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

The γ decay in the quasicontinuum is used to study the order-to-chaos transition in the thermally excited 163 Er nucleus. The experimental analysis is performed on high-statistics EUROBALL data, focusing on the spin region I ≈ (20–40) ¯ h, and internal excitation energy up of ≈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 ≈ 1.2 MeV, whereas 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. Second, the line-shape analysis of γ -γ coincidence spectra provides a direct experimental measurement of the rotational and compound damping widths (� rot and � µ), yielding values of 200 and 20 keV, respectively, in good agreement with theory