Fluctuations and multifragmentation in intermediate energy heavy ion collisions

The mean field approaches to studies of reactions between heavy ions have proved to be very successful in describing the dynamical evolution of the interacting systems in regions of stability of nuclear matter. However, if the nuclear matter enters a region of instability, the fluctuations which are not present in the average field treatment are of great importance. We discuss here the occurence of spinoidal (volume) instabilities which may develop when, after an initial compression, the expansion brings the system in the overlapping zone (neck area) below the critical density. If the interaction time and the time for growing of instabilities are of the same order, the fluctuation will be amplified by the mean field. In a statistically consistent way, we introduce density fluctuations into the phase space distribution. The variance of these fluctuations is determined by assuming a Fermi gas at local thermal equilibrium. The Boltzmann-Uheling-Uhlenbeck (BUU) formalism is used to follow the further evolution of the system, since it is the mean field that will determine whether the fluctuation will evolve. We first discuss the role of density fluctuations in the case of stable dynamics in the incomplete fusion reaction of the Ni + Ni system. We then consider a deep inelastic scattering of Ni on Zr. Density fluctuations were introduced when the system started to expand but before it entered an instability region. In the case of Ni + Zr the fluctuations grew, leading to a variety of different final configurations. We observe evidence of neck instabilities leading to a production of intermediate mass fragments, as observed in many heavy ion experiments.