Comparison of simulated and measured energy response spectra for a Medipix2 detector using CdTe as sensor material

The new generation of photon counting pixel X-ray detectors like the Medipix2 and the Medipix3 does not measure energy deposition directly. Instead the measured observable is the number of photons, which deposited an amount of energy larger than a given threshold. The implementations in the simulation are the physics processes in the sensor layer, diffusion and repulsion of charge carriers during drift and lifetime of the charge carriers, all taking into account intrinsic doping of the sensor material. While the charge carriers drift, mirror charges are induced at the electrodes leading to a signal even if not all of the electrons and holes reach the electrode. This results in a partial charge collection and therefore has an impact on the energy response. Noise is modelled with Fano noise during energy deposition, and several noise contributions of the pixel electronics. Furthermore, the charge summing mode of the Medipix3 is implemented. 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. This contribution compares simulated spectra from this detector class with measured spectra from a Medipix2-MXR ASIC bump-bonded to a CdTe sensor with different pixel and electrode sizes. Due to the shift of the photo peak caused by the limited lifetime of the charge carriers an improved energy calibration is needed to compare the energy response spectra.