In-orbit Spectral Calibration Prospects for the COSI Space Telescope

The Compton Spectrometer and Imager is an upcoming NASA space telescope in the MeV range. COSI's primary science goals include precisely mapping nuclear line and positron annihilation emission in the Milky Way galaxy through Compton imaging. This relies on our ability to maintain COSI's spectral performance over its mission lifetime. Changes to the detectors' gain characteristics over time will result in a non-linear stretching of the entire energy range. Moreover, observations from past MeV telescopes and proton-beam experiments have shown that radiation damage in space causes photopeak shifts and spectral line broadening. These necessitate a plan for regular, in-orbit calibration. In this study, we demonstrate a method to monitor and recalibrate the COSI detectors using background line emissions produced by the space radiation environment. We employ Monte Carlo simulations of particle background and show that strong background lines arise from nuclear excitation of COSI's detectors (germanium) and cryostat (aluminum) materials. These span COSI's entire bandwidth for single-site interactions and can be used to monitor the effects of radiation damage and gain shifts every eight hours at the full instrument level and every 24 days at the individual detector level. Methods developed by Pike et al. to correct the effects of hole trapping and gain characteristics can then be applied to recover the original spectral performance. These results inform COSI's telemetry requirements for calibration and housekeeping data, and rule out the need for an on-board radioactive calibration source which would have increased the complexity of the spacecraft.