Regularity and chaos in low-lying 2(+) states of even-even nuclei

Using all the available empirical information, we analyse the spacing distributions of low-lying 2+ levels in even-even nuclei by comparing them with a theoretical distribution characterized by a single parameter (the chaoticity parameter f). We use the method of Bayesian inference. We show that the necessary unfolding procedure generally leads to an overestimate of f. We find that f varies strongly with the ratio R-4/2 of the excitation energies of the first 4+ and 2+ levels and assumes particularly small values in nuclei that have one of the dynamical symmetries of the Interacting Boson Model.