Characterization of GaAs:Cr Sensors for High Flux X-Ray Imaging Applications M. C. Veale1, S. J. Bell1,2, D. D. Duarte1,2, M. J. French1, M. Hart1, A. Schneider1, P. Seller1, M. D. Wilson1, G. Georgiou2, P. J. Sellin2, A. D. Lozinskaya3, V. A. Novikov3, O. P. Tolbanov3, A. Tyazhev3 and A. N. Zarubin3 STFC Rutherford Appleton Laboratory, Oxfordshire. OX11 0QX Faculty of Engineering and Physical Sciences, University of Surrey. GU2 7XH. Siberian Physical-Technical Institute of Tomsk State University, Tomsk, Russia

Outline Why GaAs:Cr? Material Characterization Initial X-Ray Performance 1mm Thick Devices Dislocations

Large Pixel Detector (LPD) 52cm LPD detector for XFEL Based on Hamamatsu Si 500 mm thick sensors 500 mm x 500 mm pixels Final system 1M pixels Expected lifetime dose >1MGy. M. Hart et al. “Development of the LPD, a high dynamic range pixel detector for the European XFEL”. IEEE NSS 2012 (534-537)

Silicon Radiation Damage A single LPD module Diamond B16 Beamline 12 keV irradiation 4x1010 photons s-1 mm-2 Pixel currents increased Radiation damage D A

Chromium Compensated GaAs N-type GaAs Evaporation of Cr Annealing GaAs:Cr G. I. Azyenshtat t al. NIMA 466 25-32 (2001)

Materials Characterization 3 Simple Planar devices + Guard Ring. Ni/GaAs/Ni contacts. 7.0 x 7.0 x 0.5 mm3 Current-Voltage (IV) measurements show quasi-ohmic response. -150nA @ - 200V. r = 2.4x109 W cm @293K

Electron Transport Alpha particle spectroscopy used to measure mete. Detectors held under vacuum and bias varied. Spectra collected for 300s. tshape = 2 ms. Count rate ~ 10 cps. Peak centroids fitted with the SC Hecht equation. mete ~ 1x10-5 cm2 V-1.

Hole Transport Spectra collected for 900s. Count rate ~ 2 cps. Reduction in count rate despite identical conditions. Odd behaviour at high voltages. Reduction in peak centroid with increased voltage?! mhth <= 5x10-6 cm2 V-1.

Initial Spectroscopic Tests 3x 500 mm thick devices. Assembly of detectors at RAL. Low-T Ag epoxy bonding. Bonded to HEXITEC ASIC. 80 x 80 pixels, 250 mm pitch. Fully spectroscopic readout. Tested using HEXITEC DAQ. M. Veale et al. Nuclear Instruments and Methods A, 752, July 2014.

500mm Device Performance 300K Resistivity ~ 3 x 109 W cm Vbias = - 300 V Leakage current (4cm2) = 2 mA Electron sensing FWHM@60keV = 3.0 +/- 0.5 keV

New 1mm Thick Devices

Non-Uniformities HEXITEC (250mm) 1mm Thick Medipix (55mm) 0.5mm Thick* *E. Hamann t al. R07-02

Microbeam Line Scans Diamond B16 Beamline. 15keV Monoenergetic X-ray. Line scans over pixels. GaAs step size 10 mm. CdTe step size 25 mm . Processed using charge sharing discrimination. Observed 1st, 2nd & 3rd harmonics.

Microbeam Line Scans 15 keV counts compared at each scan position. Variation in counts is evidence for sub-pixel structure. sCdTe = 1.3 % sGaAs = 12.2 % Suggests greater non-uniformity in GaAs:Cr on the 10 mm scale.

Field Non-Uniformities Spectroscopic data used to study non-uniformities. No variation in FWHM observed! Only the number of counts per position varies. Suggestive of lensing effects around dislocations. Consistent with previous observations in CdTe. B C A

Next Steps Strip detectors fabricated 50 mm and 100 mm pitch 512 and 256 channels Wire bonded to XCHIP ASICs Readout using Ultra system Maximum flux = 109 s-1 mm-2 High flux measurements! 100 mm 50 mm

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No Correction: Highly Uniform Response! GaAs:Cr Performance 6,400 Pixels No Correction: Highly Uniform Response!

GaAs:Cr Performance Map of X-Ray Counts

GaAs:Cr Performance Spectroscopic Data Contains Material Information

Radiation Tollerance GaAs B16 beam line 108 – 109 photons s-1 mm-2 12 keV mono 8 irradiation areas Dose rate ~ 10 Gy s-1 Total dose ~ 1 MGy Cu Slits

Flux Calculation (1) (8) Irradiation area varied Areas: 35.75 – 3.25 mm2 Leakage current measured A = 35.75 mm2 t = 10 s A = 3.25 mm2 t =10,000 s s < 0.5 % Increasing Dose

Flux Calculation (1) (8) Irradiation area varied Areas: 35.75 – 3.25 mm2 Leakage current measured Fit to Current Vs. Area W-Value = 4.2 eV ehp-1 X-ray flux ~ 3 x 108 s-1 mm-2 Si diode ~ 5 x 108 s-1 mm-2 A = 35.75 mm2 t = 10 s A = 3.25 mm2 t =10,000 s

Dose Calculation Energy = 12 keV l12keV = 11 mm Volume = Area x l Area = 35.75 – 3.25 mm2 Flux = 3 x 108 s-1 mm-2 Time = 101 – 105 s Density = 5.32 g cm-3 Area Doses = 102 – 106 Gy X-Rays Dose 𝑫𝒐𝒔𝒆 𝑮𝒚 = 𝑭𝒍𝒖𝒙×𝑨𝒓𝒆𝒂×𝑬𝒏𝒆𝒓𝒈𝒚×𝑻𝒊𝒎𝒆 𝑫𝒆𝒏𝒔𝒊𝒕𝒚×𝑽𝒐𝒍𝒖𝒎𝒆

Irradiation Results TOTAL DOSE ~ 1 MGy NO CHANGE

Flux Calculation - Photons Cu absorber placed in beam Spectroscopic data collected Single photon counting 12 keV peak observed Small Cu florescence signal Flux per pixel calculated X-ray flux ~ 3 x 108 s-1 mm-2