Statistical and Dynamical Aspects of Intermediate Energy Nuclear Collisions

Abstract: Studies of intermediate energy heavy ion reactions have revealed the existence of a number of reducibility and thermal scaling properties in nuclear multifragmentation. In particular, the probability of emitting n-fragments is reducible to the probability of emitting a single fragment through the binomial distribution. The resulting one-fragment probability shows a dependence on the thermal energy that is characteristic of statistical decay. Similarly, the charge distributions associated with n-fragment emission are reducible to the one-fragment charge distribution, and thermal scaling is observed. Binomial and Poisson simulations of multifragment decay confirm the above experimental findings. The reducibility equation for the n-fragment charge distributions contains a quantity with a value that starts from zero, at low transverse energies, and saturates at high transverse energies. This evolution may signal a transition from a coexistence phase to a vapor phase. In the search for a signal of liquid-gas phase transition, the appearance of intermittency is reconsidered. Percolation calculations, as well as data analysis, indicate that an intermittent-like signal appears from classes of events that do not coincide with the critical one. Information about the dynamical evolution of nuclear sources formed in intermediate energy heavy ion collisions can be gained by means of intensity interferometry. In particular, simultaneous neutron-neutron, proton-neutron and proton-proton interferometry measurements are interesting, since different effects participate differently in the correlations between different types of particle pairs. The simultaneous measurement of particles with nearly equal momenta is a very delicate experimental task, and in particular two-neutron interferometry is plagued by instrumental problems, such as low detection efficiency, background noise and neutron rescattering between detectors (cross-talk). The strength of the measured correlation functions is found to depend on the energy of the particle pair. The measured proton-neutron correlation function shows less strength than expected, possibly due to the Coulomb interaction of the proton with the emitting source. The dynamical aspects of nuclear collisions can be probed by studying meson production. Complete pion production excitation functions can be measured in ramped-beam experiments at storage rings. In such experiments, particular attention must be payed to the delicate problem of translating the measured yields into absolute cross sections. Information on the pion production mechanism can be obtained by comparison of the measured yields with predictions from different model calculations.