Level densities and radiative strength functions

摘要

This paper presents a summary of the latest results from experiments performed at the OsloCyclotron. The Oslo group has developed a technique to extract simultaneously the level densityand radiative strength function from primary γ-ray spectra. A small (pygmy) resonance in theradiative strength function has been observed at around 3 MeV in several deformed rare earthnuclei. This resonance is shown to be of M1 character and the origin is thought to be the scissorsmode. This resonance vanishes for the spherical Sm nuclei, as expected, since the scissors modeis 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~(116,117)Sn an increase in the slope of theradiative strength function around E_γ = 4.5 MeV indicates the onset of resonance-like structuresin both nuclei, resulting in a significant enhancement of the radiative strength functions comparedto standard models in the energy region 4.5 < E_γ< 7.7 MeV. Another interesting phenomenon isthe unexpected enhancement of the y-strength function below 4 MeV which has been observed inFe, Mo, V, and Sc isotopes. The level density and radiative strength function of 96Mo have beenreanalyzed, and the enhanced γ strength for E_γ< 3 — 4 MeV is confirmed. This enhancement ispresently not understood and remains a challenge for theoretical models.