We study the neutron and proton dynamical behavior along the fragmentation path in semi-peripheral collisions: 58 Fe+ 58 Fe (charge asymmetric, N/Z = 1.23) and 58 Ni+ 58 Ni (charge symmetric, N/Z = 1.07), at 47 AMeV. We observe that isospin dynamics processes take place also in the charge-symmetric system 58 Ni+ 58 Ni, that may produce more asymmetric fragments. A neutron enrichment of the neck fragments is observed, resulting from the interplay between pre-equilibrium emission and the phenomenon of isospin-migration. Both effects depend on the EoS (Equation of State) symmetry term. This point is illustrated by comparing the results obtained with two different choices of the symmetry energy density dependence. New correlation observables are suggested, to study the reaction mechanism and the isospin dynamics. Collisions between heavy ions with large isospin asymmetries , made possible by the recent radioactive beam developments, represent a very efficient way to probe the structure of nuclear-EoS (Equation of State) symmetry term. In particular the symmetry energy behavior is influencing very dissipative reaction mechanisms, such as fragmentation processes, leading to important effects on fragment composition. In central heavy ion collisions at intermediate energies the spinodal decomposition has been proposed as a possible mechanism for fragment formation, see Ref.[1] for a recent review. According to this description, fragments should reflect the properties of the low density phase, where they are formed. In charge asymmetric systems , the isospin distillation, i.e. the formation of more symmetric fragments surrounded by a neutron richer dilute phase, takes place. Here we will focus on fragmentation in semi-peripheral collisions, where intermediate mass fragments (IM F) are mostly produced in the overlap zone (the neck region) between projectile-like and target-like fragments (P LF/T LF , the spectator region), [2, 3, 4, 5, 6, 7] [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. We will discuss the influence of the symmetry energy on the features of this fragmentation mechanism. The presence of a density gradient between the neck (low density) and the spectator (high density) regions affects the N/Z of fragments in a different way with respect to the spin-odal decomposition mechanism [20, 21, 22]. Moreover since the isoscalar density gradients are ruling the isospin transfer through the density dependence of the symmetry energy, we see that measurements of isospin observables in semiperipheral collisions will directly probe the slope of the symmetry term around saturation, of large impor-* On leave from HH-NIPNE and University of Bucharest, Romania. tance for the structure of neutron-rich nuclei [23]. We consider the reactions 58 Fe + 58 Fe (charge asym-metric N/Z = 1.23) and 58 Ni + 58 Ni (charge symmetric N/Z = 1.07). In fact in these collisions, due to the uniform N/Z distributions, we do not have isospin gradients initially. We show that the study of the full reaction dynamics and of the possible occurrence ...
