Monte Carlo simulations of stacked x-ray detectors as designed for SIMBOL-X

Simbol-X is a next generation X-ray telescope with spectro-imaging capabilities over the 0.5 to 80 keV energy range. The combination of a formation flying mirror and detector spacecraft allows to extend the focal length to 20 m, resulting in a so far unrivaled angular resolution and sensitivity in the hard X-ray range. The focal plane detector system for Simbol-X is planned to consist of an array of so-called Macro Pixel Detectors (MPD) on top of a 2 mm thick CdZnTe pixellated detector array. Photons of energy less than about 17 keV will be primarily absorbed in the MPDs, whereas higher energy photons will be detected in the CdZnTe array below. A computer model of such stacked detectors and its interaction with the radiation environment encountered by the spacecraft in orbit is currently being developed by our group using the Monte Carlo toolkit GEANT4. We present results of the simulation and an outlook for possible optimizations of future detector geometry and shielding.