Measurements with Si and GaAs pixel detectors bonded to photon counting readout chips

Detectors fabricated with SI-GaAs and Si bulk material were bonded to Photo n Counting Chips (PCC), developed in the framework of the MEDIPIX Collabora tion. The PCC consists of a matrix of 64 x 64 identical square pixels (170 mum x 170 mum) with a 15-bit counter in each cell. We investigated the imag ing properties of these detector systems under exposure of a dental X-ray t ube at room temperature. The image homogeneity and the mean count rate were determined via hood exposure images and compared. Exposures for GaAs detec tors exhibit a 3 times larger spread in count rate per image in comparison to Si detectors. This also results in a 3 times worse signal to noise ratio . IV-characteristics and X-ray images at different values of the detectors bias voltage were also taken and show a 30 times higher leakage current for GaAs, The Si detector is fully active beginning from 70 V, whereas the GaA s detector does not reach full charge collection. The presampling modulatio n transfer function of both assembly types was measured via slit images and gives a spatial resolution of 4.3 lp/mm for both detector systems. (C) 200 1 Elsevier Science B,V. All rights reserved.