Strong influence of the entrance channel on the formation of compound nuclei $^{216,222}$Th and their evaporation residues
Résumé
The dynamical effects of the entrance channel on the formation of the evaporation residues are studied by analyzing the $^{40}$Ar+$^{176}$Hf, $^{86}$Kr+$^{130,136}$Xe, $^{124}$Sn+$^{92}$Zr, and $^{48}$Ca+$^{174}$Yb reactions leading to the $^{216}$Th and $^{222}$Th compound nuclei. We find that the difference between the evaporation residue cross sections for the reactions leading to the same compound nucleus is caused by the different angular momentum distributions of the partial fusion cross sections $\sigma_fus^l$($E_c.m.$). The strong dependence of the fusion angular momentum distribution on the mass (charge) asymmetry and shell structure of reactants is demonstrated. The effect of the A/Z ratio for the $^{86}$Kr+$^{130,136}$Xe reactions is discussed. The dynamical conditions of capture affect the competition between complete fusion and quasifission and, consequently, the shape of the angular momentum distribution of the compound nucleus. By this way the peculiarities of the entrance channel also affect the fission-evaporation competition of the excited intermediate nuclei along the deexcitation cascade of the compound nucleus and, consequently, the evaporation residue formation.