Dynamical signals in fragmentation reactions: Time scale determination from three fragments correlations by using the 4π Chimera multidetector

For fragments emitted in the reactions (124)Sn + (64)Ni and (112)Sn + (58)Ni at 35 A MeV, isotopic composition and velocity correlations have been studied as a function of the centrality of the collision, using the 4 pi Chimera multidetector. We have investigated the time scale for fragments formation, in order to distinguish between prompt dynamical and sequential statistical emission. Promptly emitted light fragments (Z <= 9) produced in the mid-rapidity domain are characterised by larger N/Z ratio and stronger angular anisotropies than those produced in sequential statistical emission. Results are compared with stochastic BNV code simulations obtained for primary fragments. Valuable information on the symmetry term of the nuclear equation of state at sub-saturation densities are obtained.