부산대학교 도서관

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 π = 0 + , β 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 156Gd 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,γ),(n,e-),(e,e′),(p,p′),(d,p), and (d,t) reactions with six known excited Kπ=0+ bands. We measured level lifetimes of 156Gd in order to determine the nature of the low-lying excited bands. Method: The lifetimes of the excited states in the 156Gd 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 156Gdnucleus including the lifetimes of three of the Kπ=0+ bands. Conclusions: There are two Kπ=0+ bands in this nucleus connected to the ground-state band. Transitions from the Kπ=0+2 band at 1049.5 keV to the ground-state band are more collective than the ones from the Kπ=0+3 band at 1168.2 keV. The moments of inertia of the various Kπ=0+, the Kπ=2+, and the Kπ=4+ bands show that all the bands except the Kπ=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 Kπ=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 Kπ=2+ band to the ground-state band supports the assignment of this band as the γ band. Also, the Kπ=0+4 and the Kπ=4+1 bands at 1715.2 and 1510.6 keV, respectively, are shown to be strongly connected to the Kπ=2+γ band presenting evidence for the observation of a second set of two-phonon γγ vibrational excitations albeit with greatly varying degrees of anharmonicity in comparison to the case of 166Er. [ABSTRACT FROM AUTHOR]