Level structure of ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$

The level structure and configurations of ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$ have been studied by observing the alpha decay of ${}_{}{}^{220}{}_{}{}^{}\mathrm{Fr}$, which is in secular equilibrium with ${}_{}{}^{224}{}_{}{}^{}\mathrm{Ac}$. Six of the 16 observed states are tentatively assigned to the configuration π(${\mathit{h}}_{9/2}$${)}^3$υ(${\mathit{g}}_{9/2}$${)}^5$, and four to the mixed configuration π(${\mathit{h}}_{9/2}$${)}^3$υ(${\mathit{g}}_{9/2}$${)}^4$(${\mathit{i}}_{11/2}$)+π(${\mathit{h}}_{9/2}$${)}^2$(f ${}_{7/2}{}^{}{}_{}{}^{}$)υ(${\mathit{g}}_{9/2}$${)}^5$. These two configurations in ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$ are compared with the corresponding configurations in ${}_{}{}^{212}{}_{}{}^{}\mathrm{Bi}$ and ${}_{}{}^{210}{}_{}{}^{}\mathrm{Bi}$. The shape transitions are traced from the quadrupole-octupole deformed ${}_{}{}^{224}{}_{}{}^{}\mathrm{Ac}$ through ${}_{}{}^{220}{}_{}{}^{}\mathrm{Fr}$ and ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$, to ${}_{}{}^{212}{}_{}{}^{}\mathrm{Bi}$, described by the spherical shell model, using the alpha decay sequence beginning with ${}_{}{}^{228}{}_{}{}^{}\mathrm{Pa}$. © 1996 The American Physical Society.