3D Reconstruction of Ion-Beam Implantation Profile in Superconducting Tunnel Junctions using Atom Probe Tomography*

Recent use of superconducting tunnel junctions (STJs) to perform precision, low-energy nuclear decay experiments from implanted radioactive ions have shown tremendous promise in our search for BSM physics.  In particular, the Beryllium Electron capture in Superconducting Tunnel junction (BeEST) experiment searches for BSM neutrino-mass physics in the electron capture decay of ⁷Be.  The search is currently limited by unexpected peak broadening, which may be due to interaction of the implanted ⁷Be within the STJ detector materials.  In this talk, I will present the working progress for transmission electron microscopy (TEM) imaging and atom probe tomographic (APT) 3D reconstruction of ⁷Be and ⁷Li implanted into Ta-based and Al-based STJs.  Such reconstructions will be compared to density functional theory modeling of the electronic structure of lithium atoms in different atomic environments of polycrystalline Ta and Al.  This comparison and associated data will clarify in-medium effects to better understand the observed peak broadening towards higher sensitivity searches for new physics using STJs.