PROTON NEUTRON SYMMETRY IN BOSON MODELS OF NUCLEAR-STRUCTURE

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.