CdZnTe Focal Plane Polarimetric Performances for a Laue Lens Point Spread Function

To date in astrophysics, X- and $\gamma$ -ray celestial emissions have been studied almost exclusively through spectral and timing variability analysis. By measuring the angle and degree of linear polarization of the detected radiation, it is possible to double the number of observational parameters, allowing better discrimination between different astrophysical models. Therefore, future $\gamma$ -ray telescope instrumentation should include the capability to perform high sensitivity polarimetric measurements. This idea has recently become even more appealing in the perspective of new very sensitive $\gamma$ -ray focusing missions equipped with a CdZnTe focal plane, such as the Gamma-Ray Imager and DUAL missions submitted to the European Space Agency's Cosmic Vision calls. By combining the most recent simulation developments and the latest experimental results we present the best polarization assessment so far of such a CdZnTe focal plane when coupled to wide band-pass Laue optics (up to 1 MeV). Firstly, the Laue lens response to different source types (Crab type and monochromatic) was simulated by a Monte Carlo code generating a characteristic PSF (point spread function). Afterwards this PSF served as the input to the focal plane mass model simulation. We have calculated the respective modulation $Q$ factor for on- and off-axis (2') sources. Finally, experimental polarimetric performances of a CdZnTe prototype operating together with Laue lens crystal samples, tested at the European Synchrotron Radiation Facility, validated the focal plane simulation results ( $Q\sim 0.30$ to 0.40, 200 keV to 600 keV range). A minimum detectable polarization of 2% $(3\sigma,10^{6}~{\rm s})$ was estimated for a 100 mCrab, type source, which is compatible with the high polarimetric sensitivity required for space $\gamma$ -ray polarimetry.