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

摘要

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 Imaging 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 32*32 individual pixels, each with a surface area of 2*2 mm~2, corresponding to about 160 cm~2 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 optimize 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.