Polarimetric performance of a prototype hard X-ray telescope

Polarimetry in astronomy for the hard X- and soft /spl 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 micro-spectrometers 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/spl times/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 double event efficiency and distribution, for beams of nearly 100% linearly polarised 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.