A Step toward Large-Amplitude Description of the Ground Band around Band Crossing

Abstract

The possible occurrence of large-amplitude motion in the band-crossing region is shown numeri­ cally for the ground rotational band. Calculations are based on the formalism by means of the self-consistent collective coordinate method. The phenomena in which the energy curve of the ground-state band crosses that of the rotation-aligned band (so-called s-band) have been mostly analyzed in the framework of mean-field theory. In particular, the roles of residual interactions have not yet been investigated in the band-crossing region, where the excitation energy of an aligned two-quasiparticle state is very low. The ground-state band (g-band) is not expected to be purely rotational around the band crossing, where it is thought to begin generating a new intrinsic mean field. Residual interaction is considered to play an essential role in this dynamics. For example, in 164Er the g-band and the s-band cross around the angular momentum 1=15. Just after the crossing, the yrast 1=16 state decays to two 1=14 states with nearly the same B(E2) value. l ) This fact implies that the structures of these states are not simple, though they are usually considered to be rotational states built on some intrinsic state. It is an important subject to study the dynamics of the transition (reorganization) process from the intrinsic state of the g-band to that of the s-band. As it is our aim to describe the transition process as large-amplitude motion, we first extract the relevant subspace connecting old and new intrinsic states, in which the large­ amplitude motion is properly described. Marumori and his collaborators attempted to define such a relevant subspace of the large-amplitude motion by making use of the selfconsistent collective coordinate (SeC) method. 2 ).3) In this paper we report the results of numerical application of the description given in the appendix of Ref. 3), which is now extended to the fourth power of an amplitude. 4 ) As we do not attempt to reproduce experimental values in this report, we adopt the model Hamiltonian of the pairing-plus-quadrupole force and the model space of two major shells for proton and neutron, respectively. In this model the g-band and the s-band of 164Er cross around 1=7. The main purpose of this paper is to study the dynamical roles of residual interaction at 1=6. In order to take new degrees of freedom for large-amplitude motion into account, we follow Ref. 3) and express a state of the g-band in the form