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 $^{178}$Hf compound nuclei, with excitation energies between 54 and 124 MeV, formed in $^{18}$O + $^{160}$Gd fusion reactions. Gammasphere provided the ability to construct channel-specific $\alpha $-particle spectra and thus allow for a comparison to statistical model calculations on this basis. The expected clear signature of a rapid fade out of a collective enhancement is not found. The data are best reproduced by calculations which do not explicitly consider a collective enhancement and its fade out.