MAGNETIC DIPOLE SPIN EXCITATIONS IN RARE-EARTH AND ACTINIDE NUCLEI

Magnetic dipole spin excitations around E(x) congruent-to 6-8 MeV are described for rare-earth nuclei, ranging from Ce-140 to Pt-198. Starting from an unperturbed basis of 1+ two-quasiparticle states, constructed within the Nilsson model and applying the BCS formalism, one can quantitatively estimate the total spin M1 strength. We include Coriolis mixing and residual quadrupole and spin separable forces and study the spin excitations in more detail for a number of representative examples. Far from closed shells, a double-humped structure in the spin M1 strength results from the calculations and is found to be related to separate proton and neutron spin-flip excitations. We furthermore show that quantitative discrepancies between theoretical and experimental M1 strength distributions are resolved to a large extent when accounting for fragmentation of the strength into a background of more complex configurations. Our results are compared with those of other theoretical studies and the different interpretations are evaluated, starting from a schematic model calculation.