The hard X-ray response of epitaxial GaAs detectors

We report on hard X-ray measurements with two epitaxial GaAs detectors of a ctive areas 2.22 mm(2) and thicknesses 40 and 400 mu m at the ESRF and HASY LAB synchrotron research facilities. The detectors were fabricated using hi gh-purity material and in spite of an order of magnitude difference in depl etion depths, they were found to have comparable performances with energy r esolutions at - 45 degrees C of similar to 1 keV ffhm at 7 keV rising to si milar to 2 keV fwhm at 200 keV and noise floors in the range 1-1.5 keV. At energies < 30 keV, the energy resolution was dominated by leakage current a nd electromagnetic pick-up, while at the highest energies measured, the res olutions approach the expected Fano limit (e.g., - 1 keV near 200 keV. Both detectors are remarkably linear, with average rms non-linearities of 0.2% over the energy range 10-60 keV, which, taken in conjunction with Monte-Car lo results indicate that charge collection efficiencies must be in excess o f 98%. This is consistent with material science metrology which show that t he material used to produce them is of extremely high purity with impurity concentration < 10(13) cm(-3). (C) 2000 Elsevier Science B.V. All rights re served.