Level Density of Hot Nuclei With A-less-than-or-equal-to-40

The light charged particles emitted from hot Ca-40 compound nuclei, populated at excitation energy E(x) = 94 MeV and <J> = 20.5HBAR by the reaction 130 MeV O-16 on Mg-24, have been studied. Energy spectra of protons and alpha particles, measured in coincidence with evaporation residues and with a selection of multiple-alpha chains, have been compared with the predictions of Monte Carlo statistical model calculations. The comparison shows that the level density of hot nuclei with A less-than-or-equal-to 40, needed to account for the measured quantities, is well predicted by the Fermi-gas model using a level-density parameter a(eff) = A/8 MeV-1 up to excitation energy <E(th)/A> approximately 1.7 MeV. In agreement with theory, light hot nuclei do not show the transition from a(eff) = A/8 MeV-1 to a(eff) approximately A/13 MeV-1 evidenced in the A approximately 160 mass region for the same range of excitation energies E(th)/A. The analysis of the alpha-particle spectra shows that the effects associated with the angular-momentum-induced deformations depend on the entrance channel characteristics and, in this case, are very small compared with those evidenced in the past for compound nuclei in the region A = 50-70.