Effect of HgI2 crystal nonuniformities on gamma-ray response

As-grown HgI2 crystals exhibit regions of varying optical clarity indicative of varying defect density within the crystal bulk. It has been reported in the past that the energy resolution available from HgI2 gamma ray detectors is affected by these variations in crystal quality. The present work is a study correlating the crystal quality, mechanical response, and impurity variations of large-area detector slices with gamma response variations. It was found that regions of higher optical clarity exhibit a 'dead layer' at their suface that reduces the amplitude of their low energy gamma response peak. The hole lifetime is longer and the high energy gamma resolution better, however, in the regions that contain fewer optically visible defects. Chemical and structural analysis of the different crystal regions was done. Impurity content was not found to correlate directly with optical clarity nor were major variations in stoichiometry apparent between regions of different optical density. Microhardness measurements demonstrate that the clear regions exhibit a significantly higher Knoop microhardness than optically hazy regions. (orig.)