Collinear-laser spectroscopy with the bunched-beams technique was used for the study of neutron deficient Rb isotopes, out to ${}^{74}\mathrm{Rb}$ ($N=Z=37$) at TRIUMF. The measured hyperfine coupling constants of ${}^{76,78m}\mathrm{Rb}$ were in agreement with literature values. The nuclear spin of ${}^{75}\mathrm{Rb}$ was confirmed to be $I=3/2$, and its hyperfine coupling constants were measured for the first time. The mean-square charge radius of ${}^{74}\mathrm{Rb}$ was determined for the first time. This result has improved the isospin symmetry breaking correction term used to calculate the $\mathcal{F}t$ value, with implications for tests of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

• Received 6 April 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.212502