Radiation Damage Effects and Charge Trapping Correction Techniques in the COSI Germanium Detectors

The Compton Spectrometer and Imager (COSI) is a wide-field gamma-ray (0.2-5 MeV) imager and spectrometer utilizing germanium strip detectors. COSI had a successful super pressure balloon flight from New Zealand in 2016 (COSI-APRA), producing results on polarization of Gamma-Ray Bursts (GRBs), positron annihilation emission from the Galactic Center, measurement of the 26-Al flux from the Galactic Center, and detections of other celestial gamma-ray sources. COSI has recently been selected for a SMEX mission (COSI-SMEX) for launch in 2025. We are now proposing a 3-year program focused on radiation damage effects and correction techniques in our cross-strip GeDs developed for the COSI program. This work will focus on detector modeling of charge trapping effects, characterization of charge trapping resulting from radiation damage, and finally development of a detailed algorithm for correcting the effect of radiation damage to optimize and maintain the excellent spectral performance of these detectors. Objective 1: Significantly improve our physical modeling of charge transport in the COSI-APRA GeDs to include charge trapping effects. This work will allow us to better understand the effects of radiation damage on charge collection, and ultimately spectral performance. Objective 2: Perform proton beam irradiations on the COSI-APRA detectors at fluences comparable to those expected in LEO to characterize the effects of radiation damage on both the hole and electron collection. These measurements will allow us to benchmark the detector simulations for moderate-to-severe radiation damage. Objective 3: Use the detector simulations to develop detailed techniques for correcting the effects of charge trapping on the spectral response of the detectors. These techniques will be demonstrated on the radiation-damaged COSI-ARPA detectors. Objective 4: Annealing the COSI-APRA detectors to characterize the effectiveness in repairing the proton radiation damage. Objective 5: Repeat the modeling and beam tests on a non-flight version of the COSI-SMEX detectors, enabling us to develop data analysis methods that incorporate the correction techniques for optimal spectral performance for the COSI-SMEX detector geometry. Objective 6: Perform detailed background simulations of the COSI-SMEX detectors in their LEO environment to identify characteristics in the background data that we will be able to utilize to characterize the radiation damage effects and optimize the correction technique while on orbit. The proposed work covers fundamental detector physics focused on radiation damage in germanium cross-strip detectors. The correction algorithms that we develop will be useful for optimizing the spectral performance for the COSI-SMEX detectors over the lifetime of the mission. This work is not part of the funded technology line for COSI-SMEX.

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