Investigating the DQE of the Medipix detector using the multiplicity concept

Semiconductor photon counting pixel detectors like the Medipix detectors will increasingly be used in X-ray imaging. They consist of a sensor layer which allows the impinging photon to deposit energy in form of charge carriers drifting in an electric field to the pixel electrodes. For an energy deposition above an adjustable discriminator threshold a counter is incremented in the corresponding pixel. One photon can be counted in several pixels due to processes like charge sharing. This has an impact on the imaging properties, assessed for example in terms of the detective quantum efficiency DQE(f) as a function of spatial frequency. In this contribution we discuss the impact of multiple counting on the zero frequency DQE in dependence of photon energy and discriminator threshold energy. The multiplicity framework is applied in simulations and measurements to investigate the influence of multiple counting on the DQE at zero spatial frequency of the Medipix2 detector. Based on our considerations an optimum threshold setting can be determined.