SiPM Readouts for the Compton Spectrometer and Imager's Anti-Coincidence Shield

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer mission focused on exploring the "MeV Gap" (0.2-5 MeV) in the electromagnetic spectrum. Utilizing sixteen 3-D high-resolution germanium detectors (GeDs) and an Anti-Coincidence Shield (ACS) comprising of Bismuth Germanate (BGO) scintillation crystals coupled with Silicon Photomultipliers (SiPMs), COSI aims to conduct spectroscopy, imaging, and polarimetry of cosmic gamma-ray sources. The ACS, providing passive shielding and anti-coincidence monitoring, detects escaped gamma-rays and tracks background radiation. For COSI's ACS, SiPMs are the preferred read-out method to Photomultiplier Tubes (PMT) due to their low voltage, weight and volume. This presentation covers the SiPM and crystal studies used to find the optimal conditions and parameters for operation. In tests using laboratory electronics, we find that a 3x3 SiPM array centered on the narrow side of a BGO slab with a bias voltage ~3V over SiPM breakdown maximizes the threshold. To simplify the overall COSI electronics, the SiPM readout reuses the NRL4 Application Specific Integrated Circuit (ASIC) that is used to read out the GeDs. Results from coupling the NRL4 ASIC to the SiPM and BGO crystals for an ACS side wall will also be presented.