Microscopic model approaches to fragmentation of nuclei and phase transitions in nuclear matter

The properties of excited nuclear matter and the quest for a phase transiti on which is expected to exist in this system are the subject of intensive i nvestigations. High-energy nuclear collisions between finite nuclei which l ead to matter fragmentation are used to investigate these properties. The p resent report covers effective work done on the subject over the two last d ecades. The analysis of experimental data is confronted with two major prob lems, the setting up of thermodynamic equilibrium in a time-dependent fragm entation process and the finite size of nuclei. The present status concerni ng the first point is presented. Simple classical models of disordered syst ems are derived starting with the generic bond percolation approach. These lattice and cellular equilibrium models, like percolation approaches, descr ibe successfully experimental fragment multiplicity distributions. They als o show the properties of systems which undergo a thermodynamic phase transi tion. Physical observables which are devised to show the existence and to f ix the order of critical behaviour are presented. Applications to the model s are shown. Thermodynamic properties of finite systems undergoing critical behaviour are advantageously described in the framework of the microcanoni cal ensemble. Applications to the designed models and to experimental data are presented and analysed. Perspectives of further developments of the fie ld are suggested. (C) 2001 Elsevier Science B.V. All rights reserved.