Next Generation Ta-STJ Sensor Arrays for BSM Physics Searches

The Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment uses superconducting tunnel junction (STJ) sensors to search for physics beyond the standard model (BSM) with recoil spectroscopy of the 7Be EC decay into 7Li. A pulsed UV laser is used to calibrate the STJs throughout the experiment with ∼20 meV precision. Phase III of the BeEST experiment revealed a systematic calibration discrepancy between STJs. We found these artifacts to be caused by resistive crosstalk and by variable substrate heating due to intensity variations of the calibration laser. For phase-IV of the BeEST experiment, we have removed the crosstalk by designing the STJ array so that each pixel has its own ground wire. We now also use a more stable UV laser for calibration. The new STJ arrays were fabricated at STAR Cryoelectronics and tested at LLNL and FRIB. They have the same high energy resolution of ∼1–2 eV in the energy range of interest below 100 eV as before, and significantly reduce the presence of both calibration artifacts. We discuss the design changes and the STJ array performance for the next phase of the BeEST experiment.