Space-based gamma-ray spectrometers utilize active anticoincidence shielding to reduce the background caused by charged-particle interactions. Shielding improves the performance of gamma-ray spectrometers by reducing the effect of charged particle interactions which can not be distinguished from true gamma-ray interactions by the spectrometer. Active shields produce a blanking signal when a charged particle is detected, so that the signal from the spectrometer can be ignored during the spectrometer's charged-particle interaction. Anticoincidence shielding for space-born gamma-ray detectors requires a cylindrical-shell geometry and charged-particle sensitivity. To reduce the size, weight, and cost of the shielding we utilize a new direct-conversion charged-particle detector material, polycrystalline mercuric iodide. We present the results from planar film growth techniques for the particle-counting detection capabilities necessary for anti-coincidence shielding. We also show that films with similar detection properties were grown on curved substrates with the size and curvature needed to surround space-based spectrometer main detectors.
Mercuric iodide (HgI<sub>2</sub>) polycrystalline films are being developed as a new detector technology for digital x-ray imaging. Films have been grown with areas up to 80 cm<sup>2</sup> (4' diameter) and thickness of 20-250 micrometers using sublimation. The growth techniques used can be easily extended to produce much larger film areas (>10'x10'). Thickness of the grown layers and size of the grains can be regulated over a wide range by adjusting the growth parameters. The films were characterized with respect to their electrical properties and in response to ionizing radiation. Leakage current as low as 40 pA/cm<sup>2</sup> at the operating bias voltage of ~50 V has been observed. High sensitivity and excellent linearity in the response to x-rays was measured. Signals from these HgI<sub>2</sub> polycrystalline detectors, in response to ionizing radiation, compare favorably to the best published results for all high Z polycrystalline films grown elsewhere, including TlBr, PbI<sub>2</sub> and HgI<sub>2</sub>. The low dark current, good sensitivity, and linearity of the response to x-rays put HgI<sub>2</sub> polycrystalline semiconductor detectors in position as a leading candidate material for use in digital x-ray imaging systems. Our future efforts will concentrate on optimization of film growth techniques specifically for deposition on a-Si:H flat panel readout arrays.