GEANT4 Modeling of eV-scale Photon Backgrounds in STJ Sensors for the BeEST Experiment

The BeEST (Beryllium Electron Capture in Superconducting Tunnel Junctions) experiment searches for physics beyond the standard model (BSM) in the neutrino sector by utilizing the electron capture (EC) decay of ⁷Be. The rare isotopes are embedded in superconducting tunnel junction (STJ) quantum sensors such that the low-energy (< 0.1keV) radiation from EC decay is absorbed with unit efficiency – except for the neutrino and a 478 keV gamma ray that occurs in ~10% of decays. While the gamma ray does not interact in the thin STJ (~0.5um), it can interact and deposit energy in the silicon substrate that the STJ is deposited on. This is currently the dominant source of background in the BeEST experiment and is thus important to model and understand. In this talk I will present preliminary work towards using specific low-energy physics packages within GEANT4 to model this background and compare the simulations with data collected from Lawrence Livermore National Laboratory.