Dynamical effects in nuclear collisions in the Fermi energy range: alignedbreakup of heavy projectiles

Recent experimental results concerning heavy systems (Pb + Au, Pb + Ag, Pb + Al, Gd + C, Gd + U, Xe + Sn,...) obtained at GANIL with the INDRA and NAU TILUS 4 pi arrays will be presented. The study of reaction mechanisms has s hown the dominant binary and highly dissipative character of the process. T he two heavy and excited fragments produced after the first stage of the in teraction can decay into various decay modes from evaporation to multifragm entation including fission. However, deviations from this simple picture ha ve been found by analyzing angular and velocity distributions of light char ged particles, and fragments. Indeed, there is a certain amount of matter i n excess emitted between the two primary sources suggesting either the exis tence of a mid-rapidity source similar to the one observed in the relativis tic regime (participants) or a strong deformation induced by the dynamics o f the collision (neck instability). This last possibility has been suggeste d by analyzing in detail the angular distributions of the fragments. More p recisely, we observe an isotropic component which is compatible with the pr ediction of statistical models and a second one corresponding to breakup al igned with the recoil direction of the projectile like source which should be compared with the predictions of dynamical calculations based on microsc opic transport models. (C) 2000 Elsevier Science B.V. All rights reserved.