Experimental polarimetric study of a hard X-ray energy telescope prototype

Polarimetry in astronomy for the hard X- and soft (gamma)-ray energy range has not been developed much to date. Indeed no dedicated polarimeters have ever been launched either in space or as a balloon-borne experiment. We propose a polarimetry project for hard X-ray and soft gamma-ray astrophysics known as the Coded Imager and Polarimeter for High Energy Radiation (CIPHER) telescope. The novel design of this telescope, using a matrix of thick CdTe microspectrometers as the detection plane, will allow this type of measurement to be performed for the first time by a dedicated instrument. In order to compare the results already obtained from simulations with experiment, we have constructed a prototype detector of 4*4 pixels, to study the response of this concept of polarimeter to linearly polarized radiation in the 100 keV to 1 MeV energy range. The use of a beamline at the European Synchrotron Radiation Facility (ERSF) allowed us to study double event efficiency and distribution, for beams of nearly 100% linearly polarized radiation in the energy range from 100 keV to 1 MeV. Polarimetric Q factors higher than 0.4 for relative efficiencies higher than 20% were observed. The photon beam energy and detector thickness dependencies will be discussed, comparing these experimental results with those calculated by Monte Carlo simulations.