The electron-capture decay of ${}_{85}{}^{204}{}_{}{}^{}\mathrm{At}$ (9 min) has been studied by means of $\gamma$ and $e^{-}$ multiscaled singles and $\gamma -\gamma$ coincidence measurements with Ge(Li) and Si(Li) detector systems. The samples were produced by $\mathrm{Ir}({}_{}{}^{16}{}_{}{}^{}\mathrm{O}, xn)$ reactions and on-line isotope separation. About 50 $\gamma$-ray transitions have been assigned to the ${}_{}{}^{204}{}_{}{}^{}\mathrm{At}$ decay, ∼35 of which have been placed in a proposed level scheme for ${}_{84}{}^{204}{}_{}{}^{}\mathrm{Po}$. The energy of the $8^{+}$→$6^{+}$ isomeric transition has been deduced to be 12.1 keV, and the half-life of the $8^{+}$ isomer has been measured to be 150±10 ns. From these data, the $B\left(E2\mathrm{}\right)\mathrm{}$ value of this isomeric transition is found to be 3.7 Weisskopf units. The ${}_{}{}^{204}{}_{}{}^{}\mathrm{At}$ ground state is assigned an $I^{\pi }$ value of $7^{+}$, which eliminates the need for some low-energy levels in ${}_{}{}^{204}{}_{}{}^{}\mathrm{At}$ and ${}_{}{}^{200}{}_{}{}^{}\mathrm{Bi}$ that were tentatively proposed in recent data evaluations, where a $5^{+}$ assignment was made.

RADIOACTIVITY ${}_{}{}^{204}{}_{}{}^{}\mathrm{At}$ from $\mathrm{Ir}({}_{}{}^{16}{}_{}{}^{}\mathrm{O}, xn)$; measured $E_{\gamma }$, $I_{\gamma }$, ICC, $\gamma -\gamma -t$, isomeric lifetime in ${}_{}{}^{204}{}_{}{}^{}\mathrm{Po}$; ${}_{}{}^{204}{}_{}{}^{}\mathrm{Po}$ deduced levels, $J$, $\pi$; ${}_{}{}^{204}{}_{}{}^{}\mathrm{At}$ ground state deduced $J$, $\pi$; Ge(Li) and Si(Li) detectors; on-line isotope-separated sources.

• Received 15 November 1982

DOI:https://doi.org/10.1103/PhysRevC.27.2248