Experimental Design for Operating STJs in Dilution Refrigerators for Phase-IV of the BeEST Experiment

The Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment uses superconducting tunnel junction (STJ) sensors, operated below 100 mK, to perform high-resolution recoil spectroscopy of the two-body decay ⁷Be + e^- → ⁷Li + ν_e and search for physics beyond the standard model (BSM). In earlier BeEST phases, cooling was provided by wet adiabatic demagnetization refrigerators (ADRs), which reliably achieved operational temperatures, but only in discrete cycles, limiting the time for continuous data acquisition. This constraint reduced data-collection efficiency and required frequent sensor re-tuning between cycles. To overcome these limitations, phase-IV of the BeEST experiment employs a dilution refrigerator that provides continuous cooling to well below 100 mK. We describe the experimental design modifications required for this transition and present results from the in-situ calibration of STJ detectors in a dilution refrigerator environment. These developments pave the way for sustained high-statistics data acquisition and enhanced sensitivity in the BeEST search for new physics.