Low-lying states of the Ba 132 nucleus within the nucleon-pair approximation

In this paper we study the low-lying states of the Ba132 nucleus within the nucleon-pair approximation, with our focus on the mechanism of band-crossing in the yrast band and two side bands headed by the 51- and 61- states, respectively. Two mechanisms of the back-bending phenomenon in the yrast band previously suggested, i.e., the contribution of noncollective pairs with two h11/2 neutron holes coupled to various spins (called H pairs for short) and the coupling of two neutron pairs with negative parity, are reconciled on the same footing and are found to be complementary in the description of low-lying states of the Ba132 nucleus. This is carried out by nucleon-pair-approximation calculation in the proton configuration space constructed by collective SD pairs, coupled with the neutron configuration space constructed by SD pairs, H pairs, and collective negative-parity pairs of spin from 4 to 7. Our calculations show that back-bending exists in the 131- state in the band based on the 51- state, and in the 141- state in the band based on the 61- state, in addition to the back-bending in the 101+ state in the yrast band. These back-bendings are interpreted in terms of the (νh11/2)-2 alignment. We also perform calculations in a number of selected subspaces for neutron configuration. We show that the states above the corresponding back-bendings are reasonably represented by the negative-parity neutron pairs and better represented by the spin-10 H pair. In order to reproduce the experimental value of B(E2,101+→81+), the configurations of negative-parity neutron pairs are essential.