ….. and change with time (annealing): Radiation Damage – Charge carrier trapping Deterioration of Charge Collection Efficiency (CCE) by trapping Trapping is characterized by an effective trapping time eff for electrons and holes: where: Increase of inverse trapping time (1/) with fluence Charge trapping leads to very small le,h at Feq = 1016/cm² Consequence:Cooling does not help but:use thin detectors (~100mm) and p-type Si, listen to Gregors talk! Hamburg workshop 24-Aug-06

  10. Summary • Silicon Detectors in the inner tracking area of future colliding beam experiments have to tolerate a hadronic fluence of up to Feq = 1016/cm² • Deterioration of the detector performance is largely due to bulk damage caused by non ionizing energy loss of the particles • Reverse current increase (originating likely from both point defects and clusters) can be effectively reduced by cooling. Defect engineering so far not successful • Change of depletion voltage severe, also affected by type inversion and annealing effects. Modification by defect engineering possible, for standard devices continuous cooling essential (freezing of annealing) • Charge trapping lilely the ultimate limitation for detector application, responsible trapping centers widely unknown, cooling and annealing have little effects Hamburg workshop 24-Aug-06

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