Monte Carlo simulation of pixelated photon counting X-ray detectors like the Medipix2 and the Medipix3 using high-Z sensor materials

The new generation of pixelated photon counting X-ray detectors like the Medipix2 and the Medipix3 does not measure energy deposition directly. Instead of this the measured observable is the number of photons, which have deposited an energy larger than a given threshold. To understand the response of these detectors we developed a new detector class to be used in the Monte-Carlo-Simulation ROSI, which is based on the C++ library LSCAT-GISMO, including the well-established EGS4 algorithms with its low energy extension LSCAT. The implementations in the detector class are the physics processes in the sensor layer, diffusion and repulsion of charge carriers during their drift and lifetime of the charge carriers, all taking into account intrinsic doping of the sensor material. The drift of charge carriers induces mirror charges at the electrodes leading to a signal even if not all electrons and holes reach the electrode. This results in partial charge collection and therefore has an impact on the energy resolution. The noise is modelled with Fano noise during the energy deposition, and several noise contributions of the pixel electronics. Further, the charge summing mode of the Medipix3 is available in the detector class. Due to the hybrid design of the Medipix detectors several combinations of ASIC and sensor are possible. With the implemented physics it is possible to simulate high-Z sensor materials like CdTe or GaAs in addition to silicon. The detector class allows an arbitrary positioning and orientation of the detector with respect to the incoming beam. Therefore it is possible to simulate complex imaging systems containing several units of pixelated photon counting X-ray detectors taking into account a detailed implementation of the processes inside each detector unit.