Heavy-ion break-up processes in the Fermi energy range

Abstract

The current knowledge on collisions between complex nuclei in the range 10-100 AMeV bombarding energy is reviewed in comparison with the essential features characterizing reactions below 10 AMeV and-partly-above 100 AMeV. The data are inspected for deviations from fusion and binary reactions-quasi-elastic and deep-inelastic-prevailing below 10 AMeV and, in particular, the latter are found to persist far into the energy range considered. The sequential statistical decay of the primary fragments, due to increasing excitation energies, is important in the entire energy range and must carefully be taken into account. The break-up of the lighter collision partner into two or more fragments during the interaction phase (direct break-up) is found to set in at about 10 AMeV and to develop fully until 40 AMeV. It appears to be the typical process for the lower half of the energy range considered, while in the upper half systematic information is still lacking; however the rapidly growing knowledge allows, at best, some trends to be recognized. As an alternative to and probably competing with direct break-up we also discuss frequently used scenarios where the observed ejectiles originate from a statistical decay of an intermediate hot subsystem. We find that the experimental evidence for such a concept, though broad, is not compelling and some inconsistencies persist. Rather, the intermediate-energy regime can be understood in terms of the reaction mechanisms already established at low energies, when combined with the proper extension of the explicitly treated degrees of freedom to those of suitable constituents of the collision partners. The concept of dissipation and friction stays useful in the proper dynamical context. Microscopic modelling is still far from being perfect. We face the competition of mean-field effects and nucleon-nucleon collisions, specific many-particle correlations as intermediate-size phenomena playing an essential role.