The energies and intensities of $\gamma$ rays and internal-conversion electrons from the electron-capture decay of ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$ and from the $\alpha$ decays of ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$ and ${}_{}{}^{211}{}_{}{}^{}\mathrm{Po}$ have been measured. A new first-forbidden electron-capture branch of ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$ is reported. First-forbidden electron-capture transition rates are discussed with reference to the $2g_{\frac{9}{2}}$ and $1i_{\frac{11}{2}}$ neutron states in ${}_{}{}^{211}{}_{}{}^{}\mathrm{Po}$. $\gamma$ rays corresponding to the $l$-forbidden $M1\mathrm{}$ transitions $\nu {\left(3\mathrm{}{p}_{\frac{3}{2}}\right)}^{-1\mathrm{}}\to \nu {\left(2\mathrm{}{f}_{\frac{5}{2}}\right)}^{-1\mathrm{}}$ in ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ and $\nu \left(1\mathrm{}{i}_{\frac{11}{2}}\right)\to \nu \left(2\mathrm{}{g}_{\frac{9}{2}}\right)$ in ${}_{}{}^{211}{}_{}{}^{}\mathrm{Po}$ have been observed. The $\alpha$ spectra of ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$ and ${}_{}{}^{211}{}_{}{}^{}\mathrm{Po}$ and the energies and relative intensities of $\gamma$ rays from the electron-capture decay of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Bi}$ have been remeasured. A level scheme incorporating all of these decays is given.

RADIOACTIVITY ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$ [from ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}(\alpha , 2n)$]; measured $E_{\gamma }$, $I_{\gamma }$, $I_{\mathrm{ce}}$, $I_{\alpha }$; deduced $log\mathrm{ft}$, EC branching, $\alpha$ branching. ${}_{}{}^{211}{}_{}{}^{}\mathrm{Po}$, ${}_{}{}^{207}{}_{}{}^{}\mathrm{Bi}$, ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ deduced levels, $J$, $\pi$, ICC, $\gamma$ multipolarity, $\gamma$ branching. Ge(Li), Si(Li), Au-Si detectors.

• Received 6 January 1975

DOI:https://doi.org/10.1103/PhysRevC.11.1385