Fine pitch CdTe-based Hard-X-ray polarimeter performance for space science in the 70-300 keV energy range

摘要

X-rays astrophysical sources have been almost exclusively characterizedthrough imaging, spectroscopy and timing analysis. Nevertheless, moreobservational parameters are needed because some radiation mechanisms presentin neutrons stars or black holes are still unclear. Polarization measurementswill play a key role in discrimination between different X-ray emission models.Such a capability becomes a mandatory requirement for the next generation ofhigh-energy space proposals. We have developed a CdTe-based fine-pitch imagingspectrometer, Caliste, able to respond to these new requirements. With a580-micron pitch and 1 keV energy resolution at 60 keV, we are able toaccurately reconstruct the polarization angle and polarization fraction of animpinging flux of photons which are scattered by 90{\deg} after Comptondiffusion within the crystal. Thanks to its high performance in both imagingand spectrometry, Caliste turns out to be a powerful device for high-energypolarimetry. In this paper, we present the principles and the results obtainedfor this kind of measurements: on one hand, we describe the simulation tool wehave developed to predict the polarization performances in the 50-300 keVenergy range. On the other hand, we compare simulation results withexperimental data taken at ESRF ID15A (European Synchrotron Radiation Facility)using a mono-energetic polarized beam tuned between 35 and 300 keV. We showthat it is possible with this detector to determine with high precision thepolarization parameters (direction and fraction) for different irradiationconditions. Applying a judicious energy selection to our data set, we reach aremarkable sensitivity level characterized by an optimum Quality Factor of 0.78in the 200-300 keV range. We also evaluate the sensitivity of our device at 70keV, where hard X-ray mirrors are already available; the measured Q factor is0.64 at 70 keV.