Nuclear multifragmentation within the framework of different statistical ensembles

Abstract

The sensitivity of the Statistical Multifragmentation Model to the underlying statistical assumptions is investigated. We concentrate on its micro-canonical, canonical, and isobaric formulations.As far as average values are concerned, our results reveal that all the ensembles make very similar predictions, as long as the relevant macroscopic variables (such as temperature, excitation energy and breakup volume) are the same in all statistical ensembles. It also turns out that the multiplicity dependence of the breakup volume in the micro-canonical version of the model mimics a system at (approximately) constant pressure, at least in the plateau region of the caloric curve. However, in contrast to average values, our results suggest that the distributions of physical observables are quite sensitive to the statistical assumptions. This finding may help deciding which hypothesis corresponds to the best picture for the freeze-out stage.

Effects of geometric constraints on the nuclear multifragmentation process

“…The fragment energies E A,Z have contributions from the translational motion, as well as from the nuclear bulk, surface, asymmetry, and Coulomb energies [8]. The latter, is calculated through the Wigner-Seitz approximation [41,46].…”

“…For instance, there has been intensive debate on whether negative heat capacities should be observed in nuclear systems [1,2,3,4,5,6,7,8], not to mention the fundamental question of whether there are any clear signatures of a liquid-gas phase transition in nuclear multifragmentation [9,10,11,12,13]. Experimental studies have proven to be essential in providing insight into the main properties of the phenomenon (see [14] and references therein).…”

Many-particle correlations and Coulomb effects on temperatures from fragment momentum fluctuations

“…where F f (T, V bk ) is the Helmholtz free energy associated with the fragmentation mode [25]. For each event, the fragments' momenta are assigned according to the Metropolis simulation method.…”

“…Our Monte-Carlo simulation uses the canonical implementation of SMM [24,25] to generate the M fragments, whose multiplicity and composition of species vary for each fragmentation mode f . The statistical weight of the latter is given by:…”

“…In one of them, the canonical version of the Statistical Multifragmentation Model (SMM) [21][22][23] is used to generate fragments, at a given temperature T [24,25], on an event by event basis. These fragments are then placed inside the breakup volume and allowed to propagate until they are sufficiently separated from each other, so that the Coulomb energy of the system is negligible.…”