Level densities and radiative strength functions

This paper presents a summary of the latest results from experiments performed at the Oslo Cyclotron. The Oslo group has developed a technique to extract simultaneously the level density and radiative strength function from primary γ-ray spectra. A small (pygmy) resonance in the radiative strength function has been observed at around 3 MeV in several deformed rare earth nuclei. This resonance is shown to be of M1 character and the origin is thought to be the scissors mode. This resonance vanishes for the spherical Sm nuclei, as expected, since the scissors mode is dependent on deformation. Experiments performed in Oslo found the resonance in Dy isotopes to be twice as wide as the width extracted in an indirect way by comparing simulations with two-step cascade spectra from neutron capture experiments. In 116,117Sn an increase in the slope of the radiative strength function around Eγ = 4.5 MeV indicates the onset of resonance-like structures in both nuclei, resulting in a significant enhancement of the radiative strength functions compared to standard models in the energy region 4.5<Eγ<7.7 MeV. Another interesting phenomenon is the unexpected enhancement of the γ-strength function below 4 MeV which has been observed in Fe, Mo, V, and Sc isotopes. The level density and radiative strength function of 96Mo have been reanalyzed, and the enhanced γ strength for Eγ<3-4 MeV is confirmed. This enhancement is presently not understood and remains a challenge for theoretical models.