Towards a response function for the COSI anticoincidence system: preliminary results from Geant4 simulations Article Swipe
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· 2024
· Open Access
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· DOI: https://doi.org/10.1117/12.3020252
· OA: W4399599563
The Compton Spectrometer and Imager (COSI) is an upcoming NASA Small Explorer\nsatellite mission scheduled for launch in 2027 and designed to conduct an\nall-sky survey in the energy range of 0.2-5 MeV. Its instrument consists of an\narray of germanium detectors surrounded on four sides and underneath by active\nshields that work as anticoincidence system (ACS) to reduce the contribution of\nbackground events in the detectors. These shields are composed of bismuth\ngermanium oxide (BGO), a scintillator material, coupled with Silicon\nphotomultipliers, aimed to collect optical photons produced from interaction of\nionizing particles in the BGO and convert them into an electric signal. The\nreference simulation framework for COSI is MEGAlib, a set of software tools\nbased on the Geant4 toolkit. The interaction point of the incoming radiation,\nthe design of the ACS modules and the BGO surface treatment change the light\ncollection and the overall shielding accuracy. The use of the Geant4 optical\nphysics library, with the simulation of the scintillation process, is mandatory\nfor a more realistic evaluation of the ACS performances. However, including the\noptical processes in MEGAlib would dramatically increase the computing time of\nthe COSI simulations. We propose the use of a response function encoding the\nenergy resolution and 3D light yield correction based on a separate Geant4\nsimulation of the ACS that includes the full optical interaction. We present\nthe verification of the Geant4 optical physics library against analytical\ncomputations and available laboratory measurements obtained using PMTs as\nreadout device, as a preparatory phase for the simulation of the COSI ACS\nresponse.\n
