Direct Mass Measurements on the Superallowed Emitter Rb74 and Its Daughter Kr74: Isospin-Symmetry-Breaking Correction for Standard-Model Tests

The decay energy of the superallowed $\beta$ decay ${}^{74}\mathrm{R}\mathrm{b}({\beta }^{+}){}^{74}\mathrm{K}\mathrm{r}$ was determined by direct Penning trap mass measurements on both the mother and the daughter nuclide using the time-of-flight resonance technique and was found to be $Q=10416.8(4.5)\mathrm{k}\mathrm{e}\mathrm{V}$. The exotic nuclide ${}^{74}\mathrm{R}\mathrm{b}$, with a half-life of only 65 ms, is the shortest-lived nuclide on which a high-precision mass measurement in a Penning trap has been carried out. Together with existing data for the partial half-life as well as theoretical corrections, the decay energy yields a comparative half-life of $Ft=3084(15)\mathrm{s}$ for this decay, in agreement with the mean value for the series of the lighter nuclides from ${}^{10}\mathrm{C}$ to ${}^{54}\mathrm{C}\mathrm{o}$. Assuming conserved vector current, this result allows for an experimental determination of the isospin-symmetry-breaking correction ${\delta }_{\mathrm{C}}$.