Pre-compound neutron evaporation in low energy heavy ion fusion reactions

Inclusive and exclusive neutron evaporation spectra have been studied from the fusion reactions at 80 MeV C-12 on Ti-46 and 131 MeV 31 p on Al-27, populating the compound nucleus Ni-58 at the excitation energy of 79.5 MeV but in different angular momentum ranges. The evaporation residues Fe-53, Fe-55 and (CO)-C-56 were identified by their characteristic gamma-rays. The measured energy spectra of evaporated neutrons were compared with the predictions from statistical-model calculations. In case of the asymmetric system C-12 + Ti-46, the neutron spectra are well reproduced, when compared with the results from statistical-model calculations with level density parameter a = A/8 MeV-1. On the contrary, the experimental spectra for the symmetric system 31 p + Al-27 are found to be harder than the theoretical predictions with a = A/8 MeV-1. In this case, a lower value of the level density parameter a = A/10 MeV-1 seems to be required to reproduce the experimental results. The dependence of the energy spectra on the entrance channel is taken as an indication that, in case of the symmetric system, the neutrons are in part emitted from a temperature equilibrated di-nuclear complex at a higher temperature before the formation of the compound nucleus.