In this paper we study the structure of even-even Ba isotopes with neutron number $N=84–90$ within the framework of nucleon-pair approximation of the shell model. We use a phenomenological Hamiltonian, which includes single-particle energies, monopole, quadrupole, and octupole pairing energies, as well as quadrupole-quadrupole and octupole-octupole interactions. Our calculated energy levels and $B\left(E2\right)$ values are in good agreement with experimental data. The alternating parity doublets are well reproduced by considering $F^{-}$ nucleon pairs with spin-three and negative parity, which demonstrates the importance of $F^{-}$ pair in octupole deformation. The octupole-octupole interaction is found to favor stronger $B\left(E3\right)$ transitions and suppress $B\left(E2\right)$ transitions in the low-lying states of nuclei in these even-even Ba isotopes.

• Received 22 October 2023
• Revised 25 December 2023
• Accepted 29 January 2024

DOI:https://doi.org/10.1103/PhysRevC.109.024322