Equilibration of the neutron-to-proton ratio for heavy asymmetric systems in heavy-ion collisions

Some results on the charge equilibration process for the ²⁰⁵Pb (1550 MeV) + ⁷⁴Ge and ⁸⁶Kr (550 MeV) + ¹⁹⁷Au reactions obtained by on-line measurements are discussed and compared with the previous results of the ¹³²Xe (890 MeV) +¹⁹⁷Au reaction investigated using radiochemical techniques. In all cases global and isobaric charge widths were measured as a function of the excitation energy. It is shown that the corrections associated with both the evaporation process and the experimental mass or charge resolution are most important in obtaining correct primary distributions. The results confirm a preferential neutron exchange at the beginning of the reaction, as was suggested by the Brosa and Gross model. Further attempts to analyse the data with existing dynamical models have not succeeded in their present stage of development in discriminating between the assumptions of statistical versus quantal fluctuations as observed in the study of the Kr + Mo system.