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We report the room-temperature α-particle response of a detector fabricated from high-purity polycrystalline mercuric iodide thin films. The films are deposited by physical vapor technique using multipassed zone refined high-purity starting material. Films approximately 15 μm thick were deposited on indium-tin-oxide (ITO) coated glass substrate and were shown to be reasonably compact and have uniform surface morphology by cross-sectional scanning electron microscopy study. Photoluminescence at 10 K showed two distinct peaks at 533 nm and around 560 nm. A previously reported peak at ∼620 nm, mainly attributed to the presence of impurities, was not present. Room-temperature α-particle response measurements were carried out under vacuum and demonstrated that efficient charge collection can be obtained with thin polycrystalline HgI2. We have demonstrated that a full energy peak is developed, although first indications are that the performance is significantly poorer than a silicon charged particle detector. Charge collection was found to improve with voltage, as expected.