The search for sterile neutrinos is among the brightest possibilities in our quest for understanding the microscopic nature of dark matter in our universe. Experiments that hunt for these particles using large-volume direct-detection methods, however, have an inherent disadvantage in these searches since sterile neutrinos are predicted to have much weaker couplings to the Standard Model (SM) than the active neutrinos. As a result, the existence of these elusive particles are best probed indirectly via momentum conservation with SM particles during their creation in weak-interaction processes. One way to observe these momentum recoil effects experimentally is through high-precision measurements of nuclear electron-capture (EC) decay, where the final state only contains the neutrino and a recoiling atom. This approach is a powerful method in our search for beyond Standard Model (BSM) physics since it relies only on the existence of a heavy neutrino admixture to the active neutrinos and not on the model-dependent details of their interactions. In this talk, I will describe our Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment that uses the decay-momentum reconstruction technique to precisely measure the 7Li recoil spectrum following 7Be decay in sensitive superconducting tunnel junctions (STJ). I will also present our ongoing work for dramatically increasing the sensitivity of the BeEST, which includes scaling the experiment to thousand-pixel arrays and generating an atom-by-atom map of the rare-isotopes in our sensors using state-of-the-art material characterization methods combined with theoretical quantum simulations.
