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

Abstract

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 $\ensuremath{\alpha}$ particles emitted from $^{178}\mathrm{Hf}$ compound nuclei, with excitation energies between 54 and 124 MeV, formed in $^{18}\mathrm{O}+^{160}\mathrm{Gd}$ fusion reactions. The Gammasphere spectrometer provided us with the ability to construct channel-specific $\ensuremath{\alpha}$-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.