Hard X- and soft gamma-ray polarimetry with CdTe array prototypes

Up to date there have been very few developments in the field of polarimetry in hard X- and soft γ-ray astronomy, even if this measurement can give unique information about the geometry and the processes responsible of high energy emission from cosmic sources. We have recently proposed an instrument devoted to polarimetric measurements in this energy range known as the Coded Imager and Polarimeter for High Energy Radiation (CIPHER), a telescope which uses a detection plane optimized for the detection of Compton scattered events. In order to study the response of this polarimeter concept to linearly polarized radiation in the 100 keV to 1 MeV energy range, we have prepared an experiment (POLCA: POLarimetry with CdTe Arrays) based on CdTe detector prototypes of 4×4 pixels. The use of a beamline at the European Synchrotron Radiation Facility (ERSF) allowed us to study the Compton double-event efficiency and distribution, for beams of nearly 100% linearly polarized radiation in the energy range from 100 keV to 400 keV. We have obtained polarimetric modulation factors higher than 0.4 with double-event efficiencies higher than 20%. The photon beam energy and detector thickness dependencies will be discussed, comparing these experimental results with those calculated using Monte Carlo simulations.