Accurate Q value for the 74Se double-electron-capture decay

The Q   value of the neutrinoless double-electron-capture (0νECEC0νECEC) decay of 74Se was measured by using the JYFLTRAP Penning trap. The determined value is 1209.169(49) keV, which practically excludes the possibility of a complete energy degeneracy with the second 2+2+ state (1204.205(7) keV) of 74Ge in a resonant 0νECEC0νECEC decay. We have also computed the associated nuclear matrix element by using a microscopic nuclear model with realistic two-nucleon interactions. The computed matrix element is found to be quite small. The failure of the resonant condition, combined with the small nuclear matrix element and needed p-wave capture, suppresses the decay rate strongly and thus excludes 74Se as a possible candidate to search for resonant 0νECEC0νECEC processes.