Fragment Production in a Finite Size Lattice Gas Model

Abstract

It is generally believed that heavy ion collisions may provide an experimental way to explore the properties of the nuclear matter under different thermodynamical conditions. Then, an important experimental issue is to extract information on the nuclear equation of state (EOS) from reaction data. In particular, considering the analogy between the nucleon-nucleon force and a Van der Waals interaction it is expected that the EOS presents the characteristic a liquid-gas phase transition. In the recent years, different indications of such a transition have been accumulated. On one side, studying the decay of hot nuclear systems it was observed a sudden opening of the multifragmen-tation^{1, 2, 3} and vaporization⁴ channels associated with the disappearance of a large residual nucleus. These behaviors were interpreted as signatures of the transition from the survival of a liquid phase at low temperature towards the massive production of a dilute gas system at high excitation energy through the apparition of a phase mixture in the intermediate energy domain^{5, 6}. Moreover, this direct observation of various phases has been completed by the extraction of thermodynamical signals of first order phase transition. Indeed, it has been reported an anomaly of the evolution of the observed temperature as a function of the excitation energy (the so-called caloric curve)⁷ which shows a structure similar to a first order phase transition in the framework of statistical equilibrium models^{8, 6}. Moreover, statistical equilibrium models, extremely successful in reproducing multifragmentation patterns, systematically suggest low freeze out densities^{9, 6} which are in complete agreement with the idea that the transition occurs in the middle of the coexistence region.