Exclusive Mass-Energy Distributions of the Fast Fission Fragments in the Sm Reaction.

The dependence of the properties of the fission fragments formed both in the fusion-fission and fast fission processes on the angular momentum still remains very unclear from an experimental as well as theoretical point of view. To study the properties of fast fission fragments of preactinide nuclei as function of the interaction energy and introduced angular momentum, the mass-energy distributions of Pb fission fragments formed in the Sm reaction at energies above the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET at the Ca ions' energies of 212, 231, 244, and 277 MeV. The mass-energy distributions of the fast fission fragments have been extracted by subtracting the mass-energy matrices corresponding to the compound nucleus fission from those of all measured fissionlike events. The asymmetric fragments with masses 76 and 108 u were found to be the most probable in the fast fission of Pb. With increasing Ca energy from 231 to 277 MeV the mass distributions of fast fission fragments change weakly, whereas an increase in the TKE is about 20 MeV. The properties found for the fast fission fragments indicate the incomplete mass relaxation and not full energy dissipation. [ABSTRACT FROM AUTHOR]