Institut de recherche sur les lois fondamentales de l'Univers

The level density and radiative strength function are fundamental nuclear properties and important input parameters in large network calculations of the nucleosynthesis of heavy elements. The Oslo group has developed a technique to extract simultaneously the level density and radiative strength function from the same experiment. After finding the level density as a function of excitation energy, the entropy is known and we can explore thermodynamic properties of the nucleus such as the microcanonical temperature and heat capacity. A small (pygmy) resonance in the strength function has been observed at around 3 MeV in several deformed rare earth nuclei; the resonance vanishes for the spherical Sm nuclei, as expected, since the origin of this resonance is thought to be the scissors mode. An unexpected enhancement of the gamma-strength below 4 MeV in the total radiative strength function of Fe, Mo, V, Sc (and preliminary in Ti) isotopes has been observed. This enhancement is presently not understood and remains a challenge for theoretical models.