Semiconductor Detector Optimization for Trace Mercury Analysis in Environmental Samples

The counting of low energy gamma-rays from mercury in activated environmental and biological samples is often hindered by dominant activity of higher energy gamma-rays from elements such as sodium. An investigation of gamma-ray interaction theory for silicon and germanium at these energies shows that germanium is the preferred material, and that a detector thickness of about 5 mm in the direction of the incident gamma-ray is optimum for mercury counting. Measurements and Figure-of-Merit calculations for true coaxial, closed-end coaxial, and thin high resolution planar Ge(Li) detectors resulted in FOM values for this application of 2.5 to 4.6 for the large volume coaxial detectors, and 8.0 to 17.3 for the thin planar detectors. Coincidence counting of the gamma- and x-rays can yield large FOM values and separation of the gamma- and x-rays in the recorded spectrum. Certain phenomena related to coincident emission, to background efficiency, and to energy resolution are not included in the FOM, and should be considered separately in defining optimum detectors.