Evidence for near instability against gamma deformation inBa128

The decay of 5.2-min ${}_{}{}^{128}{}_{}{}^{}\mathrm{La}$ to levels in ${}_{}{}^{128}{}_{}{}^{}\mathrm{Ba}$ has been investigated by $\gamma$-ray and conversion-electron spectroscopy. Of 78 $\gamma$ rays assigned to the decay, 69 have been placed in a decay scheme consisting of 31 excited states. The ${}_{}{}^{128}{}_{}{}^{}\mathrm{La}$ ground state is probably $5^{-}$ and is interpreted as the ($\nu {\frac{7}{2}}^{-}\mathrm{}\left[523\right]+\pi {\frac{3}{2}}^{+}\mathrm{}\left[411\right]\mathrm{}$) configuration. $\mathrm{EC}+{\beta }^{+}$ decays with $log\mathrm{ft}$ values in the allowed range to states at 2425.5 and 2878.3 keV are explained by ${\left(\nu {\frac{7}{2}}^{-}\mathrm{}\right[523]+\nu {\frac{1}{2}}^{+}\mathrm{}[411\left]\right)}_{4-}$ and ${\left(\nu {\frac{7}{2}}^{-}\mathrm{}\right[523]+\nu {\frac{3}{2}}^{+}\mathrm{}[402\left]\right)}_{5-}$ configurations, respectively. A quasi-$\gamma$ band is proposed with levels at 884.5 keV ($2^{+}$), 1324.5 keV ($3^{+}$), 1372.4 keV ($4^{+}$), and 1931.4 keV ($5^{+}$). A state at 1939.4 keV is possibly the $6^{+}$ band member. The properties of ${}_{}{}^{128}{}_{}{}^{}\mathrm{Ba}$ were calculated using the microscopic boson-expansion code of Kishimoto and Tamura and found to be in rather good agreement with experiment. The small experimentally observed splitting (48 keV) between the $3_{\gamma }$ and $4_{\gamma }$ states is a signature of $\gamma$ instability. The prolate-oblate difference, consistent with the experimental data, is estimated to be 0.5 MeV, a value in good agreement with potential-surface calculations.