Monte Carlo simulations of the design of a hard X-ray telescope optimised for polarimetry

To date polarimetry in astrophysics in the energy domain from hard X-rays up to soft gamma-rays has not been pursued due to the difficulties involved in obtaining sufficient sensitivity. Indeed for those few instruments which are capable of performing this type of measurement (eg the COMPTEL instrument on the Compton Gamma-ray Observatory and soon also the IBIS instrument on INTEGRAL), polarimetry itself plays a secondary role in the mission schedule, as the efficiencies (0.5% and 10% maximum respectively) and polarimetric Q factors (0.1 and 0.3, respectively) are relatively limited. In order to perform efficient polarimetric measurements for hard X and soft gamma-ray sources, with an instrument of relatively robust and simple design, 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 comprising four modules of 32x32 individual pixels, each with a surface area of 2x2 mm(2) (about 160 cm(2) total detection area). The polarimetric performance and design optimisation of the CIPHER detection surface have been studied by use of a Monte Carlo code based on GEANT4 modules. Simulations show that we can achieve a Q factor better than 0.5 and Compton double event efficiency better than 11% between 100 keV and 1 MeV. Herein we will present and discuss the general problems that affect polarimetric measurements in space such as the inclination of the source with respect to the telescope optical axis and space background radiation. Q factor calculations for several beam inclinations as well as for space background together with simulated astronomical sources will be presented and discussed.