Polarimetry study with a CdZnTe focal plane detector

Polarimetry has been recognized as a very important observational parameter for high energy astrophysics (>100 keV). Therefore the capability to perform accurate polarimetric measurement of high energy cosmic sources should be included in future space mission proposals. This idea become very appealing in the last three years in the development of a new telescope mission concept based on the Laue focusing technique - the Gamma Ray Imager mission - for the next call for mission ideas in the ESA Cosmic Vision 2015-2025 plan framework. In order to optimize the design of a focal plane for this mission as Compton scattering hard X- and soft ¿-ray polarimeter, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under an ~100% polarized gamma-ray beam. The prototype detector tested is a 5 mm thick CdZnTe array with the anode segmented into 16×16 pixels of 2.5×2.5 mm². One of the most important factors that can affect significantly the polarimetric measurements is the direction of the incoming photons from the polarised radiation source with respect to the focal plane of the polarimeter. We have studied and analyzed the polarimetric response of the CZT prototype as a function of the direction of a polarized beam forming different inclination angles with the optical axis of the detector: 0°, 0.5°, 1°, 1.5°, 2°, 3°, 4°, 5° and 10°.