Microscopic structure of the low-lying negative-parity states inSm154

Abstract

The proton-neutron symplectic model with Sp(12,$R$) dynamical algebra is applied to the description of the microscopic structure of the low-lying negative-parity states of the ${K}^{\ensuremath{\pi}}={0}_{1}^{\ensuremath{-}}$ and ${K}^{\ensuremath{\pi}}={1}_{1}^{\ensuremath{-}}$ bands in $^{154}\mathrm{Sm}$ without the introduction of additional degrees of freedom that are inherent to other approaches to odd-parity nuclear states. For this purpose, the model Hamiltonian is diagonalized in a U(6)-coupled basis, restricted to state space spanned by the fully symmetric U(6) irreps of the lowest odd irreducible representation of Sp(12,$R$). In this way, the positive- and negative-parity collective bands are treated on equal footing within the framework of the microscopic symplectic-based shell-model scheme. A good description of the energy levels of the two bands under consideration, as well as the reproduction of some energy splitting quantities which are usually introduced in the literature as a measure of the octupole correlations, is obtained. The microscopic structure of low-lying collective states with negative-parity in $^{154}\mathrm{Sm}$ shows that practically there are no admixtures from the higher shells and hence the presence of a very good U(6) dynamical symmetry. Additionally, the structure of the collective states under consideration shows also the presence of a good SU(3) quasidynamical symmetry.