Evidence for near instability against gamma deformation inBa128

Abstract

The decay of 5.2-min $^{128}\mathrm{La}$ to levels in $^{128}\mathrm{Ba}$ has been investigated by $\ensuremath{\gamma}$-ray and conversion-electron spectroscopy. Of 78 $\ensuremath{\gamma}$ 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 ${5}^{\ensuremath{-}}$ and is interpreted as the ($\ensuremath{\nu}{\frac{7}{2}}^{\ensuremath{-}}[523]+\ensuremath{\pi}{\frac{3}{2}}^{+}[411]$) configuration. $\mathrm{EC}+{\ensuremath{\beta}}^{+}$ decays with $log\mathrm{ft}$ values in the allowed range to states at 2425.5 and 2878.3 keV are explained by ${(\ensuremath{\nu}{\frac{7}{2}}^{\ensuremath{-}}[523]+\ensuremath{\nu}{\frac{1}{2}}^{+}[411])}_{4\ensuremath{-}}$ and ${(\ensuremath{\nu}{\frac{7}{2}}^{\ensuremath{-}}[523]+\ensuremath{\nu}{\frac{3}{2}}^{+}[402])}_{5\ensuremath{-}}$ configurations, respectively. A quasi-$\ensuremath{\gamma}$ 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 ${3}_{\ensuremath{\gamma}}$ and ${4}_{\ensuremath{\gamma}}$ states is a signature of $\ensuremath{\gamma}$ 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.[RADIOACTIVITY $^{128}\mathrm{La}$ [from $^{118}\mathrm{Sn}(^{14}\mathrm{N},4n)$], measured ${T}_{\frac{1}{2}}$, ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${I}_{\mathrm{ce}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coin; deduced $log\mathrm{ft}$. $^{128}\mathrm{Ba}$ deduced levels, ICC, $J$, $\ensuremath{\pi}$. Enriched target, Ge(Li) and Si(Li) detectors.]