Collective excitations in hot nuclei - Up to which excitation energy do they persist?

Direct evidence for collective excitations in nuclei is obtained e.g. by ob servation of collective rotational bands in nuclear spectroscopy. But alrea dy at law excitation energies of a few MeV above the yrast line discrete tr ansitions can no longer be observed, but evidence for collective motion can be found from quasi-continuum gamma-ray spectroscopy. Stretching out to mu ch higher excitation energy the level density of a compound nucleus is supp osed to be enhanced by the collective degrees of freedom depending on the s ymmetry class of the nuclear shape [1]. From this collective enhancement of the level density and its damping with excitation energy we will conclude on the persistence of collective excitations in hot nuclei. The fission probability and hence its complement - the survival probability against fission - provide a unique experimental information on the relativ e level densities of two neighbouring nuclei in very different shapes: The fissioning nucleus at the saddle-point deformation and the daughter nucleus after neutron emission above the ground state.