3D-CZT Polarimeter project

High-energy polarized emissions are expected in a wide variety of gamma-ray sources such as pulsars, solar flares, active galactic nuclei, galactic black holes and gamma-ray bursts, but polarimetry in this energy regime is an almost unexplored field mainly due to two facts. In the first place, the expected polarized portion of hard X/γ-rays flux from cosmic sources is in general only a small percentage of the already low incoming flux (few % to 10-20 %), and only in few cases can represent a large fraction of it (>40%). This implies that a significant detection requires instrument with very high sensitivity. 

Second, X/γ ray polarimetric measurements require the implementation of the complex of detection, electronic and signal processing systems, on-board to high-altitude balloon or satellite missions in space. Therefore, until few years ago, no dedicated hard X\γ-ray polarimetric missions have been launched into space, and X- and γ-ray source emissions have been studied almost exclusively through spectral, timing and analysis of the measured fluxes and by using imaging techniques. On the other hand, polarization measurements will increase the number of observational parameters of a γ-ray source by two: the polarization angle and the level of linear polarization. These additional parameters should allow a better discrimination between different emission models characterizing the same object. 

Polarimetry observations can provide important information’s about the geometry of the emission region, the intensity of the magnetic field, and other important parameters of the emission mechanisms. Up to now, in the soft γ-ray domain (0.1 –1 MeV), only a small number of polarimetric measurements were performed by space instruments. SPI and IBIS instruments on-board the INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) mission, have reported positive polarization observations on the Crab Pulsar, on the galactic black hole Cygnus X-1 and on some high flux gamma-ray bursts (GRB). On GRB’s other results on the same energy range have been reported recently by observations of the ASTROSAT CZT-Imager. 

Furthermore, in the last year, new polarization measurements on the Crab source have been obtained by the same CZT-Imager and by the balloon borne payload PoGO+ in the energy band between few tenths up to few hundreds of keV. The development of CdZnTe (CZT)/CdTe spectrometers with high 2D/3D spatial resolution and fine spectroscopy represent a challenge to the realization of a new class of high performance instruments, for hard X/γ-rays, able to fulfil the current and future requirements in several applications fields. The choice of CZT/CdTe spectroscopic imager as scattering polarimeter, allows optimizing the detection efficiency, due to the high atomic number of the material, and simultaneously ensuring good spectroscopic performance and high spatial resolution (in 2D or 3D). Furthermore, 3D CZT/CdTe spectro-imagers, because their high spectroscopic resolution, (few % FWHM at 60 keV and <1% above 600 keV) and the high 3D spatial resolution (0.2-0.5 mm) achievable, allow to operate not only in full energy mode, but also as Compton scattering detectors if equipped with an appropriate electronics providing a suitable coincidence logic to handle multi-hit events. Because of these possibilities, such sensors are suitable to realize wide field detector for γ-ray (> 100 keV) both in ground and space applications. 

In the framework of several funded projects (INAF,MIUR,ASI) AOB-INAF Bologna and IASF-INAF Palermo, since several years are involved, with different responsibilities levels, in the design and development of a fully functional 3D CZT spectro-polarimeter with the characteristics described above, in collaboration with other Italian and International research groups.