In a reinvestigation of the $\beta$ decay of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$, formed in the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}(t, p){}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ reaction at $E_t=3.0\mathrm{}$ MeV, the unique first-forbidden $\beta$-ray transition to the 4445 keV, $J^{\pi }={\frac{5}{2}}^{-}$ state of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ has been found with a branching intensity of 0.054±0.004% and $log{f}_{1}t=10.93\mathrm{}\pm 0.03$. The previously unobserved 692-keV $\gamma$-ray branch in the 7978→7286→0 cascade following the $\beta$ feeding of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ 7978-keV state has also been measured as 0.85% and limits were placed on other $\gamma$ branches from the 7978-keV level. Revised values are given for various other $\beta$- and $\gamma$-ray branching ratios. Shell model calculations are made using the full $1\hslash \omega$ basis for the positive-parity states and $0\hslash \omega +2\mathrm{}\hslash \omega$ for the negative-parity states. The calculated electromagnetic decays of positive-parity states in ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ are compared with experiment. The first-forbidden $\beta$-decay matrix elements involved in the decay of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ to the lowest four states of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ are calculated and the resultant decay rates are compared with experiment. The question of meson exchange corrections to the ${\mathrm{½}}^{+}$ → ${\mathrm{½}}^{-}$ $\beta$ decay is considered.

RADIOACTIVITY ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ [from ${}_{}{}^9{}_{}{}^{}\mathrm{Be}(t, p)$]: measured $E_{\gamma }$, $I_{\gamma }$, $\gamma -\gamma$ coincidence; Ge(Li) and NaI(T1) detectors: deduced $\beta$- and $\gamma$-ray branches, $log\mathrm{ft}$ values; compared with theory.

• Received 3 May 1982

DOI:https://doi.org/10.1103/PhysRevC.26.1167