Entrance Channel Effect on the Formation of Heavy and Superheavy Nuclei

We study the effect of the entrance channel and the shell structure of reacting massive nuclei on the fusion mechanism and the formation of evaporation residues of heavy and superheavy nuclei. In the framework of the combined dinuclear system concept and advanced statistical model, we analyze the Ar-40 + Hf-176, Kr-86 + Xe-130 and Sn-124 + Zr-92 reactions leading to Th-216*; the S-32 + W-182, Ti-48 + Er-166, and Ni-60 + Sm-154 reactions leading to Th-214*; the Ar-40 + Ta-181 reaction leading to Pa-221*; the Ca-48 + Cm-248 reaction leading to the (296)116 compound nucleus. In our calculations of the excitation functions for capture, fusion and evaporation residues we use the relevant variables such as mass-asymmetry of nuclei in the entrance channel, relative distance between nuclear centers, shell effect and shape of colliding nuclei and such characteristics of the reaction mechanism as potential energy surface, driving potential, the dependence of capture, fusion cross sections and survival probability of compound nucleus on the orbital angular momentum. As a result we obtain a beam energy range for the capture of the nuclei before the system fuses and the Gamma(n)/Gamma(f) ratio at each step along the de-excitation cascade of the compound nucleus. Calculations allow us to reach useful conclusions about the mechanism of the fusion-fission process, that is in competition with the quasifission process, and the production of the evaporation residues.