Studies of the Radioactive Decays ofEu152andEu154

Abstract

$\ensuremath{\gamma}$ rays emitted in the radioactive decays of $^{152}\mathrm{Eu}$ and $^{154}\mathrm{Eu}$ have been studied with Ge(Li) and NaI detectors. Energies and relative intensities were derived from singles experiments with large volume Ge(Li) detectors and assignments of $\ensuremath{\gamma}$ rays to transitions in the level schemes were made with the help of Ge(Li)-NaI coincidence measurements. In all, 71 $\ensuremath{\gamma}$ rays were observed in the $^{152}\mathrm{Eu}$ decay and 51 in the $^{154}\mathrm{Eu}$ case. Three members of each of the $\ensuremath{\beta}$- and $\ensuremath{\gamma}$- vibrational bands were observed. In addition the results of the coincidence experiments are used to assign a level at 1649.8 keV as the probable ${2}^{\ensuremath{-}}$ member of the $K=2$ band in $^{152}\mathrm{Sm}$. The properties of this state and of the ${K}^{\ensuremath{\pi}}={0}^{\ensuremath{-}} \mathrm{and} {1}^{\ensuremath{-}}$ bands are compared with the properties of the corresponding states in $^{154}\mathrm{Gd}$. It is found that there is evidently less Coriolis coupling between the negative-parity states in $^{152}\mathrm{Sm}$ than in $^{154}\mathrm{Gd}$. The members of the ${K}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ band in the former are not inverted as in the latter, and the $B(E1)$ ratios from the ${K}^{\ensuremath{\pi}}={0}^{\ensuremath{-}}$ states in the former are in better agreement with the predictions of the rotational model. The low-energy levels in $^{152}\mathrm{Gd}$ are considered in light of two possible interpretations. We find that it is preferable to consider these states as resulting from quasirotations and quasivibrations than to treat them as members of one-, two-, and three-phonon vibrational excitations about spherical equilibrium shapes. Finally, a new ${2}^{+}$ level at 1293 keV in $^{152}\mathrm{Sm}$ is discussed in light of recent experiments involving two-neutron-transfer reactions to levels in this nucleus.