eV-Scale Gamma-Recoil Coincidence Spectroscopy in ⁷Be EC Decay using Superconducting Sensors

The BeEST experiment uses rare-isotope-doped Superconducting Tunnel Junction (STJ) detectors to measure the daughter recoil energy following the EC decay of ⁷Be to search for heavy neutrino mass states. In the resulting eV-scale nuclear recoil spectrum, the region of interest for a heavy neutrino signal has a standard-model-allowed background that results from decay into the excited state of ⁷Li and the subsequent doppler-broadening. We have used high-efficiency NaI detectors surrounding the STJ detector to isolate the decay branch to the excited nuclear state by tagging on the 478 keV γ-ray. This talk will present the first direct nuclear recoil-γ coincidence measurement in EC decay and present preliminary results on the recoil stopping time of the daughter nucleus in the tantalum lattice and shake-up and shake-off spectral features.