Cadmium Zinc Telluride (CZT ) is a high Z, large bandgap, semiconductor which became standard hard X-ray detectors in many fields. The material has good electron transport properties, and the spectroscopic performance surpasses that of regular scintillators. By adding othogonal strip electrodes on two faces, or an array of pixel electrodes on one face, a single wafer can be segmented into a position sensitive detector for use in an imaging telescope. A vigorous program of CZT development (see Papers) is being carried out by several groups (Harvard, GSFC, Caltech and UCSD) within the EXIST collaboration. At the Harvard-Smithsonian Center for Astrophysics, we are developing pixellated CZT detectors for EXIST under a balloon-borne hard X-ray survey project - ProtoEXIST. ProtoEXIST is a pathfinder for EXIST in both science and technology.
In order to build a large arrays of CZT detectors for EXIST, we take the following modularization scheme. First, we start with 2x2x0.5 cm3 CZT crystal bonded onto 2 of 128 readout ASICs possibly through an interposer board. This basic building block is called a Detector-Crystal-Unit (DCU). Second, we combine a 2x4 array of DCUs into a Detector-Crystal-Array (DCA), which is 4x8 cm2. Third, we put 7 DCAs into a Detector Module (DM), which is 8x28 cm2. Then, 2x7 array of DMs make a sub-telescope detector plane (56x56 cm2).
In ProtoEXIST1 we are building 1000 cm2 CZT detectors with 2.5 mm pixels, and in ProtoEXIST2 , we plan to refine the pixel size to 1.25 mm. The current batch of DCUs (2006) exhibit about 4.5 keV FWHM resolution at 60 keV (on average and 2% at 356 keV, see below), which meets the requirements for EXIST CZT detectors. We can improve the resolution down to 3 keV in the upcoming batch, and we plan to achieve 2 keV FWHM for EXIST CZT detectors.