New band mechanism of doubly-odd nuclei around mass 130

Abstract

The nuclear structure of the $\ensuremath{\Delta}I=1$ doublet bands in doubly-odd nuclei $^{130}\mathrm{Cs}$, $^{132}\mathrm{Cs}$, $^{132}\mathrm{La}$, and $^{134}\mathrm{La}$ is studied in terms of a pair-truncated shell model, in which the collective nucleon pairs with angular momenta of zero and two are its basic ingredients. The effective interactions consist of single-particle energies and monopole and quadrupole pairing plus quadrupole-quadrupole interactions, whose strengths are determined so as to describe the level schemes of even-even nuclei in the mass $A~130$ region. The calculation reproduces well the energy levels of the doublet bands and the electromagnetic transitions, especially the staggering of the ratios $B(M1;I\ensuremath{\rightarrow}I\ensuremath{-}1)/B(E2;I\ensuremath{\rightarrow}I\ensuremath{-}2)$ for the yrast states. Through the analysis of the wave functions, the doublet bands turn out to be made of different angular momentum configurations of an unpaired neutron and an unpaired proton, weakly coupled with the quadrupole collective excitations of the even-even part of the nucleus.