Lifetime measurements in Gd156

Abstract

Background: The nature of low-lying oscillations or excitations around the equilibrium deformed nuclear shape remains an open question in a nuclear structure. The question revolves around the possible degrees of freedom in deformed nuclei. Rotational motion is an expected feature of deformed nuclei; the open challenge is whether the ``granularity'' of the nuclei allows single or multiple quanta of vibrational oscillations or excitations superimposed on the equilibrium deformed shape of the nucleus. Special emphasis is placed on the ${K}^{\ensuremath{\pi}}={0}^{+},\phantom{\rule{0.28em}{0ex}}\ensuremath{\beta}$ vibration whose existence is open to debate some 40 years after Bohr-Mottelson-Rainwater's Nobel prize for connecting nucleon motion to the emergence of collectivity.Purpose: The $^{156}\mathrm{Gd}$ nucleus is an excellent test case for the search of the predicted oscillations since it has one of the most developed level schemes up to 2.35 MeV and it lies in the well-deformed rare-earth region of the chart of nuclides. This nucleus has previously been studied by $(n,\ensuremath{\gamma}),\phantom{\rule{0.28em}{0ex}}(n,{e}^{\ensuremath{-}}),\phantom{\rule{0.28em}{0ex}}(e,{e}^{\ensuremath{'}}),\phantom{\rule{0.28em}{0ex}}(p,{p}^{\ensuremath{'}}),\phantom{\rule{0.28em}{0ex}}(d,p)$, and ($d,t$) reactions with six known excited ${K}^{\ensuremath{\pi}}={0}^{+}$ bands. We measured level lifetimes of $^{156}\mathrm{Gd}$ in order to determine the nature of the low-lying excited bands.Method: The lifetimes of the excited states in the $^{156}\mathrm{Gd}$ nucleus were measured following neutron capture using the grid technique at the Institut Laue-Langevin in Grenoble, France.Results: Twelve level lifetimes were measured from four excitation bands in the $^{156}\mathrm{Gd}$ nucleus including the lifetimes of three of the ${K}^{\ensuremath{\pi}}={0}^{+}$ bands.Conclusions: There are two ${K}^{\ensuremath{\pi}}={0}^{+}$ bands in this nucleus connected to the ground-state band. Transitions from the ${K}^{\ensuremath{\pi}}={0}_{2}^{+}$ band at 1049.5 keV to the ground-state band are more collective than the ones from the ${K}^{\ensuremath{\pi}}={0}_{3}^{+}$ band at 1168.2 keV. The moments of inertia of the various ${K}^{\ensuremath{\pi}}={0}^{+}$, the ${K}^{\ensuremath{\pi}}={2}^{+}$, and the ${K}^{\ensuremath{\pi}}={4}^{+}$ bands show that all the bands except the ${K}^{\ensuremath{\pi}}={0}_{3}^{+}$ band at 1168.2 keV have nearly identical moments of inertia with the ground-state band pointing to the fact that all of the bands discussed here with the exception of this indicating ${K}^{\ensuremath{\pi}}={0}_{3}^{+}$ band seem to be collective excitations built on the ground state. This result is consistent with various theoretical predictions. $B(E2)$ calculations for transitions from the ${K}^{\ensuremath{\pi}}={2}^{+}$ band to the ground-state band supports the assignment of this band as the $\ensuremath{\gamma}$ band. Also, the ${K}^{\ensuremath{\pi}}={0}_{4}^{+}$ and the ${K}^{\ensuremath{\pi}}={4}_{1}^{+}$ bands at 1715.2 and 1510.6 keV, respectively, are shown to be strongly connected to the ${K}^{\ensuremath{\pi}}={2}^{+}\phantom{\rule{0.28em}{0ex}}\ensuremath{\gamma}$ band presenting evidence for the observation of a second set of two-phonon $\ensuremath{\gamma}\ensuremath{\gamma}$ vibrational excitations albeit with greatly varying degrees of anharmonicity in comparison to the case of $^{166}\mathrm{Er}$.