Statistics of 2+ levels in even-even nuclei

Using all the available empirical information, we analyze the spacing distributions of low-lying 2(+) levels of even-even nuclei. To obtain statistically relevant samples, the nuclei are grouped into classes defined by the ratio R-4/2 of the excitation energies of the first 4(+) and 2(+) levels. This ratio serves as a measure of collectivity in nuclei. With the help of Bayesian inference, we determine the chaoticity parameter for each class. This parameter is found to vary strongly with R-4/2 and takes particularly small values in nuclei that have one of the dynamical symmetries of the interacting boson model. (C) 2003 Published by Elsevier B.V.