Temperature Dependence of the Response of Lithium-Drifted Germanium Detectors to Gamma Rays

While it has generally been stated that lithium-drifted germanium detectors must be operated at liquid-nitrogen temperature for optimum performance, no systematic investigation to prove or disprove this statement has been reported. The closest experiment is that of Tavendale who, however, only investigated the resolution as a function of temperature for a monoenergetic low-energy gamma-ray source. We have investigated the resolution and efficiency of two lithium-drifted germanium detectors, approximately 20 mm in diameter and 3 mm thick, as a function of detector temperature in the range from 85° to 160°K. Gamma-ray energies ranged from 203Hg (279 keV) to 60Co (1333 keV). We found that, contrary to the results of Tavendale, the resolution of the detectors had a pronounced peak at about 105°K. The origin of this peak is, at present, unknown, but is not due to changes in diode capacitance or leakage current. Since both detectors were cut from the same germanium ingot, it is possible that these results are peculiar to these detectors. It is therefore proposed to repeat the experiment with a detector produced from a different germanium ingot, as soon as such a detector becomes available.