Model for phase transition based on statistical disassembly of nuclei at intermediate energies

Consider a model of particles (nucleons) that has a two-body interaction, which leads to bound composites with saturation properties. These properties are: all composites have the same density and the ground-state energies of composites with k nucleons are given by -kW+σk2∕3, where W and σ are positive constants. W represents a volume term and σ a surface-tension term. These values are taken from nuclear physics. We show that in the large N limit where N is the number of particles, such an assembly in a large enclosure at finite temperature shows properties of liquid-gas phase transition. We do not use the two-body interaction but the gross properties of the composites only. We show that (a) the p-ρ isotherms show a region where pressure does not change as ρ changes just as in the Maxwell construction of a Van der Waals gas, (b) in this region the chemical potential does not change, and (c) the model obeys the celebrated Clausius-Clapeyron relations. A scaling law for the yields of composites emerges. For a finite number of particles N (up to some thousands) the problem can be easily solved on a computer. This allows us to study finite particle number effects, which modify phase-transition effects. The model is calculationally simple. Monte Carlo simulations are not needed.