Characterisation of a GaAs(Cr) Medipix2 hybrid pixel detector

A semi-insulating GaAs(Cr) sensor of 300 μm thickness with a bulk resistivity of 109 Ω cm and no guard ring structures has been bump bonded to a Medipix2 readout chip. The room temperature spectroscopic response of this device to fluorescence photons in the energy range from 8 to 28 keV at different sensor biases is measured and compared to the response of a reference detector with a 300 μm thick Si sensor. The measured responses show that the full volume of the GaAs(Cr) is active and indicate a charge collection efficiency of 90%. The spatial resolution if this device is 8 lp/mm.