Ternary events measured with CR-39 in the U-238+(197)AU reaction at 15 MeV/nucleon

We report the study of the reaction 15 MeV/nucleon U-238 + (197)AU, in, whi ch up to 7 final heavy fragments have been detected, using a CR-39 plastic nuclear track detector. The measured multiplicity distributions indicate th at the binary fission of one (U-like) or both (U-like and Au-like) primary products clearly dominates for peripheral and intermediate impact parameter s exhausting more than 80% of the total cross section. Multifragment emissi on sets in for the most central collisions. Reactions with 3 heavy final fr agments, accounting for half of the total cross section, have been analysed event-by-event after measuring the geometrical parameters of each detected track, assuming momentum conservation and using empirical range-energy-cha rge curves. Most of these ternary processes originate in a two-step mechani sm: a initial peripheral projectile-target interaction plus a deep-inelasti c process followed by the subsequent fission-like decay of one of the fragm ents (mainly the quasiprojectile). Indeed, the relative velocity between th e two fastest fragments are clearly peaked at values close to the Viola sys tematics for the fission of a very-heavy nucleus.