GENERALIZED ENTROPY AND TEMPERATURE IN NUCLEAR MULTIFRAGMENTATION

Tn the framework of a two-dimensional Vlasov model, we study the time evolution of the ''coarse-grained'' generalized entropy (GE) in a nucl ear system which undergoes a multifragmentation (MF) phase transition. We investigate the GE both for the gas and the fragments (surface and bulk part, respectively). We find that the formation of the surface c auses the growth of the GE during the process of fragmentation. This q uantity then characterizes the MF and confirms the crucial role of det erministic chaos in filling the new available phase space: at variance with the exact time evolution, no entropy change is found when the li near response is applied. Numerical simulations were used also to extr act information about final temperatures of the fragments. From a fitt ing of the momentum distribution with a Fermi-Dirac function we extrac t the temperature of the fragments at the end of the process. We calcu late also the gas temperature by averaging over the available phase sp ace. The latter is a few times larger than the former, indicating a ga s not in equilibrium. Though the model is very schematic, this fact se ems to be very general and could explain the discrepancy found in expe rimental data when using the slope of light particles spectra instead of the double ratio of isotope yields method in order to extract the n uclear caloric curve.