Multifragmentation: The success of statistical models

The determination of fundamental properties of nuclear matter far from the ground state is one of the principal motivations for the study of collisions between two complex nuclei. At densities slightly above the normal nuclear density transverse flow phenomena are regarded as fingerprints of the nuclear equation of state. At subsaturation densities, on the other hand, the fragmentation pattern of the nuclear system is expected to be influenced by the bulk properties of nuclear matter. The special interest in this low energy region arose from the idea that a nuclear many-body system might undergo a phase-transition during the disassembly leading to multifragmentation - the simultaneous formation of several complex fragments - of the system. Multifragment emission is observed in central symmetric heavy ion collisions as well as in peripheral collisions. During the central collision of a symmetric system the whole system forms a hot and dense fireball. Due to the thermal pressure, this excited and highly compressed nuclear matter will expand. If the thermal energy of the system is big enough, the system may reach the region of mechanical instability. In this region, density fluctuations are not damped by the mean nuclear field. They will grow exponentially - which leads to the simultaneous formation of several fragments. Due to indications for both a chemical and a thermal equilibrium in collision data, a detailed comparison between the observed distributions and the predictions of statistical models was pursued