First direct measurement of the ⁷Be electron capture Q value to aid experimental searches for sterile neutrinos with the BeEST

The Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment is searching for the signature of sterile neutrinos via momentum reconstruction in the EC decay of ⁷Be. In EC decay, there are only two final state products, the daughter atom, and neutrino. Rather than having an energy distribution of final products, as in beta decay, in EC decay the neutrino and recoiling daughter atom have well defined energies that depend on the kinematics of the decay. In the case of ⁷Be EC decay, the ground-state to ground-state Q-value of 861.89(71) keV results in a recoil energy for the ⁷Li daughter atom of 56.83(1) eV. Although the uncertainty in the recoil energy is only 10 meV, this is already comparable to the uncertainty in the ⁷Li recoil energy obtained in Phase II operation of the BeEST experiment. A new, direct measurement of the ⁷Be Q value is called for to check its accuracy, to reduce the uncertainty for a meaningful check of systematics in the BeEST experiment, and to meet the anticipated precision of the ⁷Li recoil energy in future Phases of BeEST operation. We have performed such a measurement via Penning trap mass spectrometry with the LEBIT facility at the NSCL and have reduced the Q value uncertainty by a factor of about three.