Spatiotemporal Energy Relaxation Simulation in Superconducting Tunnel Junction Detectors

The BeEST experiment uses Superconducting Tunnel Junction (STJ) radiation detectors to search for sub-MeV neutrino mass states by precisely measuring the daughter recoil energy following the EC decay of ⁷Be. In order to improve the modeling of the energy response of STJ sensors, we are developing detailed spatiotemporal Monte-Carlo simulations of the energy relaxation process within the detector. The goal is to understand the non-equilibrium physics by reproducing the measured response of the sensors. In particular, the simulations aim to model the physical processes that create observed low-energy tails, line splitting artifacts, and other effects that can limit the sensitivity of the sterile neutrino search. This talk presents recent progress and comparison with experimental data.