Distribution of ${}^{7}$Li Implants in Nb Films for a Sterile Neutrino Search

Abstract

The BeEST experiment is measuring the ${}^{7}$Li recoil spectrum from the decay of ${}^{7}$Be implanted into Ta-based sensors to provide the most stringent limits on the existence of sterile neutrinos in the sub-MeV mass range. Its sensitivity is limited by spectral broadening due to interactions between the atomic shell of the ${}^{7}$Be/${}^{7}$Li and the Ta sensor film. This study is investigating the suitability of Nb as an alternative sensor material by scanning transmission electron microscopy (STEM) and atom probe tomography (APT) of ${}^{7}$Li-implanted Nb films. STEM does not detect any change in the microstructure of polycrystalline Nb due to ${}^{7}$Li implantation. APT reveals some segregation of ${}^{7}$Li at grain boundaries in Nb. While this may slightly alter the ${}^{7}$Li binding energies, the effect on the recoil spectrum is not expected to be strong enough to preclude using Nb-based detectors in future phases of the BeEST experiment.