Preflight Background Estimates for COSI

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 yr. The observation of MeV gamma rays is dominated by background. Thus, background simulation and identification are crucial for predicting the sensitivities of instruments. 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 yr in orbit in order to determine the buildup of the activation due to long-lived isotopes. The simulations account for the known background components and include time-dependent rate variations due to the geomagnetic cutoff and South Atlantic Anomaly (SAA) passages. In addition, they include detailed modeling of the delayed activation due to short- and long-lived isotopes. We determine the rates of events induced by the background that are reconstructed as Compton events in the simulated COSI data. We find that the extragalactic background photons dominate at low energies (<660 keV), while delayed activation from cosmic-ray primaries (proton/alpha) and albedo photons dominate at higher energies. As part of this work, a comparison at low latitude (divided by b divided by <= 1 degrees) between recent measurement of the SAA and the AP9/AE9 model has been made, showing an overestimation of the flux by a factor similar to 9 by the model. The systematic uncertainties associated with these components are quantified.