Testing the level density of excited nuclei from evaporative particle spectra

The transition of the level density parameter aeff from the low excitation energy value aeff=A/8 MeV-1 to the Fermi Gas value aFG∼ A/15 MeV-1 was discovered a few years ago in studying particle spectra evaporated from hot compound system of A∼160. A number of experiments have been performed to confirm the earlier findings and extend the investigation to other mass regions and to higher excitation energies. Difficulties have been evidenced in extracting the nuclear temperature from the slope of the particle spectra because of angular momentum induced effect at relatively low excitation energy and of the onset of cluster and IMF emission at excitation energies ε≥3 MeV/u. In the mass region A∼110, the transition to the FG value has been found at an excitation energy per nucleon larger than that for A∼160, as expected from theoretical predictions. For lighter mass nuclei, experiments do not show a departure from the aeff=A/8 MeV-1 value. On the contrary the need of an excitation energy dependent level density parameter has been evidenced in the mass region A∼200. The effect of this level density on the statistical model predicted pre-scission multiplicities is very important. Deformation induced effects on proton spectra have been also searched for. Experimental proton spectra do not show remarkable differences setting gates on bands characterized by different deformation (oblate, prolate, SD) in the 153Dy residual nucleus. (author). 25 refs, 8 figs