Discussion on the lowest energy positron decay

Positrons (e+) from 59Ni decay have a maximum energy of 51 keV, the lowest known energy for a e+ transition. The identifying signal is e+e− annihilation into two 511 keV γ-rays that can be readily observed. Modern Ge detector arrays offer the opportunity to accurately check the 59Ni result (to resolve an apparent error in the intensity) and, further, to search for even lower energy e+ decays. The extremely small e+ branching ratios associated with such decays: (1) require efficient detection and suppression of backgrounds and (2) limit the practical utility of low energy e+ transitions. Branching ratio results for allowed low energy e+ decays could be compared to atomic theory calculations and, at high precision, test weak interaction theory.