90 20 v 2 1 D ec 2 00 0 Classification of the Nuclear Multifragmentation Phase Transition

The phenomenon of heavy-ion collision induced nuclear multifragmentation (NMF) has been extensively studied over the past decade. Experimentally it is found that nuclei with excitation energies of E/A = 5-10 MeV, i.e. in the region of the binding energy, expand forming a number of intermediate mass fragments [1]. Using subtle and advanced experimental techniques from a number of collision experiments with different nuclei and at different collision energies an experimental caloric curve has been derived[2, 3], which supports the interpretation that NMF is the small particle number onset of the nuclear liquid-gas phase transition. Theoretically NMF has been described by a number of different approaches ranging from simple percolation models[4, 5], dynamical models with different levels of sophistication, e.g. quantum molecular dynamics of different flavors [6, 7, 8, 9, 10] and fermion molecular dynamics [11], and a large variety of statistical models [12, 13, 14, 15, 16, 17, 18, 19]. Among the statistical models two mainstream-theories, namely the statistical multifragmentation model (SMM) [12] using the heat bath or canonical ensemble and the micro-canonical multifragmentation model (MMM) [13] using the constant energy or micro-canonical ensemble should be mentioned. For different ensembles the shape of the caloric curve may differ significantly. However, because all ensembles are uniquely connected by simple integral transforms the qualitative features of the phase transition should be the same in all ensembles. We therefore omit at this point the (surely) interesting question which ensemble is more appropriate for the description of nuclear multifragmentation.