New High-Spin Isomer 2.3-DayAu198mand theAu198Level Structure

Abstract

In 18-MeV deuteron bombardments of enriched $^{200}\mathrm{Hg}$, we have produced a new isomer 2.27 \ifmmode\pm\else\textpm\fi{} 0.05-day $^{198m}\mathrm{Au}$, which is believed to be analogous to the $({h}_{\frac{11}{2}}\ensuremath{\pi},{i}_{\frac{13}{2}}\ensuremath{\nu}){12}^{\ensuremath{-}}$ isomer in $^{196}\mathrm{Au}$. No ${\ensuremath{\beta}}^{\ensuremath{-}}$ branching is observed in the decay of $^{198m}\mathrm{Au}$. The isomeric transition is followed by a cascade of 204.10- and 180.31-keV $\ensuremath{\gamma}$ rays to a 123-nsec level at 312.10 keV which deexcites to ground by a sequence of 97.21-keV ($E1$) and 214.89-keV ($E2$) transitions. A revised interpretation of the results of earlier ($n,\ensuremath{\gamma}$) and ($d,p\ensuremath{\gamma}$) studies of the $^{198}\mathrm{Au}$ level structure is presented and shell-model assignments for the levels populated in the isomeric decay are proposed. It is argued that the $E1$ deexcitation of the 123-nsec isomer involves the $l$-forbidden single-particle transition $({d}_{\frac{3}{2}}\ensuremath{\pi},{i}_{\frac{13}{2}}\ensuremath{\nu}){5}^{+}\ensuremath{\rightarrow}({d}_{\frac{3}{2}}\ensuremath{\pi},{f}_{\frac{5}{2}}\ensuremath{\nu}){4}^{\ensuremath{-}}$.