Systematic Study Of The Heavy-ion Fusion Dynamics At Energies Near The Coulomb Barrier

The heavy-ion fusion at energies near the Coulomb barrier can be treated as a multi-dimensional barrier penetration problem. In recent decades, the study of capture and fusion dynamics in heavy-ion collisions has been a subject of intense experimental and theoretical interests because the heavy-ion capture not only is of central importance for nucleosynthesis but also can reveal rich interplay between nuclear structure and reaction dynamics. Theoretically, the multidimensional barrier penetration process can be described by solving the coupled channel equations. Furthermore, in the eigenchannel framework, the couplings to other channels split the single potential barrier into a set of discrete barriers. Based on the concept of the barrier distribution, empirical coupled channel models have been developed to describe the capture and fusion process, in which the coupling effects are taken effectively into account by introducing a barrier distribution.We perform a systematic study of capture excitation functions for 217 reaction systems with 182 ?ZPZT ? 1640 by using an empirical coupled channel model. In this model, an asymmetric Gaussian-form barrier distribution is used to take effectively into account the effects of couplings between the relative motion and intrinsic degrees of freedom and the coupling to neutron transfer channels. Based on the interaction potential between the projectile and the target, the parameters of the barrier distribution are determined in two ways: one is to fit the experimental values and get the optimal parameter set; the other is to propose empirical formulas for calculating the parameters of the barrier distribution. The results show that our empirical coupled channel model works quite well in the description of the capture cross section at energies around the Coulomb barrier. Therefore, this model can provide prediction of capture cross section for the synthesis of superheavy nuclei as well as valuable information on capture dynamics.For the reactions including breakup channel, systematic behaviors of the breakup effects on the complete fusion cross sections are studied. The reactions induced by 6,7Li, 9Be, 10,11 B, 12,13 C and 16 O are investigated. The reduced complete fusion cross sections are compared with the universal fusion function which is used as a uniform standard reference. The results show that the complete fusion cross sections at energies above the Coulomb barrier are suppressed by the breakup of projectiles; the suppression effect for reactions induced by the same projectile is independent of the target. An exponential relation between the suppression factor and the energy corresponding to the lowest breakup threshold is held, which suggests that the influence of the breakup channel on the complete fusion is a threshold effect. Furthermore, only the prompt breakup can affect the fusion processes.The suppression of complete fusion at energies above the Coulomb barrier is studied by comparing the data with the predictions from the empirical coupled channel model. The results show that the suppression effect for reactions induced by the same projectile is independent of the target. In addition, the empirical coupled channel model is extended by including the breakup effect which is described by a prompt-breakup probability function. For the reactions induced by 9Be, the prompt-breakup probabilitiy functions have been extracted from the measured prompt-breakup probabilities, the fact of good agreement between calculated complete fusion cross and the data supports to some extent the conclusion that only the prompt breakup affects the fusion processes. For reactions induced by 6,7Li and 10,11 B, the prompt-breakup probabilities have been determined by making a fit to the complete fusion data. The results show that the prompt-breakup probabilities for reactions induced by the same projectile are independent of the target. An exponential relation between the prompt-breakup probability and the energy corresponding to the lowest breakup threshold is held, which indicates that the prompt breakup of the weakly bound nuclei is also a threshold effect.