From multifragmentation to neck fragmentation: Mass, isospin, and velocity correlations

We present new features of fragmentation dynamics at the transition from semicentral to semiperipheral heavy-ion collisions at Fermi energies, as obtained within a microscopic transport model, the stochastic mean field. We show that, along this transition, specific hierarchy phenomena of some kinematic observables associated with the intermediate mass fragments develop. Their correlations with the dynamics of the isospin degree of freedom, predicted by our calculations, open new possibilities to learn about the density dependence of nuclear symmetry energy below saturation, as well as about the relevant fragmentation mechanisms. Detailed results are presented for mass symmetric Sn + Sn reactions with different isospin content at 50 MeV/nucleon.