Collective E2 transitions of midshell Ba isotopes in the boson expansion theory -: art. no. 024303

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 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.