Damping mechanisms and order-to-chaos transition in the warm rotating Er-163 nucleus

The gamma decay in the quasicontinuum is used to study the order-to-chaos transition in the thermally excited Er-163 nucleus. The experimental analysis is performed on high-statistics EUROBALL data, focusing on the spin region I approximate to(20-40)h, and internal excitation energy up of approximate to 2.5 MeV. The results are compared to cranked shell model calculations for this nucleus, taking into account the dependence on the K quantum number. Two main topics are investigated. First, the validity of the selection rules associated with the K quantum number are studied 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 approximate to 1.2 MeV, whereas in the interval U approximate to 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. Second, the line-shape analysis of gamma-gamma coincidence spectra provides a direct experimental measurement of the rotational and compound damping widths (Gamma(rot) and Gamma(mu)), yielding values of 200 and 20 keV, respectively, in good agreement with theory.