Investigation of GaAs:Cr Timepix assemblies under high flux irradiation

High resistivity, chromium compensated gallium arsenide (HR-GaAs: Cr) has recently shown to be a promising sensor material for X-ray detectors due to its high resistivity, its fully active volume, the good electron transport properties and good absorption properties for photon energies up to around 40 keV. These properties make this material a favorable candidate for producing sensors for photon counting X-ray imaging detectors. Such detector systems have also gained increasing attention to be used for high flux applications as found at synchrotrons or in medical applications like computed tomography. Whereas other interesting high-Z materials such as CdTe have already been studied under high flux irradiation, the behavior of HR-GaAs: Cr under such conditions has yet to be investigated. In this work, we study the performance of a 500 m m thick GaAs: Cr sensor, bump-bonded to Timepix readout electronics, with a main emphasis laid on the behavior under the irradiation of high X-ray fluxes using a monochromatic synchrotron beam. We find that, at high X-ray fluxes, the detector performance degrades, and that the dislocation network typically present in melt-grown GaAs influences the detection properties due to local variations of the charge transport properties. However, we also demonstrate that the high flux behavior can be substantially improved by optimized chip settings, namely by tuning the Timepix' preamplifier output pulse shape. In this way, the photon counting capabilities of the detector assembly are restored to a certain extent and it is possible to operate the detector assembly at impinging fluxes as high as 1010 s(-1) mm(-2).