The critical exponent of nuclear fragmentation

Abstract

Nuclei colliding at energies in the MeV's break into fragments in a process that resembles a liquid-to-gas 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 mide, 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 grand 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.