Monte Carlo Modeling of eV-Scale Backgrounds in Superconducting Tunnel Junctions from Gamma-Ray Interactions

The BeEST experiment searches for physics beyond the standard model (BSM) in the neutrino sector by utilizing the electron capture (EC) decay of 7Be. The 7Be is embedded in superconducting tunnel junction (STJ) sensors such that the low-energy (eV-scale) radiation is detected with high efficiency. In 10% of the decays, a 478 keV gamma-ray is emitted in the dexcitation of the 7Li daughter nucleus. While the gamma ray does not interact in the STJ (~0.5um), it scatters in the silicon substrate that the STJ is deposited on. This then results in a low-energy background seen on the energy spectra read from the quantum sensor. In this talk I will discuss our modeling of this background using a python based Monte Carlo of light-matter interactions and subsequent phonon propagation.