Low-energy magnetic dipole radiation in open-shell nuclei

Low-energy M1 strength functions of 60,64,68Fe are determined on the basis of large-scale shell-model calculations with the goal to study their development from the bottom to the middle of the neutron shell. We find that the zero-energy spike, which characterizes nuclei near closed shells, develops toward the middle of the shell into a bimodal structure composed of a weaker zero-energy spike and a scissors-like resonance around 3 MeV, where the summed strengths of the two structures change within only 8% around a value of 9.8 nuclear magnetons. The summed strength of the scissors region exceeds the total absorption strength from the ground state by a factor of 2 or more, which explains the discrepancy between total strengths of the scissors resonance derived from (gamma,gamma') experiments and from experiments using light-ion induced reactions.