Production and de-excitation modes of hot nuclei formed in the reactions 84Kr + 12C, 84Kr + 27Al, 84Kr + NatTi at 26.4, 34.4 and 45.4 MeV/nucleon

Formation and de-excitation of hot nuclei at intermediate energies was studied, using inverse kinematics to measure the characteristics of fragments of mass > 10 atomic units produced in the reactions Kr + C, Al and Ti at 25, 35 and 45 MeV/u. Spectrum analysis of the velocity and distributions of complex fragments (mass <40u) shows an emission component from a source in equilibrium, well reproduced by Monte Carlo simulations, except for Kr + Al and Ti at 45.4 MeV. For Kr + C at 25 and 35 Mev/u, this emission component corresponds to complete fusion, which represents only a tenth of the cross section of evaporation residues. In the other cases, pulse transfer is incomplete. The emission of complex fragments therefore separates out the hottest nuclei formed in the reaction. Source velocities associated with evaporation residues and fragments decrease as the system becomes more symmetrical. Maximum excitation energies calculated assuming mass transfer increase as a function of available energy, and reach 5.8 MeV/u for available energy equal to bond energy. A statistical de-excitation model which includes complex fragment emission qualitatively reproduces these results, assuming an angular momentum of the emitting nucleus of 40 to 50 ℎ for all the systems. The excitation energy remains < 4.2 MeV/nucleon, i.e., lower than in the mass transfer hypothesis