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

There have been very few developments in the field of polarimetry in hard X- and soft γ-ray astronomy, indeed not one dedicated polarimeter has ever been launched either as a satellite or balloon borne experiment. A recent result from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) solar observatory suggests that, at least in gamma-ray bursts, high energy emission may be strongly polarised. 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 optimised for the detection of Compton scattered events. In order to compare the results already obtained from simulations with experimental data, we have prepared an experiment (POLCA: POLarimetry with CdTe Arrays) based on a prototype detector of 4 × 4 pixels, to study the response of this concept of polarimeter to linearly polarised 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 the Compton double-event efficiency and distribution, for beams of nearly 100% linearly polarised radiation in the energy range from 100 keV to 400 keV. Polarimetric Q factors higher than 0.4 and double events relative efficiencies higher than 20% were obtained. The photon beam energy and detector thickness dependencies will be discussed, comparing these experimental results with those calculated by suitable Monte Carlo simulations.