Warm Rotating Nuclei: Damping Mechanisms and the Order-To-Chaos Transition

The γ-decay in the quasi-continuum is used as a probe of nuclear structure properties in thermally excited nuclei. The experimental analysis is performed on high-statistics EUROBALL data on 163Er, and the results are compared to band mixing calculations for this nucleus. Two topics are investigated. First, a direct experimental measurement of the rotational and compound damping width (Γrot and Γμ) is given. From a line-shape analysis of γ-γ coincidence spectra values of 200 and 20 keV are obtained for Γrot and Γμ, respectively, in the spin region I ≅ 30-40ℎ, in good agreement with theory. Secondly, the validity of the selection rules associated with the K-quantum number are investigated as a function of the internal energy U above yrast. K-selection rules are found to be obeyed in the decay along discrete unresolved rotational bands up to U ≅ 1.2 MeV, while in the interval U ≅ 1.2-2.5 MeV, where the order-to-chaos transition is expected to take place, selection rules are found to be only partially valid. (author)