Probing the symmetry energy at low density using observable from neck fragmentation

Semi-peripheral collisions of Ni-64 + Sn-124 in direct and inverse kinematics, at 35 A MeV incident energy, are studied to characterize the dynamical origin and alignments properties of fragments emission at midrapidity. This emission is partly due to the fragmentation of a transient neck joining the projectile and target in the early stages of the reaction. Recently we exploited observable from neck fragmentation to constraint the density dependence of the symmetry energy. In this contribution we focus on the density evolution of the mid-rapidity source. An experimental survey is suggested to check if data are consistent with the formation of neck fragments in a dilute density region in contact with the projectile-like and target-like spectators at normal saturation density.