Phase transition in an isospin dependent Lattice Gas Model

The role of isospin as a degree of freedom and as an observable in heavy ion reaction experiments is analyzed in the theoretical framework of an isospin dependent Lattice Gas Model. We show that the critical temperature is only slightly dependent on the isospin asymmetry and finite size effects go in the direction of increasing the tau critical exponent respect to the asymptotic limit of the three dimensional Ising universality class. An interesting signature of the liquid-gas phase transition and a possible observable to localize the multifragmenting system in the T - rho plane is given by the isotopic composition of fragments. We show that in the coexistence region the vapor fraction is more asymmetric than the liquid fraction and that this signal should not be washed out by secondary decays. (C) 1999 Elsevier Science B.V. All rights reserved.