Low-Energy Enhancement of Magnetic Dipole Radiation

Magnetic dipole strength functions are deduced from averages of a large number of M1 transition strengths calculated within the shell model for the nuclides Zr-90, Mo-94, Mo-95, and Mo-96. An enhancement of M1 strength toward low transition energy is found for all nuclides considered. Large M1 strengths appear for transitions between close-lying states with configurations including proton as well as neutron high-j orbits that recouple their spins and add up their magnetic moments coherently. The M1 strength function deduced from the calculated M1 transition strengths is compatible with the low-energy enhancement found in (He-3, He-3') and (d, p) experiments. The Letter presents an explanation of the experimental findings.