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Radiation hardness of GaAs Sensors. K. Afanaciev, Ch. Grah, A. Ignatenko, W. Lange, W. Lohmann, M. Ohlerich. Very Forward Region of the ILC Detector. BeamCal. Interaction point. EM calorimeter with sandwich structure: 30 layers of 1 X 0 3.5mm W and 0.3mm sensor
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Radiation hardness of GaAs Sensors . K. Afanaciev, Ch. Grah, A. Ignatenko, W. Lange, W. Lohmann, M. Ohlerich FCAL collaboration meeting. Krakow.
Very Forward Region of the ILC Detector BeamCal Interaction point EM calorimeter with sandwich structure: 30 layers of 1 X0 • 3.5mm W and 0.3mm sensor Angular coverage from 5 mrad to 28 mrad Moliére radius RM ≈ 1cm Segmentation between 0.5 and 0.8 x RM • The purpose of the instrumentation of the very forward region is: • Hermeticity: increase the coverage to polar angles > 5mrad • Fast beam diagnostics FCAL collaboration meeting. Krakow.
The Challenges for BeamCal Creation of beamstrahlung at the ILC e- e+ Interaction Bethe-Heitler process e+e- pairs from beamstrahlung are deflected into the BeamCal 15000 e+e- per BX => Edep 10 – 20 TeV ~ 5 MGy per year strongly dependent on the beam and magnetic field configuration => radiation hard sensors Detect signatures of single high energetic particles on top of the background. => high dynamic range/linearity ≈ 1 MGy/y ≈ 5 MGy/y For 1 layer, per cell FCAL collaboration meeting. Krakow.
Irradiation facility Superconducting DArmstadt LINear ACcelerator Technical University of Darmstadt V.Drugakov 2X0 Energy spectrum of shower particles in BeamCal Irradiation up to several MGy using the injector line of the S-DALINAC: 10 ± 0.015 MeV and beam currents from 2 to 100 nA corresponding to doses about 10 to 600 kGy/h FCAL collaboration meeting. Krakow.
Irradiation facility FCAL collaboration meeting. Krakow.
The material Ga Gallium arsenide (GaAs) Compound semiconductor, direct bandgap Two sublattices of face centered cubic lattice (zinc-blende type) . As GaAs Si Diamond Density 5.32 g/cm3 2.33 3.51 Pair creation E 4.3 eV/pair 3.6 13 Band gap 1.42 eV 1.14 5.47 Electron mobility 8500 cm2/Vs 1350 1800 Hole mobility 400 cm2/Vs 450 1200 Dielectric const. 12.85 11.9 5.7 Radiation length 2.3 cm 9.4 18.8 Ave. Edep/100 m (by 10 MeV e-) 69.7 keV 53.3 34.3 Ave. pairs/100 m 13000 9200 3600 Structure p-n or insul. p-n insul. FCAL collaboration meeting. Krakow.
The material Supplied by FCAL group at JINR Produced by Siberian Institute of Technology, Tomsk Sample is semi-insulating GaAs doped by Sn (shallow donor) and compensated by Cr (deep acceptor). This is done to compensate electron trapping centers and provide i-type conductivity. Sample works as a solid state ionisation chamber Structure provided by metallisation (similar to diamond) 500 m thick detector is divided into 87 5x5 mm pads and mounted on a 0.5mm PCB with fanout Metallisation is V (30 nm) + Au (1 m) 2 samples FCAL collaboration meeting. Krakow.
Methodology. Irradiation • Irradiation under bias voltage • Monitoring of beam and sample currents, sample temperature Beam exit window of beam line collimator (IColl) Faraday cup (IFC, TFC) sensor box (Is, Ts, HV) FCAL collaboration meeting. Krakow.
Methodology. CCD Setup PA Sr90 sample ADC delay Sr90 source Scint. discr PM1 & Gate discr PM2 Preamplifier Sensor box Trigger box Typical spectrum of GaAs sensor FCAL collaboration meeting. Krakow.
Characterization: I-V and C-V Constant pad capacity no dependence on V => no structure Pad capacity about 12 pF Almost linear IV characteristics => resistor Rpad 500 MOhm Pad parameters homegenious FCAL collaboration meeting. Krakow.
Characterization: signal S8 pad4 ring 4 Clear separation of peaks from Sr90 source S8 pad4 ring 6 Quite homogeneous response over different pads Saturation of signal @ about 200V bias Collection efficiency 60% FCAL collaboration meeting. Krakow.
GaAs. Irradiation GaAs2 before irradiation Samples 7&8 (GaAs1, GaAs2) pad4, ring6 @ 200V Absorbed doses 0.85 and 1.5 MGy GaAs2 after irradiation FCAL collaboration meeting. Krakow.
GaAs. Irradiation results CCD GaAs1 pad4 ring6, 0.85 MGy IV CCE dropped sharply after irradiation but signal is visible Signal increases with bias voltage FCAL collaboration meeting. Krakow.
GaAs. Irradiation results CCD GaAs2 pad4 ring6, 1.5 MGy IV Results: CCE dropped to about 6% from 60% (by ~ 90%) but signal is still visible for absorbed dose of about 1.5 MGy Dark current increased 2 times (from 0.4 to 1 A @ 200V) FCAL collaboration meeting. Krakow.
GaAs. Neighbor pads GaAs2 pad5 ring6 Double peak - result of a partial exposure of neighbor pads pad size 5x5 mm vs. collimator size 10x10 mm Spectra measured a few weeks after the irradiation => no defect diffusion on this timescale at room temperature. FCAL collaboration meeting. Krakow.
Conclusion • Signal still visible up to absorbed doses of 1.5 MGy • Good homogeneity over the sensor area • Predictable behavior under irradiation • Signal could still be increased with increasing bias voltage • New samples with higher radiation hardness are received by • Zeuthen => new beamtest is needed. • Irradiated samples are returned to manufacturer for the • radiation damage investigation. • At the moment GaAs proved to have radiation hardness • close to the requirements of the BeamCal, but an improvement • is still needed. • Could be considered as one of the main candidates • for the BeamCal prototype. FCAL collaboration meeting. Krakow.
