Direct Mass Measurements on the Superallowed Emitter $^{74}$Rb and its Daughter $^{74}$Kr: Isospin-Symmetry-Breaking Correction for Standard-Model Tests

The decay energy of the superallowed $\beta$-decay $^{74}$Rb($\beta^{+}$)$^{74}$Kr was determined by direct Penning trap mass measurements on both the mother and the daughter nuclide using the time-of-flight resonance technique and found to be $Q = 10416.8(4.5)$ keV. The exotic nuclide $^{74}$Rb, 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) s for this decay, in agreement with the mean value for the series of the lighter nuclides from $^{10}$C to $^{54}$Co. Assuming conserved vector current, this result allows for an experimental determination of the isospin-symmetry-breaking correction $\delta$C.