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 early stage of the reaction show larger values of $\langle${}N/Z$\rangle${} isospin asymmetry, stronger angular anisotropies, and reduced odd-even staggering effects in neutron to proton ratio $\langle${}N/Z$\rangle${} 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 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 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.

Semiperipheral collisions in the ${}^{124}\mathrm{Sn}+{}^{64}\mathrm{Ni}$ reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes.

Some properties of fast, nonequilibrium splitting of projectiles in the ${}^{124}\mathrm{Sn}+{}^{64}\mathrm{Ni}$ reaction at 35A MeV were determined using the $4\pi$ CHIMERA detector system. In particular the charge distributions, in- and out-of-plane angular distributions, and relative velocities of projectilelike fragments were measured. The time scale of the process was estimated and it turned out that the process is sequential but much faster than the ordinary, equilibrated fission.