Configuration changes and hindered decays in four- and six-quasiparticle isomers in 178Ta.

Abstract

A six-quasiparticle isomer with ${K}^{\ensuremath{\pi}}={21}^{\ensuremath{-}}$, a half-life of 290(12) ms, and the ${\ensuremath{\pi}}^{3}{\ensuremath{\nu}}^{3}$ configuration has been identified in the odd-odd nucleus $^{178}\mathrm{Ta}$, at an excitation energy of 2902 keV. The rotational bands built on the known ${K}^{\ensuremath{\pi}}={15}^{\ensuremath{-}}$ isomer and on the newly found ${16}^{+}$ four-quasiparticle and ${22}^{+}$ six-quasiparticle states, have also been identified, allowing characterization of the configurations. The ${15}^{\ensuremath{-}}$ band is predominantly of $\ensuremath{\pi}{\ensuremath{\nu}}^{3}$ character with a ${\ensuremath{\pi}}^{3}\ensuremath{\nu}$ admixture. When the mixing is taken into account the excitation energies of the main yrast multi-quasiparticle states can be reproduced. The multi-quasiparticle states observed are related essentially through the addition of the two-quasiparticle component ${\ensuremath{\nu}}^{2}[{6}^{+}]$ or ${\ensuremath{\pi}}^{2}[{6}^{+}]$. Depending on whether the transition between the states involves the change ${\ensuremath{\nu}}^{2}[{6}^{+}]\ensuremath{\rightarrow}[0]$ or ${\ensuremath{\pi}}^{2}[{6}^{+}]\ensuremath{\rightarrow}[0]$, the $E2$ hindrance factors for decays between the six- and four-quasiparticle states are relatively large or small. This dependence mimics the pattern observed in the two-quasiparticle core transitions and, because the ${15}^{\ensuremath{-}}$ isomer is mainly $\ensuremath{\pi}{\ensuremath{\nu}}^{3}$, the magnitude sequence is inverted compared to that observed in $^{176}\mathrm{Ta}$.