Damping, motional narrowing, and chaos in rotational nuclei

We have studied the unresolved gamma-ray spectrum of three ytterbium nuclei produced in heavy-ion fusion reactions and measured the two damping widths, Gamma(mu), the spreading (compound-damping) width, and Gamma(rot), the rotational damping width, together with P-nar, the probability that the population enters and leaves a given state via the same component of the wave function. Our results for Gamma(mu) and Gamma(rot) are in good agreement with theoretical expectations, as well as with those of other groups using different methods. Our results for P-nar lead to a new method for quantitative evaluation of the full order-to-chaos transition in these Yb nuclei. Finally, we provide the first evidence for the motional narrowing of Gamma(rot) in nuclei. The basic physics occurring in this region is level mixing, which our simulation technique reproduces surprisingly well.