3D‐CZT Module for spectroscopic imaging, timing and polarimetry in hard X‐/ soft gamma‐rays satellite mission
ASI-INAF call 2017 - “Attività di studio per la comunità scientifica di astrofisica delle alte energie e fisica astro-particellare”
  completed project [2018-2020]
Project Leader: Ezio Caroli (ezio.caroli@inaf.it) - INAF-OAS Bologna
Head of IMEM activities: Andrea Zappettini (andrea.zappettini@imem.cnr.it)

The project has the final goal of realizing a complete hard-X/soft-𝛾 rays detection system in the energy range 10‐1000 keV based on CZT spectroscopic detectors with sub-millimeter spatial resolution in three dimensions and a digital electronics acquisition chain. This represents an important upgrade in respect to current space instrumentations and it will be essential in the next decades to solve several still open hot scientific issues regarding the observation of cosmic ray sources (e.g., pulsars, black holes, binary systems).

The detector must fulfil contemporarily several requirements: 
  • high detection efficiency (>80% at 500 keV)
  • fine spectroscopy (1% FWHM at 511 keV)
  • high spatial resolution (<0.5 mm)
  • fine timing resolution (<1 μs)
  • scattering polarimetry capability
Moreover, high modularity and compactness are mandatory since the device must be compatible for a large variety of satellite class. The core of the detection system, realized by the combined efforts of IMEM-CNR and due2lab s.r.l., consists in four 20×20×5 mm3 CZT crystals with orthogonally segmented electrodes which have been realized in IMEM. The large volume allows to stop a large fraction of the incoming radiation in the energy range of interest. The anode layout is characterized by strip electrodes with a width of 0.15 mm and inter-strip gap of 0.25 mm whereas the cathode side is divided into ten strips with a width of 1.9 mm and inter-strip gap of 0.1 mm. The anode strips are organized in collecting strips and drift strips that are negatively biased in order to drift the photogenerated charges towards the collecting ones. The final device showed impressive performance in term of energy resolution (1.3% FWHM @661.7 keV) without any spectral correction which demonstrates the technological improvements in device fabrication that can be reached thanks to the research developed at IMEM-CNR. 
detector IMEM 3DCaTM

This drift strips configuration allows to obtain a high sensor segmentation (∼3000 voxels) with few readout channels (25). In fact, the position of the interaction of the cosmic ray with the detector can be calculated by reading the signals induced on all the electrodes.
Furthermore, the analysis of the signals allows to extract also the depth  of interaction, thus realizing an actual 3D detector. The capability of reading the signal from the anodes, cathodes and drifting electrodes combined with digital pulse processing guarantees a large flexibility to different operative conditions and to further improve the spectroscopic performance of the device.

In summary, the prototype could be the basic element for the realization of high performance detectors in spectroscopy, imaging, timing and polarimetry both as focal planes on space telescopes implementing new high energy optics (e.g. Broadband Laue lens), and for small wide field instruments to be used in the future on clusters of micro satellites.