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, beta, and gamma bands in Sm-154. 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 Sm-154 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 beta and 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) . Q(n) interaction.
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