Evaluation of Mercuric Iodide Detectors for SNM Monitoring

Detectors made from single crystals of mercuric iodide are used in increasing numbers for the detection and analysis of gamma radiation. The large density and high atomic numbers of the constituent elements of mercuric iodide result in a high efficiency compared with other solid state detectors. The material has a very high threshold for damage by neutrons and charged particles, and its high resistivity makes it possible to fabricate large area detectors with low noise levels at ambient temperatures. The technology to fabricate these detectors makes it possible to manufacture large volume counters and sensitive spectrometers with a resolution of 3 % FWHM or less at the gamma ray energy of 662 keV. They have been operated between –20 to 60 degrees Celsius, and the performance is stable over periods of years. These properties make the mercuric iodide detectors ideally suited for the routine monitoring of stored materials, hold-up measurements, and in unattended systems. The spectra of several concentrations of Plutonium 239, Highly Enriched Uranium, and Depleted Uranium have been measured at facilities where samples of these materials were available. To perform these measurements the detectors were installed in a light-tight module, together with a preamplifier and a voltage stabilizing system. Modified NIM power and signal processing electronics were used. The detailed features of the spectra will be shown and discussed