Charged-particle induced radiation damage of a HPGe gamma-ray detector during spaceflight

The Mars Observer spacecraft was launched on September 26, 1992 with a plan ned arrival at Mars after an 11-month cruise. Among the scientific instrume nts carried on the spacecraft was a Gamma-Ray Spectrometer (GRS) experiment to measure the composition of Mars. The GRS used a passively cooled high-p urity germanium detector for measurements in the 0.2-10 MeV region. The sen sor was a closed-end co-axial detector, 5.5 cm diameter by 5.5 cm long, and had an efficiency along its axis of 28% at 1332 keV relative to a standard NaI(Tl) detector. The sensor was surrounded by a thin (0.5 cm) plastic cha rged-particle shield. This was the first planetary mission to use a cooled Ge detector. It was expected that the long duration in space of three years would cause an increase in the energy resolution of the detector due to ra diation damage and could affect the expected science return of the GRS. Sho rtly before arrival, on August 21, 1993, contact was lost with the spacecra ft following the pressurization of the propellent tank for the orbital-inse rtion rocket motor. During much of the cruise to Mars, the GRS was actively collecting background data. The instrument provided over 1200 h of data co llection during periods of both quiescent sun and solar flares. From the ch arged particle interactions in the shield, the total number of cosmic ray h its on the detector could be determined. The average cosmic ray Aux at the MO GRS was about 2.5 cm(-2) s(-1). The estimated fluence of charged particl es during cruise was about 10(8) particles cm(-2) with 31% of these occurri ng during a single solar proton event of approximately 10 days duration. Du ring cruise, the detector energy resolution determined from a background ga mma-ray at 1312 keV degraded from 2.4 keV full-width at half-maximum shortl y after launch to 6.4 keV 11 months later. This result agrees well with mea surements from ground-based accelerator irradiations (at 1.5 GeV) on a simi lar size detector. (C) 1999 Elsevier Science B.V. All rights reserved.