Effect of Prolonged Annealing on the Performance of Coaxial Ge Gamma-Ray Detectors

The effects of prolonged annealing at elevated temperatures have been investigated in a 53 cm(3) closed-end coaxial high purity germanium detector in the reverse electrode configuration. The detector was multiply annealed at 100 degrees C in block periods of 7 days. After each anneal cycle it was cooled to 77 K and the relative efficiency, peak channel location and FWHM energy resolution measured at 6 gamma-ray energies. At the present time, the detector has completed 16 anneal cycles. It was found that above similar to 662 keV the photopeak efficiency decreased almost linearly at a rate of similar to 1.5% per anneal cycle, although the energy resolution and centroid (and therefore charge collection efficiency) remained unchanged. The change in detection efficiency is attributed to the expansion of the inner n+ contact due to the thermal drive-in of Li ions into the bulk. The rate is found to follow a power-law dependence in agreement with that expected from Fick's diffusion laws. Using these data, we have derived a simple 1-D phenomenological model in which the n+ contact thickness is simply related to the Li diffusion length. For annealing at 100 degrees C, the thickness of the n+ contact, d, as a function of the annealing time, t, can be described semi-empirically by d(mm)=0.231xt(days)(1/2).