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 (Z50 and N82) 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.