Surface Event Pulse Shape Simulation for the Legend Experiment

The Large Enriched Germanium Experiment for Neutrinoless double-beta Decay (LEGEND) collaboration plans to search for neutrinoless double-beta (0νββ) decay in ⁷⁶Ge using modular arrays of enriched germanium detectors. Since 0νββ candidate events happen at a single site in the germanium detector, pulse shape simulations that model movement of charge carriers in the detector are key to cuts that can reject background from multi-site and surface events. Most events happening in the bulk of the detector, such as gamma-ray events, can be easily simulated by using established models of charge carriers inside Ge. However, surface events such as those caused by alpha incidents on the detector are complex since they generate a large charge cloud, and thus their signal is influenced by effects such as diffusion and self-repulsion. Only the p+ contact and passivated surfaces of the detector are sensitive to alpha events. While these events can be easily rejected using analysis cuts, their behavior before cuts, including their energy spectrum and their distribution on the detector surface, is difficult to model. In this talk we describe a novel simulation of charge carriers in germanium that incorporates diffusion and self-repulsion to model surface alpha events. We also show how such simulations can be sped up using parallel calculations on GPUs, and how they can be used to improve our modeling of alpha backgrounds in ⁷⁶Ge-based 0νββ searches.