Towards a response function for the COSI anticoincidence system: preliminary results from Geant4 simulations

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer mission set to launch in 2027, designed for an all-sky survey in the 0.2-5 MeV band. Its instruments are Germanium detectors surrounded by bismuth germanium oxide (BGO) shields, serving as an anticoincidence system (ACS) to reduce background events. MEGAlib is the Geant4-based reference simulation framework for COSI. Simulating scintillation processes within the shields is crucial for a realistic evaluation of the ACS detection efficiency, but including them in MEGAlib would dramatically increase the simulation time. We therefore propose the use of a response function derived from a dedicated Geant4 ACS simulation with full optical interaction. Verification and validation against analytical computations and laboratory measurements are presented. This setup is then applied to evaluate energy resolution and 3D light collection of the ACS, guiding the design of a prototype model for the response function.