Polycrystalline mercuric iodide photodetectors coupled to scintillators

We are developing polycrystalline mercuric iodide (HgI₂) films for novel use as optical photo detectors. Single crystal HgI₂ operated as a photo detector has demonstrated a full width at half maximum (FWHM) energy resolution of 4.58% at 662 keV when coupled to a thallium doped cesium iodide (CsI[Tl]) scintillator irradiated with cesium-137 (¹³⁷Cs). However, HgI₂ single crystal fabrication requires a lengthy growth time followed by post-growth processing and special packaging and it is therefore cost prohibitive to use them in large field of view applications. In contrast, polycrystalline films of HgI₂ can be fabricated quickly and cheaply on a variety of surfaces including curved ones. Polycrystalline HgI₂ films with thicknesses of about 200 and 400 mum have been deposited by chemical vapor deposition onto indium-tin-oxide (ITO) coated glass substrates. A 7 mm by 7 mm thin film of palladium (Pd) was deposited by thermal evaporation onto the top of the polycrystalline HgI₂ films. The ITO and Pd contacts are biased up to 80 V and a very low dark current, typically about 10 pA/mm², is obtained. A CsI(Tl) scintillator, 7 mm by 7 mm and 4 mm thick, is coupled with optical grease to the back side of each glass substrate and irradiated with ¹³⁷Cs. The biased polycrystalline HgI₂ photodetector is operated with a custom charge sensitive preamplifier connected to a 12 mus shaping stage and a multi channel analyzer (MCA). Initial results demonstrate a FWHM energy resolution at 662 keV of 12% and 9% for 200 mum and 400 mum thick films, respectively. These results indicate promise for the development of inexpensive large area photodetectors based on polycrystalline HgI₂ films. Such photodetectors would offer significant advantages over existing silicon (Si) photodiode and photo multiplier tube (PMT) technologies by offering inexpensive large area, compact size- , low operating voltage, and small power consumption.