Polarimetric performance of a Laue lens gamma-ray CdZnTe focal plane prototype

A gamma-ray telescope mission concept [gamma ray imager (GRI)] based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning (Cosmic Vision 2015-2025). In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an ∼100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (>100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11x11 active pixel matrix (pixel area of 2.5x2.5 mm2). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (<1 deg.) Laue diffraction inside a crystal