Charge and mass exchange in 56Fe-induced reactions at 8.3 MeV/nucleon

Projectilelike fragments from reactions of 465-MeV 56Fe projectiles with targets of 56Fe, 165Ho, 209Bi, and 238U have been measured as a function of energy loss, and neutron and proton number. The mass and charge of each fragment were uniquely determined with discrete resolution. Data were analyzed as a function of energy loss by fitting a two-dimensional Gaussian function to the neutron-proton distributions. The fit parameters provide a convenient basis for comparison with theoretical predictions. In addition, centroids and variances derived from more detailed one-dimensional Gaussian fits to isobaric charge distributions are presented. Results are given for measured data as well as for the primary fragment distributions after correction for the dominant neutron evaporation. The centroid data show that in the asymmetric reactions proton transfer from projectile to target nucleus is preferred while the average fragment neutron number is largely preserved. This produces a charge equilibration process which is dependent on energy loss within the combined systems. The neutron and proton variances indicate an early dominant neutron flow followed by a constant relative neutron and proton exchange rate at energy losses above about 30 MeV. The degree of correlation of the neutron-proton distributions increases with energy loss and is found to be strongly related to the potential energy surface. Consistently observed negative correlation coefficients at low energy losses suggest the possible importance of processes other than particle exchange in the early stages of the interaction. Variances for constant N, Z, and A are found to saturate at energy losses and with magnitudes dependent on the target nucleus