The atomic masses of ${}^{74}\mathrm{Ge}$, ${}^{74}\mathrm{Se}$, ${}^{76}\mathrm{Ge}$, and ${}^{76}\mathrm{Se}$ have been determined from cyclotron frequency ratios of pairs of ions simultaneously trapped in a cryogenic Penning trap. Allowing for cancellation of systematic errors in the mass differences between isobars, we determine the $Q$ value for double-electron capture of ${}^{74}\mathrm{Se}$ to be 1209.240(7) keV, and the $Q$ value for double-electron emission of ${}^{76}\mathrm{Ge}$ to be 2039.061(7) keV. The new $Q_{2\mathrm{EC}}$ value for ${}^{74}\mathrm{Se}$ precludes a large resonant enhancement for neutrinoless double-electron capture.

• Received 12 November 2009

DOI:https://doi.org/10.1103/PhysRevC.81.032501