Radiation damage and charge collection effects in Si(Li) gamma-ray detectors

The spectroscopy performance of 6-mm thick Si(Li) planar detectors was studied as a function of operating temperature and electric field. The energy resolution of the 662-keV gamma-ray peak from a 137Cs source was used to monitor the spectroscopy performance of the detectors. The efficiency, depletion voltage, leakage current, and noise were also monitored. The effects of radiation damage caused by 200-MeV protons were studied to determine the viability of operation in space. Four detectors, two maintained at 88 K and two maintained at 212 K, were irradiated to a fluence of 8.7x108 p/cm2. No effects were observed. The two detectors irradiated at 212 K were subsequently irradiated with an additional 8.7x109 p/cm2, again at 212 K. These detectors then exhibited slight energy resolution degradation. No other radiation damage effects were observed. The resolution degradation increased at higher operating temperature and decreased with higher electric field. Cycling the detectors to room temperature for 14 h eliminated the resolution degradation. The resolution of these detectors is limited by the combination of ballistic deficit and parallel noise in the ∼220 K range. A significant decrease in the gamma-ray peak count rate, almost certainly caused by surface channel effects, was the most dramatic temperature-dependent effect observed