Experimental polarimetric study of a gamma-ray CZT focal plane prototype

A new gamma-ray telescope mission concept (GRI: Gamma Ray Imager) based on Lane focussing techniques has been proposed in reply to the ESA 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, a CZT detector prototype has been tested at the European Synchrotron Radiation Facility under a ∼100% polarised gamma-ray beam. The spectroscopic, imaging and timing performances over the energy range of 150 keV to 750 keV were studied and in particular its potential as a polarimeter was measured. Polarisation has been recognized as being a very important observational parameter in high energy astrophysics (≫100 key) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CZT array with an 11x11 active pixel matrix (pixel area of 2.5×2.5 MM
²
). The detector was irradiated by a monochromatic linearly polarised beam with a spot diameter less than 0.5 mm over the energy range between 150 keV 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 Lane monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated, for the first time, that a polarized beam does not change its polarisation after undergoing a small angle (≪1°) Lane diffraction inside a crystal.