Monte Carlo polarimetric simulations of a hard x-ray energy telescope

The polarisation of astrophysical source emissions in the energy range from a few tens of keV up to the MeV region is an almost unexplored field of high energy astrophysics. In order to improve the capabilities of performing polarimetric studies of hard X and soft gamma ray sources through Compton polarimetry, a CdTe based telescope (CIPHER: Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position sensitive spectrometer made of four modules of 32x32 individual pixels, each with a surface area of 2x2 mm2, corresponding to about 160 cm2 active detection area. This detector, due to its intrinsic geometry, could allow efficient polarimetric measurements to be made between 100 keV and 1 MeV. In order to predict the polarimetric performance and to optimise the design and concept of the CIPHER detection plane, a Monte Carlo code based on GEANT4 library modules was developed to simulate the detector behaviour under a polarised photon flux. The Compton double event efficiency, as well bi-dimensional double event distribution maps and the corresponding polarimetric modulation factor will be presented and discussed. Modulation (Q) factors better than 0.30 and double event total efficiencies over 10 % for an energy range from 100 keV to 1 MeV have been obtained.