Charge Sharing in Cross-Strip Germanium Detectors

High-purity cross-strip germanium detectors (GeDs) for gamma-ray detection have excellent energy resolution and fine 3D position resolution. These capabilities have facilitated detailed studies of solar physics with the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) and a variety of astrophysical sources with the Compton Spectrometer and Imager (COSI). A deleterious effect intrinsic to the GeDs in GRIPS and COSI is charge sharing, which is the triggering of multiple strips by a single photon interaction. Understanding the causes and effects of charge sharing is critical to creating a realistic model of instrument response. Here, we outline our studies of charge sharing in a spare GRIPS GeD (150 strips per side of 0.5 mm strip pitch) and recent efforts to numerically simulate the transport of charge carriers inside the GeD. The fine strip pitch of GRIPS GeDs enhances the impact of charge sharing and is therefore a useful proxy of this effect. We will combine the results from real data and simulations to build a physically-motivated model of charge sharing which can improve the accuracy of our instrument response. An instrument response encapsulating the effects of charge sharing will be used in developing well-benchmarked simulations of COSI as a NASA Small Explorer satellite mission, recently selected for launch in 2025.