Level statistics of near-yrast states rapidly rotating nuclei

The nearest neighbour level spacing distribution and the Delta(3) statistic of level fluctuations associated with very high spin states (I greater than or similar to 30) in rare-earth deformed nuclei are analysed by means of a cranked shell model, The many particle-many hole configurations created in the rotating Nilsson potential are mixed by the surface-delta two-body residual interaction, The levels in the near-yrast region show a Poisson-like level spacing distribution. As the intrinsic excitation energy U increases, the level statistics shows a gradual transition from order to chaos, reaching at U greater than or similar to 2 MeV the Wigner distribution typical of the Gaussian orthogonal ensemble of random matrices, This transition is caused by the residual two-body interaction, On the other hand, the level spacings between the yrast and the first excited state show a peculiar behaviour, displaying a Wigner-like distribution instead of the Poisson-like distribution seen for the other near-yrast rotational states, The lowest spacings reflect the properties of the single-particle orbits in the mean-field, and are only weakly affected by the residual two-body interaction. (C) 1997 Elsevier Science B.V.