Heavy particles emitted in the $\beta$ decay of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ were studied by forming the activity in the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}\mathrm{}(t,p)\mathrm{}{}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ reaction at $E_t=3.4\mathrm{}$ MeV and measuring the radiations by means of a helium-jet system together with Si and NaI(Tl) detectors. $\alpha$ particles and corresponding ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ recoil ions were observed both in singles and in coincidence with 478-keV $\gamma$ rays. It is deduced that ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ branches 2.9±0.4% by $\beta$-ray emission to the 9875-keV $J^{\pi }={\frac{3}{2}}^{+}$ excited state of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, which then decays by $\alpha$ emission 87.4±1.2% to the ground state of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and 12.6±1.2% to the 478-keV excited state. The experimental $log\mathrm{ft}$ of 4.04±0.08 for the ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ $\beta$ branch to the 9875-keV state agrees with a theoretical estimate of ≃4.0, and the ambiguity in an earlier result is thereby resolved. The $\alpha$ branching intensity to the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ 478-keV state is in reasonable agreement with a theoretical estimate.

[RADIOACTIVITY ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$; measured delayed $\alpha$, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, and $\alpha -\gamma$ coin; He-jet system, Si and NaI(Tl) detectors; deduced $\beta$ and $\alpha$ decay branches; compared with theory.]

• Received 9 September 1980

DOI:https://doi.org/10.1103/PhysRevC.23.473