First-forbidden rank-one beta-decay of $\sp{20}$F

Date of Completion

January 1997


Physics, Nuclear|Physics, Radiation




Weak interactions, e.g. beta-decay, are an important tool for investigating nuclear structure. Selection rules for beta-decay (i.e. isospin, angular momentum and parity) provide nuclear spectroscopy information. Beta-decay has also become an important tool in elucidating subnucleonic (i.e. meson) degrees of freedom; of particular interest are forbidden (i.e. parity changing) beta-decays since they probe pionic currents in nuclei. First-forbidden rank-one beta-decays ($\rm\Delta\pi=-,\ \Delta J=1)$ are hindered and hard to measure, thus only a few are known in spite of their importance.^ We measured upper limits for the branching ratio of the first-forbidden rank-one beta-decay from $\sp{20}$F 2$\sp+$ ground state to the 3$\sp-$ and 1$\sp-$ excited states of $\sp{20}$Ne at 5.62 MeV and 5.79 MeV respectively. The branching ratio upper limits for these decays are $1.5\times10\sp{-6}$ to the 3$\sp-$ state and $7\times10\sp{-7}$ to the 1$\sp-$ state at 2$\sigma.$ These upper limits represent factors of 320,1400 improvements over previous branching ratio limits to the lowest 3$\sp-$,1$\sp-$ excited states of $\sp{20}$Ne, and are a factor of 10 smaller than predictions based on constant ft (comparative half-life) between the first-forbidden rank-zero beta-decay from the 2$\sp+$ ground state of $\sp{20}$F to the 2$\sp-$ state at 4.97 MeV in $\sp{20}$Ne and first-forbidden rank-one beta-decays of $\sp{20}$F. We measured this first-forbidden rank-zero beta-decay branching ratio to the 2$\sp-$ state of $\sp{20}$Ne to be $1.2(6)\times10\sp{-4},$ consistent with Alburger and Warburton's measurement of $9.0(4\times10\sp{-5}$ (AL81a). This information, when combined with beta-decay data of $\sp{20}$Na, can also serve to study the beta-decays of the mirror nuclei $\sp{20}$F and $\sp{20}$Na to the same states in $\sp{20}$Ne. ^