Low-lying states of theBa132nucleus within the nucleon-pair approximation

Abstract

In this paper we study the low-lying states of the $^{132}\mathrm{Ba}$ 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 ${5}_{1}^{\ensuremath{-}}$ and ${6}_{1}^{\ensuremath{-}}$ states, respectively. Two mechanisms of the back-bending phenomenon in the yrast band previously suggested, i.e., the contribution of noncollective pairs with two ${h}_{11/2}$ 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 $^{132}\mathrm{Ba}$ 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 ${13}_{1}^{\ensuremath{-}}$ state in the band based on the ${5}_{1}^{\ensuremath{-}}$ state, and in the ${14}_{1}^{\ensuremath{-}}$ state in the band based on the ${6}_{1}^{\ensuremath{-}}$ state, in addition to the back-bending in the ${10}_{1}^{+}$ state in the yrast band. These back-bendings are interpreted in terms of the ${(\ensuremath{\nu}{h}_{11/2})}^{\ensuremath{-}2}$ 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 $B(E2,{10}_{1}^{+}\ensuremath{\rightarrow}{8}_{1}^{+})$, the configurations of negative-parity neutron pairs are essential.