Onset of Rotational Damping in Superdeformed Nuclei

We discuss damping of the collective rotational motion in $A\sim 150$ superdeformed nuclei by means of a shell model combining the cranked Nilsson mean-filed and the surface-delta two-body residual force. It is shown that, because of the shell structure associated with the superdeformed mean-field, onset energy of the rotational damping becomes $E_x \sim 2-3$ MeV above yrast line, which is much higher than in normal deformed nuclei. The mechanism of the shell structure effect is investigated through detailed analysis of level densities in superdeformed nuclei. It is predicted the onset of damping varies in different supedeformed nuclei along with variation in the single-particle structure at the Fermi surface.