Proton-neutron symmetry in boson models of nuclear structure

Abstract

The present review deals with nuclear models which consider collective proton and neutron motion separately in terms of bosons. Extensions of the geometric Bohr-Mottelson model are sketched, followed by a more extensive discussion of the algebraic interacting boson approximation (IBA-2). In the latter the concept of F spin plays a central role, and detailed attention is paid to its tensorial properties. A general projection method is presented whereby any IBA-2 calculation for states of maximal F spin can be transformed into a simpler one in IBA-1, where the proton-neutron distinction is ignored. Data are presented on F-spin multiplets, and global fits with the IBA are discussed. A major portion of the review is concerned with M1 transitions. Strong M1s in deformed and spherical nuclei, with emphasis on the latter, are discussed as resulting from a change in proton-neutron symmetry. Weak M1s are considered as reflecting such symmetry changes in small amplitudes of the wave function. Attention is given to one-fluid boson models (IBA-1, geometric) simulating these two-fluid effects.