LEVEL STATISTICS OF NEAR-YRAST STATES RAPIDLY ROTATING NUCLEI

The nearest neighbour level spacing distribution and the Delta(3) stat istic of level fluctuations associated with very high spin states (I g reater than or similar to 30) in rare-earth deformed nuclei are analys ed by means of a cranked shell model, The many particle-many hole conf igurations created in the rotating Nilsson potential are mixed by the surface-delta two-body residual interaction, The levels in the near-yr ast region show a Poisson-like level spacing distribution. As the intr insic excitation energy U increases, the level statistics shows a grad ual transition from order to chaos, reaching at U greater than or simi lar to 2 MeV the Wigner distribution typical of the Gaussian orthogona l ensemble of random matrices, This transition is caused by the residu al two-body interaction, On the other hand, the level spacings between the yrast and the first excited state show a peculiar behaviour, disp laying a Wigner-like distribution instead of the Poisson-like distribu tion seen for the other near-yrast rotational states, The lowest spaci ngs reflect the properties of the single-particle orbits in the mean-f ield, and are only weakly affected by the residual two-body interactio n. (C) 1997 Elsevier Science B.V.