With an isospin- and momentum-dependent transport model, we find that the degree of isospin diffusion in heavy-ion collisions at intermediate energies is affected by both the stiffness of the nuclear symmetry energy and the momentum dependence of the nucleon potential. Using a momentum dependence derived from the Gogny effective interaction, recent experimental data from NSCL-MSU on isospin diffusion are shown to be consistent with a nuclear symmetry energy given by $E_{\mathrm{s}\mathrm{y}\mathrm{m}}(\rho )\approx 31.6(\rho /{\rho }_0{)}^{1.05}$ at subnormal densities. This leads to a significantly constrained value of about $-550\mathrm{M}\mathrm{e}\mathrm{V}$ for the isospin-dependent part of the isobaric incompressibility of isospin asymmetric nuclear matter.

• Received 12 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.032701