Hard X-ray polarimetry with Caliste, a high performance CdTe based imaging spectrometer

摘要

Since the initial exploration of soft gamma-ray sky in the 60's, high-energycelestial sources have been mainly characterized through imaging, spectroscopyand timing analysis. Despite tremendous progress in the field, the radiationmechanisms at work in sources such as neutrons stars and black holes are stillunclear. The polarization state of the radiation is an observational parameterwhich brings key additional information about the physical process. This is whymost of the projects for the next generation of space missions covering thetens of keV to the MeV region require a polarization measurement capability. Akey element enabling this capability is a detector system allowing theidentification and characterization of Compton interactions as they are themain process at play. The hard X-ray imaging spectrometer module, developed inCEA with the generic name of Caliste module, is such a detector. In this paper,we present experimental results for two types of Caliste-256 modules, one basedon a CdTe crystal, the other one on a CdZnTe crystal, which have been exposedto linearly polarized beams at the European Synchrotron Radiation Facility.These results, obtained at 200-300 keV, demonstrate their capability to give anaccurate determination of the polarization parameters (polarization angle andfraction) of the incoming beam. Applying a selection to our data set,equivalent to select 90 degrees Compton scattered interactions in the detectorplane, we find a modulation factor Q of 0.78. The polarization angle andfraction are derived with accuracies of approximately 1 degree and 5%. Themodulation factor remains larger than 0.4 when essentially no selection is madeat all on the data. These results prove that the Caliste-256 modules haveperformances allowing them to be excellent candidates as detectors withpolarimetric capabilities, in particular for future space missions.