Polycrystalline Mercuric Iodide Photodetectors for Cesium Iodide Scintillators

We have fabricated thin films of polycrystalline mercuric iodide for use as photodetectors coupled to cesium iodide scintillators. The compound semiconductor mercuric iodide has a large band gap (2.13 eV) causing a peak in its quantum efficiency (570 nm) which is well matched to the output of cesium iodide (550 nm). Single crystal mercuric iodide photodetectors coupled to cesium iodide have demonstrated a full width at half maximum (FWHM) energy resolution of 4.58% at 662 keV. Single crystals providing sufficient surface areas for coupling with scintillators are difficult to grow and are therefore cost prohibitive to use in large field of view applications. However, polycrystalline films can be fabricated quickly and cheaply on a variety of surfaces including curved ones. We have therefore developed a thermal vapor transport method of growing polycrystalline mercuric iodide films onto transparent glass substrates coated with indium tin oxide. A palladium thin film is thermally evaporated on top of the film and the photodetectors are operated at low voltage (up to 0.4 V/mum) demonstrating the first use of polycrystalline mercuric iodide as a photodetector. The polycrystalline mercuric iodide films have a high resistivity producing a very low dark current typically about 1 nA per square cm at 0.4 V/mum. When coupled to cesium iodide a FWHM energy resolution of 9% at 662 keV is obtained. The quantum efficiency at 570 nm is about 40% as compared to single crystal mercuric iodide which is 80%. We are currently optimizing the growth method to improve the quantum efficiency with the ultimate goal of obtaining an energy resolution close to that of single crystal performance.