The lowest energy positron decay

Vo, et al have presented {gamma}-ray coincidence spectra from the HERA Ge array using a long-lived {sup 68}Ge-{sup 68}Ga, source. A well known level at 1883 keV in {sup 68}Zn, believed to be directly populated only by a 0.218% electron capture decay, appears on the basis of Ref. 1 to also have a {approx}10{sup -7} positron (e{sup +}) branch. The maximum energy of this feed would be (15.9{plus_minus}1.2) keV, a very low energy transition. The evidence the e{sup +} branch is the presence of 511 keV {gamma}-rays from e{sup +} e-{sup -} annihilation in coincidence with 1883 keV {gamma} rays. Systematic effects due to {open_quote}pile-up{close_quote} are believed to be responsible for at least part, if not all of the e{sup +} decay signal. Further, a theoretical calculation of the e{sup +} branching ratio yields {approx}10{sup -11} for the 1883 keV level, in strong disagreement with the apparent {approx}10{sup -11} observation. A list of long-lived candidate isotopes for lowest energy e{sup +} decay will be presented. A tagged source of extremely low energy neutrinos is one possible future use for such decays.