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

We report the study of the reaction 15 MeV/nucleon 238U + 197Au, in which up to 7 final heavy fragments have been detected, using a CR-39 plastic nuclear track detector. The measured multiplicity distributions indicate that the binary fission of one (U-like) or both (U-like and Au-like) primary products clearly dominates for peripheral and intermediate impact parameters exhausting more than 80% of the total cross section. Multifragment emission sets in for the most central collisions. Reactions with 3 heavy final fragments, 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-charge curves. Most of these ternary processes originate in a two-step mechanism: a initial peripheral projectile-target interaction plus a deep-inelastic process followed by the subsequent fission-like decay of one of the fragments (mainly the quasiprojectile). Indeed, the relative velocity between the two fastest fragments are clearly peaked at values close to the Viola systematics for the fission of a very-heavy nucleus