We investigated a red to yellow color change observed in α-HgI<sub>2</sub> when cooled below 150 K. A phase transformation to β-HgI<sub>2</sub>, which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperature the gap was 2.11 ± 0.03 eV which is significantly smaller than the most recently published values of ~ 2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.
The recent technological developments and availability of mercuric iodide detectors have made their application for astronomy a realistic prospect. Mercuric iodide, because of its high resistivity and high density, can be used in a variety of astronomy instrumentation where high spectral resolution, low noise levels, stability of performance, resistance to damage by charged particles and overall ruggedness are of critical importance. X-ray detectors with areas of 12 to 100 mm square and 1 mm thickness have absorption efficiencies approaching 100% up to 60 keV. The spectral resolution of these detector's ranges from 400 eV to 600 eV at 5.9 keV, depending on their area, and the electronic noise threshold is less than 1.0 keV. Gamma ray detectors can be fabricated with dimensions of 25 mm x 25 mm x 3 mm. The spectral resolution of these detectors is less than 4% FWHM at energies of 662 keV. Because of the high atomic numbers of the constituent elements of the mercuric iodide, the full energy peak efficiency is higher than for any other available solid-state detector that makes measurements up to 10 MeV a possibility. The operation of gamma ray detectors has been evaluated over a temperature range of -20 through + 55 degrees Celsius, with only a very small shift in full energy peak observed over this temperature range. In combination with Cesium Iodide scintillators, mercuric iodide detectors with 25 mm diameter dimensions can be used as photodetectors to replace bulky and fragile photomultiplier tubes. The spectral resolution of these detectors is less than 7% FWHM at 662 keV and the quantum efficiency is larger than 80 % over the whole area of the detector.
An evaluation of the spectral performance of eight planar mercuric iodide (HgI<SUB>2</SUB>) gamma-ray detectors under continuous bias voltage for a duration of up to 2000 hours has demonstrated the high degree of long-term stability of mercuric iodide as a radiation detector material. Spectral parameters determined in this evaluation include the %FWHM, the peak-to-valley and peak-to-background ratios, the gain stability of the full energy peak, and the preamplifier offset voltages. Isotopes with three distinct energies were used for these measurements: <SUP>137</SUP>Cs (662 keV), <SUP>57</SUP>Co (122 keV) and <SUP>241</SUP>Am (59 keV). The spectra were analyzed and spectral parameters were generated using Robwin<SUP></SUP>, a spectral analysis program developed by Constellation Technology. Robwin<SUP></SUP> performs simultaneous non-linear fitting of several key elements of the spectrum, emphasizing the continuum for the entire spectrum, the photopeak response function of all lines in the spectrum, the relative intrinsic efficiency of the detector and the photopeak resolution width. These findings provide further support for the widespread use of mercuric iodide as a room temperature semiconductor radiation detector material for energy spectrometry.