Performance improvement of 3-D position-sensitive pixellated HgI2 detectors when cooled from room temperature to 10 °C

An experiment to measure the performance of 3-D position-sensitive pixellated HgI₂ crystals cooled below room temperature was carried out using a new ASIC from GammaMedica-Ideas. This ASIC is capable of reading out the digitized pre-amplifier signal waveforms of 121 pixels and the cathode. A significant improvement in energy resolution from 1.48% FWHM at 662keV for the whole detector to 1.09% FWHM is observed on a 18×18×11mm³ crystal at -4800V cathode bias when the ambient temperature is lowered from 70°F to 50°F (about 10°C). This resolution improvement is partly due to electronic noise reduction in the readout system. However, analysis of the pre-amplifier waveforms showed that changes in crystal properties played a more important role. A correlation between total electron trapping and energy resolution was identified. However, the amount of trapping was found irrelevant to crystal performance. Instead, the non-uniformity of charge collection, namely non-uniformity of the material defects, was believed to be the main contributor to crystal performance.