Octupole vibrational excitations built on shell-model states in Xe, Cs, and Ba isotopes up to mass 142

Large-scale nuclear shell-model calculations are performed in Xe, Cs, and Ba isotopes up to mass 142 (Z≥50 and N≥82) assuming ¹³²Sn as a doubly magic core. All the single-particle levels in the one-major shells, six neutron (1f7/2, 2p3/2, 2p1/2, 0h9/2, 1f5/2, and 0i13/2) orbitals, and five proton (0g7/2, 1d5/2, 1d3/2, 0h11/2, and 2s1/2) orbitals, are considered. For an effective two-body interaction, only one set of the multipole pairing and quadrupole-quadrupole interactions between neutrons and protons is employed and the strengths of the two-body interactions are set constant for all the nuclei considered. These interactions are phenomenologically determined to reproduce the experimental energy spectra in two-body systems. Single-particle energies are set constant for all the nuclei except the neutron 0i13/2 intruder orbital. Some of the isomeric states are analyzed in terms of the shell-model configurations. In this mass region, octupole correlated states are found in the low-lying energy, for which the collective octupole vibrational motion is involved. These states are constructed by phenomenologically introducing a collective octupole-phonon built on top of each shell-model state. Octupole vibrational bands naturally emerge in this treatment.