Polycrystalline mercuric iodide photodetectors coupled to scintillators

We are developing polycrystalline mercuric iodide (HgI
₂
) films for novel use as optical photo detectors. Single crystal HgI
₂
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 (
¹³⁷
Cs). However, HgI
₂
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 HgI
₂
can be fabricated quickly and cheaply on a variety of surfaces including curved ones. Polycrystalline HgI
₂
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 HgI
₂
films. The ITO and Pd contacts are biased up to 80 V and a very low dark current, typically about 10 pA/mm
²
, 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
¹³⁷
Cs. The biased polycrystalline HgI
₂
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 HgI
₂
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.