Relevance of equilibrium in multifragmentation

Abstract

The relevance of equilibrium in a multifragmentation reaction of very central $^{40}\mathrm{Ca}$ $+$ $^{40}\mathrm{Ca}$ collisions at 35 MeV/nucleon is investigated by using simulations of antisymmetrized molecular dynamics (AMD). Two types of ensembles are compared. One is the reaction ensemble of the states at each reaction time $t$ in collision events simulated by AMD, and the other is the equilibrium ensemble prepared by solving the AMD equation of motion for a many-nucleon system confined in a container for a long time. The comparison of the ensembles is performed for the fragment charge distribution and the excitation energies. Our calculations show that there exists an equilibrium ensemble that well reproduces the reaction ensemble at each reaction time $t$ for the investigated period $80\ensuremath{\leqslant}t\ensuremath{\leqslant}300 \mathrm{fm}/c$. However, there are some other observables that show discrepancies between the reaction and equilibrium ensembles. These may be interpreted as dynamical effects in the reaction. The usual static equilibrium at each instant is not realized since any equilibrium ensemble with the same volume as that of the reaction system cannot reproduce the fragment observables.