The so-called "two-octupole-phonon states" in 146,148Gd are theoretically analyzed by using the Dyson boson mapping method. In our analyses, the free ground state of 146Gd is treated as doubly closed shell since Z=64 is rather good subshell closure. The starting collective multi-phonon space that we take consists of collective octupole phonons and collective monopole pairing phonons for 146Gd. Additionally correlated particle-pair modes with J≠0 are included in the multi-phonon space for 148Gd. The effective Hamiltonian used is constituted by a Woods-Saxon-type single-particle potential, an octupole-octupole force for particle-hole modes and a surface-delta interaction for particle-pair and hole-pair modes. The numerical calculation can considerably well reproduce the octupole collectivity shown by experiments for 146,148Gd. It is shown that the Dyson boson mapping method is useful for such a complicated system as 148Gd.
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