Isospin effects in Nuclear Fragmentation

We investigate properties of the symmetry term in the equation-of-state (EOS) of nuclear matter (NM) from the analysis of simulations of fragmentation events in intermediate energy heavy ion collisions. For charge asymmetric systems a qualitative new feature in the liquid-gas phase transition is predicted: the onset of chemical instabilities with a mixture of isoscalar and isovector components. This leads to a separation into a higher density (``liquid'') symmetric and a low density (``gas'') neutron-rich phase, the so-called neutron distillation effect. We analyse the simulations with respect to the time evolution of the isospin dynamics, as well as with respect to the distribution and asymmetry of the final primary fragments. Qualitatively different effects arise in central collisions, with bulk fragmentation, and peripheral collisions with neck-fragmentation. The neck fragments produced in this type of process appear systematically more neutron-rich from a dynamical nucleon migration effect which is very sensitive to the symmetry term in regions just below normal density. In general the isospin dynamics plays an important role in all the steps of the reaction, from prompt nucleon emission to the sequential decay of the primary fragments. A fully microscopic description of the reaction dynamics including stochastic elements to treat fluctuations realistically is absolutely necessary in order to extract precise information on the fragmentation and the nuclear equation of state. We have performed simulations for fragment production events in $n$-rich ($^{124}Sn$) and $n$-poor ($^{112}Sn$) symmetric colliding systems.