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Nuclei colliding at energies in the MeV's break into fragments in a process that resembles a liquid-togas phase transition of the excited nuclear matter. If this is the case, phase changes occurring near the critical point should yield a "droplet" mass distribution of the form ≈A −τ , with τ (a critical exponent universal to many processes) within 2 ≤ τ ≤ 3. This critical phenomenon, however, can be obscured by the finiteness in space of the nuclei and in time of the reaction. With this in mind, this work studies the possibility of having critical phenomena in small "static" systems (using percolation of cubic and spherical grids), and on small "dynamic" systems (using molecular dynamics simulations of nuclear collisions in two and three dimensions). This is done investigating the mass distributions produced by these models and extracting values of critical exponents. The specific conclusion is that the obtained values of τ are within the range expected for critical phenomena, i.e. around 2.3, and the grander conclusion is that phase changes and critical phenomena appear to be possible in small and fast breaking systems, such as in collisions between heavy ions.

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Experimental studies have obtained the caloric curve of nuclear matter from heavy ion collisions as well as its dependence on the size of the fragmenting source. In particular it has been determined that smaller systems have caloric curves with higher plateau temperatures than larger systems. This work uses molecular dynamics simulations to study the thermodynamics of heavy ion collisions and to identify the main factors that determine the caloric curve. The simulations indicate that the reaction is composed of three stages: 1) an initial collision that transforms the nuclei from normal density and zero temperature and entropy, to a hot and dense blob of matter with higher values of density, temperature and entropy; 2) this is followed by a constant-entropy expansion that takes the system to the spinodal of the phase diagram; 3) where the system rapidly disassembles into fragments by the process of spinodal decomposition, and not by nucleation. These findings indicate that the plateau temperature of the caloric curve is nothing more than the temperature of the phase change and it is set by the intersection of the isentropic expansion and the spinodal. In other words, the plateau temperature is simply the temperature at which the system breaks as it enters the spinodal. This transition temperature is thus set by the entropy generated during the initial part of the collision.

Nuclei undergo a phase transition in nuclear reactions according to a caloric curve determined by the amount of entropy. Here, the generation of entropy is studied in relation to the size of the nuclear system. * Universidad Autónoma Metropolitana. Unidad Azcapotzalco. Av. San Pablo 124, Col. Reynosa-Tamaulipas, Mexico City.