Transitional Nuclei. I. Decay ofEu154to Levels ofSm154andGd154

Abstract

The $\ensuremath{\gamma}$-ray spectra accompanying the decay of $^{154}\mathrm{Eu}$ (16 yr) has been studied by using an isotopically separated $^{154}\mathrm{Eu}$ source as well as a Ge(Li) Compton suppression spectrometer. Over 150 $\ensuremath{\gamma}$ rays have been detected and can be ascribed to 32 different levels. It is suggested that the positive-parity levels can be accounted for by assignment to one- and two-phonon vibrational bands, and that the negative-parity levels form the bands of the fragmented octupole state. The level structure of the $K=0 \mathrm{and} 1$ members can be accounted for by assuming a strong Coriolis coupling between them. The relative $E2$ transition probabilities from the one- and two-phonon bands cannot be reconciled with simple two-band mixing theory. Properties associated with ${z}_{K}$ indicate that ${z}_{K}$ is not a good quantity in this nucleus, because of the strong mixing of the $\ensuremath{\beta}$- and $\ensuremath{\gamma}$-vibrational bands, and that its use in the past may have given false results for properties associated with it. The level energies in keV are: ${\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\ensuremath{-}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}\phantom{\rule{0ex}{0ex}}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{d},: 123.14({2}^{+}), 371.18({4}^{+}), \mathrm{and} 717.96({6}^{+});}{\ensuremath{\beta} \mathrm{band},: 680.71({0}^{+}), 815.55({2}^{+}), \mathrm{and} 1047.65({4}^{+});}{\ensuremath{\gamma} \mathrm{band},: 996.32({2}^{+}), 1127.90({3}^{+}), \mathrm{and} 1263.94({4}^{+});}{2\ensuremath{\beta} or M \mathrm{band},: (1292.7({0}^{+})), 1418.36({2}^{+}), \mathrm{and} (1698.2({4}^{+}));}{\ensuremath{\beta}\ensuremath{\gamma} \mathrm{band},: 1531.39({2}^{+}), 1660.94({3}^{+}), \mathrm{and} 1790.4({4}^{+});}{K=0 \mathrm{octupole}\mathrm{state},: 1241.34({1}^{\ensuremath{-}}), 1251.48({3}^{\ensuremath{-}}), \mathrm{and} 1364.2({5}^{\ensuremath{-}});}{K=1 \mathrm{octupole}\mathrm{state},: 1509.1({1}^{\ensuremath{-}}), 1397.53({2}^{\ensuremath{-}}), \mathrm{and} 1616.72({3}^{\ensuremath{-}}), \mathrm{and} 1559.68({4}^{\ensuremath{-}});}{K=2 \mathrm{octupole}\mathrm{state},: 1719.62({2}^{\ensuremath{-}}), 1796.78({3}^{\ensuremath{-}}), \mathrm{and} (1861.4({4}^{\ensuremath{-}})).}$ Other levels are at 1277.6, (1415.6), 1645.93, 1770.3, 1838, and 1894.69. The $K=2$ octupole state is consistent with Solov'ev and co-workers' assignment of a nearly pure two-quasiparticle configuration of $[411]{[523]}_{p}$, while the $K=1$ state is more consistent with its being of nearly pure $[411]{[532]}_{p}$ two-quasiparticle configuration. Consideration is given to a rotation-vibration Coriolis-type interaction between the $K=0 \mathrm{and} 1$ bands and a proposed pseudo-rotor-particle-coupling Coriolis-type interaction between the $K=1 \mathrm{and} 2$ bands of the octuple state. It is suggested that the levels at 1531.39, 1660.94, and 1790.4 keV form a $\ensuremath{\beta}\ensuremath{\gamma}$- coupled second vibrational band.