The charge and mass of the projectile-like fragments produced in the 15-MeV per nucleon ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$${+}^{209}$Bi reaction were determined for products detected near the grazing angle. Neutron number-charge (N-Z) distributions were generated as a function of the total kinetic energy loss and parametrized by their centroids, variances, and correlation coefficients. Although the initial system is very asymmetric, after the interaction, a drift of the charge and mass centroids toward further asymmetry is observed. The production of projectile-like fragments is consistent with a tendency of the projectile-like fragments to retain the projectile neutron-to-proton ratio 〈N〉/〈Z〉≃1. The correlation coefficient remains well below 1.0 for the entire range of total kinetic energy lost. Predictions of two nucleon exchange models, Randrup’s and Tassan-Got’s, are compared to the experimental results. The models are not able to reproduce the evolution of the experimental distributions, especially the fact that the variances reach a maximum and then decrease as function of the energy loss. This behavior supports the hypothesis that some form of projectile-like fragmentation or cluster emission is perturbing the product distribution from that expected from a damped mechanism.

• Received 28 February 1995

DOI:https://doi.org/10.1103/PhysRevC.52.3114