Structure of upper g9/2-shell nuclei and shape effect in the 94Ag isomeric states

Using a shell model which is capable of describing the spectra of upper $g_{9\mathrm{/}2}$-shell nuclei close to the $N=Z$ line, we study the structure of two isomeric states $7^{+}$ and ${21}^{+}$ in the odd-odd $N=Z$ nucleus ${}^{94}$Ag. It is found that both isomeric states exhibit a large collectivity. The $7^{+}$ state is oblately deformed, and is suggested to be a shape isomer in nature. The ${21}^{+}$ state becomes isomeric because of level inversion of the ${19}^{+}$ and ${21}^{+}$ states due to core excitations across the $N=Z=50$ shell gap. Calculation of spectroscopic quadrupole moment indicates clearly an enhancement in these states due to the core excitations. However, the present shell model calculation that produces the ${19}^{+}$-${21}^{+}$ level inversion cannot accept the large-deformation picture of Mukha {\it et al.} in Nature {\bf 439}, 298 (2006).