Structure of the low-lying positive-parity states in Sm154

Abstract

The proton-neutron symplectic model with Sp(12,$R$) dynamical algebra is applied to the simultaneous description of the microscopic structure of the low-lying states of the lowest ground, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ bands in $^{154}\mathrm{Sm}$. For this purpose, the model Hamiltonian is diagonalized in a U(6)-coupled basis, restricted to the state space spanned by the fully symmetric U(6) irreps. A good description of the energy levels of the three bands under consideration as well as the intraband $B(E2)$ transition strengths between the states of the ground band is obtained without the use of an effective charge. The microscopic structure of low-lying collective states in $^{154}\mathrm{Sm}$ shows that there are no admixtures from the higher shells and hence shows the presence of a very good U(6) dynamical symmetry. It is also shown that, in contrast to the Sp(6,$R$) case, the lowest excited bands, e.g., the $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ bands, naturally appear together with the ground band within a single Sp(12,$R$) irreducible representation. The obtained results is given a simple geometrical multiphonon interpretation, based on the algebraic realization of the coupled two-rotor picture, which in turn suggests an interpretation of the low-lying excited bands as relative proton-neutron excitations of the two-component nuclear system, governed by the ${Q}_{p}\ifmmode\cdot\else\textperiodcentered\fi{}{Q}_{n}$ interaction.