Beta-ray branching and half-lives ofB12andN12

Abstract

The ratio of ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ $\beta$-ray branches to the 4.44-MeV excited state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ was measured by detecting $\beta$ rays in a $4\pi$ plastic scintillator and $\gamma$ rays in two Ge(Li) detectors. Activities were made in the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}\mathrm{}(d,p)\mathrm{}{}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}({}_{}{}^3{}_{}{}^{}\mathrm{He},n){}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ reactions using a chopped Van de Graaff beam. Data were stored in two successive computer time bins of two half-lives each in order to establish by subtraction the shape of the short-lived $\beta$-ray spectrum and its net number of counts. The ratio of these branches was found to be ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$/${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ = 1.56±0.05. A measurement of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ absolute $\beta$-ray branch to the 4.44-MeV state via $\beta -\gamma$ coincidences using an NaI(T1) detector for the $\gamma$ rays gave a value of 1.276±0.050%. Half-lives, obtained by multiscaling $\beta$-ray counts, were found to be 20.20±0.02 msec for ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and 11.000±0.016 msec for ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$. The mirror asymmetries derived from these results are $\delta =+0.044\mathrm{}\pm 0.034$ for the decay to the 4.44-MeV state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and $\delta =+0.129\mathrm{}\pm 0.008$ for the branch to the ground state.