A core-excited $I^{\pi }=\left({12}^{+}\right)$ spin-gap isomer was identified in ${}^{98}\mathrm{Cd}$ in an experiment at EUROBALL IV. It was found to feed the known $I^{\pi }=\left(8^{+}\right)$ seniority isomer by an $E4$ transition. Half-lives of $T_{1∕2}=0.23$$\left(\genfrac{}{}{0ex}{}{+4}{-3}\right)\mu \mathrm{s}$ and 0.17$\left(\genfrac{}{}{0ex}{}{+6}{-4}\right)\mu \mathrm{s}$ were measured for the two states at $E_x=6635\mathrm{keV}$ and $2428\mathrm{keV}$, respectively. From the excitation energy of the core-excited isomer a ${}^{100}\mathrm{Sn}$ shell gap of $6.46\left(15\right)\mathrm{MeV}$ is inferred. The measured $E4$ and $E2$ strengths, ${}^{100}\mathrm{Sn}$ core excitations and the origin of empirical polarization charges are discussed in the framework of large-scale shell model calculations. An $E2$ polarization charge for protons of $\delta e_{\pi }<0.4e$ is found, which corresponds to the empirical value $\delta e_{\pi }=0.45\left(\genfrac{}{}{0ex}{}{+20}{-25}\right)e$ in the pure proton hole valence space.

• Received 27 February 2004

DOI:https://doi.org/10.1103/PhysRevC.69.064304