The $\beta$ decays of ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ to the 4.44-MeV first excited state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ have been studied by means of $\beta -\gamma$ coincidence measurements using a $4\pi$ plastic scintillator and a NaI(Tl) detector. The ratio of ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$/${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ $\beta$-ray branches to the 4.44-MeV state is found to be 1.52 ± 0.06 leading to $\frac{f{t}^{+}}{f{t}^{-}}=1.06\mathrm{}\pm 0.04$ for this branch. This result removes the only severe exception to the systematic trend of positive $\delta =\left(\frac{f{t}^{+}}{f{t}^{-}}\right)-1\mathrm{}$ for mirror decays and is in agreement with a recent calculation of the binding-energy effect. In separate measurements the absolute $\beta$-ray branch of ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ to the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ 4.44-MeV state is found to be (1.27 ± 0.06)%.

• Received 7 June 1972

DOI:https://doi.org/10.1103/PhysRevC.6.1167