Nucleon-pair shell model and its application to barium isotopes

The nucleon pair shell model truncated to the S-D subspace is applied to the Barium isotopes for examining the effects on collectivity due to the non-degeneracy of the single-particle energies, and to the short-range and long-range forces. It is found that both the short-range force and long-range force enhance the collectivity, while the single particle level splitting tends to destroy the collectivity, and has profound effects on the scale and pattern of the spectra as well as on the B(E2) values. The calculation shows that the collectivity survives in the "realistic" S-D subspace with s. p. energy splitting fully taken into account.