Radiation damage and annealing of three coaxial n-type germanium detectors: Preparation for spaceflight missions to asteroid 16 Psyche and Mars’ moon Phobos

Abstract

Three n-type high-purity germanium (HPGe) detectors were irradiated with protons at solar and galactic cosmic ray energies, with fluences corresponding to ≥3 years of exposure to the interplanetary radiation environment. Following 1 GeV proton irradiations (8×108 protons cm−2), annealing of the detectors at 105°and 115 °C resulted in full energy resolution recovery after 240 and 140 h of annealing, respectively. A second irradiation, consisting of 7.25x109 protons cm−2 with a solar-proton-like energy spectrum, resulted in full recovery within <316 h of annealing at 105 °C. Prior spaceflight experience raised concerns about annealing-induced leakage currents. During this study, one of the three detectors was annealed for a cumulative time of 57 days, and there was no evidence for increased leakage currents. However, gamma-ray photopeak detection efficiency was found to be strongly anti-correlated with cumulative anneal time for all three detectors. The energy- and time-dependence of the efficiency loss is consistent with the loss of active volume at the inner borehole, which we attribute to thermal diffusion of the lithium contact during annealing. The data collected during this study validate aspects of the design of two in-development planetary gamma-ray spectrometers; one to the asteroid (16) Psyche and one to Mars’ moon Phobos. The annealing strategy and operations concepts for these two instruments are informed by the results of this study.