Observation of Fermi Superallowedβ+Decays in Heavy Odd-Odd,N=ZNuclei: Evidence for0+Ground States inY78,N82b, andT86c

The ${\beta }^{+}$-decay half-lives of the neutron-deficient, odd-odd, $N=Z$ nuclei ${}^{74}\mathrm{Rb}$, ${}^{78}\mathrm{Y}$, ${}^{82}\mathrm{Nb}$, and ${}^{86}\mathrm{Tc}$ have been measured following the fragmentation of a primary ${}^{92}\mathrm{Mo}$ beam at an energy of 60 MeV per nucleon. The half-lives were measured by correlating ${\beta }^{+}$ decays with the implantation of unambigously identified fragments. The deduced $\log \mathrm{ft}$ values are consistent with $0^{+}\to 0^{+}$ Fermi superallowed transitions, which together with the measured ${\beta }^{+}$ detection efficiencies suggest $T=1\mathrm{}$, $I^{\pi }{=0\mathrm{}}^{+}$ ground states for these odd-odd, $N=Z$ nuclei. These data represent the heaviest $N=Z$ systems for which Fermi superallowed decays have been established. The results suggest that these nuclei can extend the mass range used to test the conserved vector current hypothesis of the standard model.