TOWARDS UNDERSTANDING MAGNETIC DIPOLE EXCITATIONS IN DEFORMED-NUCLEI - PHENOMENOLOGY

The pseudo SU(3) model, in an extended version in which proton and neu tron degrees of freedom are incorporated explicitly, is used to study the nature of low-lying magnetic dipole excitations in even-even rare- earth nuclei. Through a linear mapping between invariants of the rotor group and SU(3), a microscopic interpretation of the ''scissors'' mod e of the two-rotor model is realized. The model allows for, first; a c lassification of SU(3) shell-model configurations in terms of collecti ve degrees of freedom and, second, an adjustment of the relative energ y of distinct 1(+) states. Triaxiality of the parent proton and neutro n distributions is shown to add a ''twist'' degree of freedom to the u sual ''scissors'' mode picture. This new mode should be observed as: ( 1) a low-lying K=0 excitation, and (2) a second, doubly degenerate, K= 1 transition only slightly higher in energy than the usual scissors mo de. Fragmentation of the M1 strength distribution can be reproduced wi thin framework of the model by including representation mixing terms i n the Hamiltonian. [S0556-2813(98)01304-1].