HOT-NUCLEI FORMATION AND DECAY - THE AR-MEV(AG SYSTEM AT 50)U AND 70-MEV/U/

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 re sidue since the residue was detected in coincidence with all the corre sponding light charged particles by use of a 4pi multidetector. An eve nt by event analysis was performed in order to distinguish between par ticles 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 esti mating the neutron contribution. It was found that the excitation ener gy distributions obtained at 50 and 70 MeV/u were quite similar. Very large values are reached in both cases (approximately 600 MeV). This r esult means that the standard statistical decay observed at limited ex citation is still observed for very hot nuclei. The corresponding temp eratures were measured from the kinetic-energy spectra slopes. Slightl y larger values were observed at 70 MeV/u. This result is consistent w ith a slightly smaller mass of the fusion nucleus obtained in this cas e. In a last aspect of the experiment where many particles events were analysed in the 4pi set up, it turned out that the highest involved e xcitation energies can lead either to a multifragment emission, or to a standard sequential statistical decay.