Characterizing, correcting, and repairing the effects of radiation damage in the COSI germanium cross-strip detectors

TL;DR

This paper details the modeling, characterization, correction, and repair of radiation damage effects in COSI's germanium detectors, including proton irradiation experiments and annealing techniques to restore detector performance.

Contribution

It introduces a comprehensive approach combining proton irradiation, data correction, and annealing to mitigate radiation damage in space-based germanium detectors.

Findings

Proton fluence correlates with charge trapping effects.

High-temperature annealing improves spectral performance.

Correction techniques effectively mitigate radiation damage effects.

Abstract

The Compton Spectrometer and Imager (COSI) is a gamma-ray survey telescope utilizing a compact Compton imager design, enabled by an array of 16 high-resolution germanium cross-strip detectors. After its launch into an equatorial Low Earth Orbit (LEO) in 2027, COSI will experience radiation damage primarily due to energetic protons, with the proton fluence dominated by the passage of COSI through the edge of the South Atlantic Anomaly (SAA) for a few minutes each orbit. We have developed a comprehensive program focused on the modeling, characterization, data correction, and physical repair of radiation damage effects in the COSI detectors. We have performed energetic proton beam irradiations of a spare COSI detector at a proton synchrotron, with proton fluences consistent with multiple years of exposure to the COSI space radiation environment. These exposures allow us to characterize the relationship between proton fluence and induced charge trapping. We demonstrate our techniques to correct for trapping effects, as well as characterize the effectiveness of high-temperature annealing on correcting this damage, as characterized by the resulting spectral performance of the detector. We will present our efforts to characterize the effects of radiation damage in the COSI detectors, as well as our techniques for correcting these effects in the data analysis pipeline and ultimately repairing the detectors on orbit every few years through high-temperature annealing.