Observation of a core-excited<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>E</mml:mi><mml:mn>4</mml:mn></mml:mrow></mml:math>isomer in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Cd</mml:mi><mml:mprescripts/><mml:none/><mml:mn>98</mml:mn></mml:mmultiscripts></mml:math>

A core-excited ${I}^{\ensuremath{\pi}}=({12}^{+})$ spin-gap isomer was identified in $^{98}\mathrm{Cd}$ in an experiment at EUROBALL IV. It was found to feed the known ${I}^{\ensuremath{\pi}}=({8}^{+})$ seniority isomer by an $E4$ transition. Half-lives of ${T}_{1∕2}=0.23$$(\genfrac{}{}{0}{}{+4}{\ensuremath{-}3})\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ and 0.17$(\genfrac{}{}{0}{}{+6}{\ensuremath{-}4})\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ were measured for the two states at ${E}_{x}=6635\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and $2428\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, respectively. From the excitation energy of the core-excited isomer a $^{100}\mathrm{Sn}$ shell gap of $6.46(15)\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ 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 $\ensuremath{\delta}{e}_{\ensuremath{\pi}}&lt;0.4\phantom{\rule{0.3em}{0ex}}e$ is found, which corresponds to the empirical value $\ensuremath{\delta}{e}_{\ensuremath{\pi}}=0.45(\genfrac{}{}{0}{}{+20}{\ensuremath{-}25})\phantom{\rule{0.3em}{0ex}}e$ in the pure proton hole valence space.

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