Density fluctuations and multifragmentation of nuclear matter

Abstract

The density fluctuations of nuclear matter are studied within a mean-field model in which fluctuations are generated by an external stochastic field. The constraints imposed on the random force by the fluctuation-dissipation theorem are analyzed. It is shown that in the proximity of the borders of the spinodal region the assumption of a withe-noise stochastic field can be reliably used. The domain distribution of the liquid phase in the spinodal decomposition of nuclear matter is derived. The related distribution of fragment sizes compares favourably with the experimental fragment distribution observed in heavy ion collisions. PACS number

Spinodal decomposition of expanding nuclear matter and multifragmentation

“…This formalism is a generalization of that introduced in Ref. [1] and it gives a new insight into the interplay between the hydrodynamic motion and the kinetics of fluctuations. The present approach allows us to determine the time behavior of the distribution of the liquid domains, that depends on the collective expansion energy.…”

“…Thus, we extend the approach of Ref. [1] by considering fluctuations about the average time-dependent density ̺ 0 (t) of nuclear matter during a hydrodynamic expansion. The evolution of the fluctuations is described by a kinetic equation that in the absence of the stochastic field has a solution corresponding to the one-body phase-space density of the hydrodynamic motion.…”

“…In a previous paper [1] we have studied a model in which the excited nuclear system that is formed after the collision of two heavy ions at intermediate energy is described as hot unstable nuclear matter. In this system the density fluctuations can lead to a liquid-gas phase transition ( spinodal decomposition ).…”

“…In the model of Ref. [1] the density is assumed to fluctuate about a stationary mean value ̺ 0 . In order to take into account the expansion of the complex system formed in the reaction, here we generalize that approach and consider the more realistic situation in which the density ̺ 0 can be time-dependent: ̺ 0 = ̺ 0 (t).…”

“…In order to describe fluctuations about this time-dependent average behavior, we assume, like in Ref. [1], that a stochastic self-consistent mean field induces fluctuations of the density about its average value. The evolution of these fluctuations is accordingly described by means of a kinetic equation of mean-field in which a stochastic ( Langevin ) term is included.…”