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.
Jan Iwanczyk, Bradley Patt, Carolyn Tull, Lawrence MacDonald, Nathan Skinner, Edward Hoffman, Laura Fornaro, Luis Mussio, Edgardo Saucedo, Alvaro Gancharov
Mercuric iodide (HgI2) polycrystalline films are being developed as a new detector technology for digital x-ray imaging. Films have been grown with areas up to 80 cm2 (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/cm2 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 HgI2 polycrystalline detectors, in response to ionizing radiation, compare favorably to the best published results for all high Z polycrystalline films grown elsewhere, including TlBr, PbI2 and HgI2. The low dark current, good sensitivity, and linearity of the response to x-rays put HgI2 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.
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