Limiting excitation energy in fusion-evaporation reactions?

Abstract

The systematics of fusion-evaporation reactions in the mass-symmetric system ${}^{40}\mathrm{Ar}{+}^{40}$Ca are studied for incident energies of 5--30 MeV/nucleon. The measurement of the resulting evaporation residues in coincidence with emitted neutrons permits a clear identification of central collisions and a separation of equilibrium and preequilibrium contributions. The total neutron multiplicities and the total average mass loss of the system both show a tendency of soft saturation. Above about 20 MeV/nucleon the increase of the energy removal is entirely due to preequilibrium emission of light particles. The total excitation energy of the evaporation residues approaches a maximum value, but the excitation energy per nucleon rises linearly without any sign of saturation. At even higher projectile energies the production of evaporation residues is dramatically reduced and they become indistinguishable from spectatorlike fragments resulting from less central collisions. Thus, the long-standing question of a limiting excitation energy in hot nuclei can probably not be solved by experimental studies of fusion-evaporation reactions.