A study of the $\beta -\nu$ angular correlation in the first-forbidden decay of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ to the ${\mathrm{½}}^{-}$ 2125-keV level of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ was performed. The 2125-keV ground-state transition was detected in coincidence with the betas and at 180° to them. The $\beta -\nu$ asymmetry was deduced from the $\gamma$-ray Doppler shift. The result obtained as a function of $\beta$ energy indicates that the $\beta$ transition is dominated by rank 0 matrix elements and is thus of interest as a test of meson-exchange effects in nuclei. A comparison is made to the large-basis, spurious-free, shell-model predictions of Millener.

RADIOACTIVITY ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ from ${}_{}{}^9{}_{}{}^{}\mathrm{Be}(t, p)$: $\beta -\gamma$ coin.; plastic and Ge(Li) detectors, measured $\gamma$-ray Doppler shift: deduced $\beta -\nu$ asymmetry and rank 0 and 1 contributions to decay to ${\mathrm{½}}^{-}$ state, compared to theory.

• Received 3 May 1982

DOI:https://doi.org/10.1103/PhysRevC.26.1186