M1 Strength Distributions in Deformed Nuclei.

Abstract

The Elliott SU(3) Model, extended via pseudo-spin for heavy nuclei, is used to study low-lying magnetic dipole excitations in deformed nuclei which axe known as "scissors" modes. Proton and neutron degrees of freedom are handled explicitly and a system Hamiltonian that preserves SU(3) symmetry and one that includes single particle energies as well as quadrupole-quadrupole and pairing two-body interactions are considered. Starting from a basic nuclear Hamiltonian that preserves SU(3) symmetry, a microscopic interpretation of the "scissors" mode of the Two Rotor Model is realized through a linear mapping between invariants of the rotor group and SU(3). The model allows for a classification of SU(3) shell-model configurations in terms of collective degrees of freedom. Triaxiality of the parent proton and neutron distributions is shown to add a "twist" degree of freedom to the usual "scissors" mode picture. An analysis of the results for the M1 transition strength given in the SU(3) limit of the theory provides evidence for the underlying collective nature of the "scissors" mode. The low-energy spectrum and M1 strength distribution are also calculated using a realistic Hamiltonian that includes the SU(3) symmetry breaking pairing and single particle terms. These additional terms generate the experimentally observed fragmentation, that is, the breakup of the M1 strength among several levels, thus providing a natural explanation for the complex structure of the M1 transition spectra. Results for the strongly-deformed even-even 156-160Gd and 156-160Dy isotopes are shown to be in good agreement with experiment. Further results for the gamma-unstable (soft rotor) 196Pt nucleus and the even-odd 163Dy system are also discussed and found to compare favorably with the available experimental data.