Polycrystalline mercuric iodide films for novel detector applications

Polycrystalline mercuric iodide (Hgl₂) films have been in use as direct-conversion detectors for a number of applications including X-ray, gamma-ray, and charged-particle detectors. One application for particle detectors is as an anticoincidence detector used in conjunction with a large-volume scintillator employed as gamma-ray spectrometer. For space- based applications, background high-energy protons interact with the scintillator to produce spurious signals which interfere with the primary gamma-ray data. The shield detector produces a veto signal so that those detection events in the spectrometer which are due to high-energy charged particle interactions can be ignored, improving the quality of the gamma-ray spectrum collected. We are optimizing a sublimation-based film growth process to grow large-area Hgl₂ active anticoincidence shield detectors directly on planar substrates. The growth process is quick and relatively inexpensive, and results in a shield detector that is just a few hundred microns in thickness, thus saving cost and valuable space and weight for the total instrument package. We present recent results from polycrystalline Hgl₂ films used for charged-particle detection. These results demonstrate that a novel active anticoincidence shield based on polycrystalline mercuric iodide can act as an alternative to a scintillator/PMT design.