The properties of high-spin states in ${}_{}{}^{209}{}_{}{}^{}\mathrm{At}$ have been studied with the ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}({}_{}{}^6{}_{}{}^{}\mathrm{Li},3\mathrm{}n\gamma ){}_{}{}^{209}{}_{}{}^{}\mathrm{At}$ At reaction. In-beam measurements with Ge(Li) detectors of $\gamma$-ray excitation functions, $\gamma -\gamma$ coincidences, $\gamma$-ray angular distributions, and pulsed beam-$\gamma$ timing were made to establish decay schemes, level energies, spin-parity assignments, isomeric lifetimes, and magnetic moments. High-spin states involving three valence protons in the $1h_{\frac{9}{2}}$, $2f_{\frac{7}{2}}$, or $1i_{\frac{13}{2}}$ orbitals were identified. The electromagnetic properties observed in ${}_{}{}^{209}{}_{}{}^{}\mathrm{At}$ were compared with those in the closed-neutron core nucleus ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$ to examine the influence of the two neutron holes on the proton structure; microscopic calculations of this influence were carried out in perturbation theory.

NUCLEAR REACTIONS ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}({}_{}{}^6{}_{}{}^{}\mathrm{Li}, 3n\gamma )$, $E_{\mathrm{Li}}=29-34\mathrm{}$ MeV; measured $\gamma$-excitations, $\gamma -\gamma$ coincidences, $\gamma -W\left(\theta \right)$, pulsed beam-$\gamma$ timing, perturbed angular distributions; deduced level scheme, $\gamma$ multipolarities, $J^{\pi }$, $T_{\frac{1}{2}}$, $B\mathrm{}\left(E2\mathrm{}\right)\mathrm{}$, $g$ factor; perturbation calculations.

• Received 6 April 1976

DOI:https://doi.org/10.1103/PhysRevC.14.1023