Measurement of Nuclear Energy Levels For Evaluation of Ultra-Low Q Value β-Decays

Nuclear β-decay plays an important role in nuclear and particle physics. More specifically, ultra-low Q value β-decays could be used for neutrino mass determination experiments and provide a better understanding of the role of atomic interference effects in β-decay at low energies. ULQ value β-decays occur from the ground state of the parent nucleus to an excited state in the daughter with energy, E*, similar the ground-state Q value, Q_gs, such that Q* = Q_gs – E* < 1 keV. A significant number of potential ULQ value candidates exist, but more precise atomic mass and energy level data are required to determine if these decays are energetically allowed and if so, if Q* < 1 keV. Recent measurements with LEBIT and JYFL-TRAP Penning traps identified a potential decay branch of ⁷⁵Se to the 865.4 keV state in ⁷⁵As that could be ultra-low with Q* = 0.54(51) keV. However, a more precise measurement of E* in ⁷⁵As is necessary to verify this. In collaboration with NSL at Notre Dame, we plan to determine the energy of the 865.4 keV E* level in ⁷⁵As via gamma-ray spectroscopy. A proton beam produced with the Van de Graaf accelerator will impinge on a thin arsenic metal or arsenic-trisulfide foil target to induce Coulomb excitation. Gamma rays from de-excitation will be recorded using high-purity germanium detectors, with the goal of measuring the E* level to a precision of ∼100 eV.