Damping, motional narrowing, and chaos in rotational nuclei

Abstract

We have studied the unresolved γ -ray spectrum of three ytterbium nuclei produced in heavy-ion fusion reactions and measured the two damping widths, µ , the spreading (compound-damping) width, and 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 µ and 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 rot in nuclei. The basic physics occurring in this region is level mixing, which our simulation technique reproduces surprisingly well.

Projected shell model analysis of structural evolution and chaoticity in fast-rotating nuclei

“…The rare-earth nuclei are optimal for an experimental study on rotational damping [58]. The calculated n branch values as functions of the excitation energy U above the yrast states are shown in figure 5 for 164 Yb.…”

“…Experimentally, rotational damping can be studied by analyzing the rotational damping width Γ rot , which is defined as the full width half maximum (FWHM) of the distribution of transition energies for E2 transitions from a single level [58]. The Γ rot can also be calculated theoretically by analyzing the distribution of ( )…”

Rotation of warm nuclei and superdeformation

“…nuclei, both in one-dimension (mainly by the Berkeley group [54,57,58]) and in higher-coincidence histograms (in particular by the collaboration around the NBI [47,51,56,[59][60][61]). This allowed the general features of the spectrum landscape to be reproduced, and orders of magnitude to be extracted for key quantities, such as the average spreading width G rot of the damped rotation (of the order of 200 keV around spin  40 ), the compound damping width G m (rather constant and of the order of 20 keV) and the critical excitation energy U 0 at which damping sets in (about 1 MeV).…”

“…An alternative approach to the investigation of the orderto-chaos transition has been followed by the Berkeley group [57,58]. Stephens at al.…”

“…The solid (dotted) line represents the n f curve for n = 4 (n = 3), as obtained by the analytical model of (see text for details). parameter (upper and lower panels, respectively) for the measured v d ratio, as extracted from the analysis of unresolved successive g-transitions in ytterbium nuclei, at high spins [57,58]. The Poisson and Wigner limits, representative of ordered and chaotic system, respectively, are indicated by heavy lines.…”