Hot-nuclei formation and decay: The Ar + Ag system at 50 and 70 [formula omitted

Abstract

Hot-nuclei fusion properties have been studied for the Ar + Ag system at 50 and 70 MeV/u bombarding energy. The set up was very well suited to the analysis of sequential evaporation leading to an evaporation residue since the residue was detected in coincidence with all the corresponding light charged particles by use of a 4π- multidetector. An event by event analysis was performed in order to distinguish between particles evaporated from the fusion nucleus and others. It was possible to reconstruct the initial excitation energy of the fusion nucleus by adding the contributions of all the charged decay products and by estimating the neutron contribution. It was found that the excitation energy distributions obtained at 50 and 70 MeV/u were quite similar. Very large values are reached in both cases (∼ 600 MeV). This result means that the standard statistical decay observed at limited excitation is still observed for very hot nuclei. The corresponding temperatures were measured from the kinetic-energy spectra slopes. Slightly larger values were observed at 70 MeV/u. This result is consistent with a slightly smaller mass of the fusion nucleus obtained in this case. In a last aspect of the experiment where many particles events were analysed in the 4π set up, it turned out that the highest involved excitation energies can lead either to a multifragment emission, or to a standard sequential statistical decay.