Abstract
Collective $E2$ transitions of midshell Ba isotopes are studied by means of the boson expansion theory. The fermion Hamiltonian is comprised of the self-consistent $\mathrm{QQ}$ interaction with higher-order (many-body) terms, monopole- and quadrupole-pairing interactions in addition to the spherical limit of the Nilsson Hamiltonian. The Kishimoto-Tamura method of normal-ordered linked-cluster expansion of the modified Marumori boson mapping is applied to construct the microscopic boson image of the Hamiltonian and that of the $E2$ operator. It is shown that the marked increase of quadrupole collectivity, indicated by the enhancement of experimental $B(E2),$ as neutron numbers approach the midshell value of $N=66$ can be reproduced naturally in terms of the microscopic boson expansion theory by using a standard value of the effective charge.
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