Polarization degree and vector angle effects on a CdZnTe focal plane performance

To date in astrophysics, X- and gamma-ray source emissions have been studied almost exclusively through spectral and timing variability analysis. However, this analysis often allows two or more distinct models capable of explaining the same observations of a particular celestial radiation source. By measuring the polarization angle and degree of linear polarization of the source emission, it is possible to double the number of observational parameters thereby allowing better discrimination between different models. Therefore, future γ-ray telescope main instruments should allow performing polarimetric measurements. This idea has recently become even more appealing in the perspective of new hard X- and soft gamma-ray focusing instrumentation implementing Laue lenses, in particular in the framework of the preparation of a mission concept based on Laue lens - the GRI (Gamma-Ray Imager) Mission - submitted to the ESA Cosmic Vision first call. In order to optimize the design of a focal plane of this hard X- and soft γ-ray telescope, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under a ~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. The aim of these tests was to study two different effects on the polarimetric performance caused by the polarization state of a beam on a CdZnTe instrument. In particular, we intended to determine the sensitivity of the detector to the level of the beam polarisation by changing its polarization degree and to analyse the modulation of the Q factor as a function of the polarization vector angle orientation. The results obtained will be presented and discussed.