Isospin Dependence of Mechanical and Chemical Instabilities in Neutron-Rich Matter

Within nuclear thermodynamics and an isospin-dependent transport model we investigate respective roles of the nuclear mean field and the 2-body stochastic scattering on the evolution of density and isospin fluctuations in either mechanically or chemically unstable regions of neutron-rich matter. It is found that the mean field dominates overwhelmingly the fast growth of both fluctuations, while the 2-body scattering influences significantly the later growth of the isospin fluctuation only. Moreover, both fluctuations grow in mechanically unstable systems, while only the density fluctuation grows significantly in chemically unstable ones. Furthermore, the magnitude of both fluctuations decreases with the increasing isospin asymmetry because of the larger reduction of the attractive isoscalar mean field by the stronger repuslive neutron symmetry potential in the more neutron-rich matter. Finally, several experimental measurements are proposed to test these findings.