Level density of hot nuclei with<i>A</i>≤40

Abstract

The light charged particles emitted from hot $^{40}\mathrm{Ca}$ compound nuclei, populated at excitation energy ${\mathit{E}}_{\mathit{x}}$=94 MeV and 〈J〉=20.5\ensuremath{\Elzxh} by the reaction 130 MeV $^{16}\mathrm{O}$ on $^{24}\mathrm{Mg}$, 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\ensuremath{\le}40, needed to account for the measured quantities, is well predicted by the Fermi-gas model using a level-density parameter ${\mathit{a}}_{\mathrm{eff}}$=A/8 ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$ up to excitation energy 〈${\mathit{E}}_{\mathrm{th}}$/A〉\ensuremath{\sim}1.7 MeV. In agreement with theory, light hot nuclei do not show the transition from ${\mathit{a}}_{\mathrm{eff}}$=A/8 ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$ to ${\mathit{a}}_{\mathrm{eff}}$\ensuremath{\sim}A/13 ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$ evidenced in the A\ensuremath{\sim}160 mass region for the same range of excitation energies ${\mathit{E}}_{\mathrm{th}}$/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.