Low-energy M1 excitation mode in 172Yb

The multipolarity of a soft (Eγ=3.3(1)MeV) resonance in the total radiative strength function (RSF) of 172Yb is determined. For this reason, the level density and total RSF of 172Yb have been extracted from primary-γ spectra from the 173Yb(3He, αγ)172Yb reaction. In a second experiment, two-step-cascade (TSC) intensities have been measured in the 171Yb(nth,γγ)172Yb reaction. These intensities are compared to statistical-model calculations which are entirely based on experimental values of the level density and RSF from the former experiment. This comparison implies M1 assignment of the soft resonance. The strength of the M1 resonance is B(M1↑)=6.5(15)μN2.