Fast Neutron Radiation Damage of High-Purity Germanium Detectors

Abstract

Seven high resolution, high-purity planar germanium and two lithium-drifted germanium detectors have been exposed to fluences of monoenergetic fast neutrons of 1.4, 5.5 and 16.4 MeV to study radiation damage effects. Seven of the exposures were made at 5.5 MeV using detectors made from both LBL and General Electric Company material. Initial degradation of 60Co energy resolution was generally observed after fluences of 3 × 10+9 n/cm2. After fluences of 1010 n/cm2, the detector resolutions were all affected, and replacement would be required in most gamma ray spectrometry; these results are consistent with previous damage studies on germanium detectors. Considerable variability in neutron damage threshold between detectors was observed within this fluence range which must be attributable to a material parameter that is not yet fully determined. This is the major finding in this study. After irradiation, a significant increase in material resistivity was observed as a series resistance in the diodes undepleted region at low biases. The observations were made by capacitance effects and lengthened pulse rise time. Annealing of damage was observed during storage at LN2 temperature after irradiation; resulting, in some cases, in improvement of resolution and in others, further degradation. Drastic resolution degradation was observed on cycling detectors to dry ice temperatures, (200° K) with the loss of the high series resistance and an increase in acceptor concentration. Further cycling to room temperature for periods of hours resulted in improvement of the energy resolution compared with the 200° K value.