Correlations between emission timescale of fragments and isospin dynamics in 124Sn+64Ni and 112Sn+58Ni reactions at 35A MeV

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments(IMF) emitted at midrapidity in semiperipheral collisions with the emission timescale: IMFs emitted in the earlystage of the reaction show larger values of N/Z isospin asymmetry, stronger angular anisotropies, and reducedodd-even staggering effects in neutron to proton ratio N/Z distributions than those produced in sequentialstatistical emission. All these effects support the concept of isospin “migration”, that is sensitive to the densitygradient between participant and quasispectator nuclear matter, in the so called neck fragmentation mechanism.By comparing the data to a stochastic mean field (SMF) simulation we show that this method gives valuableconstraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indicationemerges for a linear density dependence of the symmetry energy.