An assessment of radiation damage in space-based germanium detectors due to solar proton events

Abstract

Radiation effects caused by solar proton events will be a common problem for many types of sensors on missions to the inner solar system because of the long cruise phases coupled with the inverse square scaling of solar particle events. As part of a study in support of the BepiColombo mission to Mercury we have undertaken a comprehensive series of tests to assess these effects on a wide range of sensors. In this paper, we report on the measurements on a large volume coaxial Ge detector which was exposed to simulated solar proton spectra of integrated fluences 8 × 10 8 , 6 × 10 9 and 6 × 10 10 protons cm - 2 . After each irradiation the detectors performance was accessed in terms of energy resolution, efficiency and activation. The detector was then annealed and the measurements repeated before the next irradiation. The minimum operational performance criteria were based on the resolution and efficiency requirements necessary to detect and separate specific radioisotope emission lines from a planetary regolith. Specifically that the energy resolution be restored to 5 keV FWHM at 1332 keV and the detection efficiency be degraded to no more than 10% of its pre-irradiation value. The key conclusion of this study is that even after a modest solar proton event the detector requires extensive annealing. After exposure to an event of integral fluence ∼ 8 × 10 8 protons cm - 2 this amounts to ∼ 1 week duration at 100 ∘ C , whereas for a fluence of ∼ 6 × 10 10 protons cm - 2 , the detector requires 3.5 months of annealing to satisfy the minimum operational performance requirements and 4.5 months to return the energy resolution to < 3 keV FWHM at 1332 keV. As a consequence such an instrument will require constant, planned and active management throughout its operational lifetime. The impact on spacecraft operations including resource management therefore needs careful consideration.