Pre-flight Background Estimates for COSI
Savitri Gallego, Uwe Oberlack, Jan Lommler, Christopher M. Karwin, Francesco Fenu, Valentina Fioretti, Andreas Zoglauer, F.M. Follega, A. Perinelli, Roberto Battiston, Roberto Iuppa, E. Steven Boggs, Saurabh Mittal, Pierre Jean, Carolyn Kierans, H. Dieter Hartmann

TL;DR
This paper presents Monte Carlo simulations of background radiation for the COSI gamma-ray telescope, analyzing its sources, rates, and uncertainties to improve data analysis for the upcoming 2027 mission.
Contribution
It provides the first detailed background simulation for COSI, including activation build-up, source contributions, and comparison with measurements, advancing background understanding for gamma-ray telescopes.
Findings
Extragalactic background dominates below 660 keV.
Activation from cosmic rays dominates at higher energies.
Model overestimates SAA flux by a factor of 9.
Abstract
The Compton Spectrometer and Imager (COSI) is a Compton telescope designed to survey the 0.2 - 5 MeV sky, consisting of a compact array of cross-strip germanium detectors. It is planned to be launched in 2027 into an equatorial low-Earth (530 km) orbit with a prime mission duration of 2 years. The observation of MeV gamma rays is dominated by background, mostly from extragalactic and atmospheric photon but also from the activation of the detector materials induced by cosmic-ray interactions. Thus, background simulation and identification are crucial for the data analysis. In this work we perform Monte Carlo simulations of the background for the first 3 months in orbit, and we extrapolate the results to 2 years in orbit, in order to determine the build-up of the activation due to long-lived isotopes. We determine the rates of events induced by the background that are reconstructed as…
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