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 mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of (N/Z) isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio (N/Z) distributions than those produced in sequential statistical emission. All these effects support the concept of isospin "migration", that is sensitive to the density gradient between participant and quasi-spectator 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 valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy. [ABSTRACT FROM AUTHOR]