Search for the fade out of a collective enhancement of the nuclear level density

The fade out of a collective enhancement of the nuclear level density is predicted (by an SU3 shell model) to give rise to a clear signature in the evolution of the spectral shape of evaporated charged particles with excitation energy. We have searched for this signature in the spectra of alpha particles emitted from Hf-178 compound nuclei, with excitation energies between 54 and 124 MeV, formed in O-18+Gd-160 fusion reactions. The Gammasphere spectrometer provided us with the ability to construct channel-specific alpha-particle spectra that could be compared to statistical-model calculations on a channel-by-channel basis. The expected clear signature of a rapid fade out of a collective enhancement was not found. The data are best reproduced by calculations that do not explicitly consider a collective enhancement and its fade out.