Compound nucleus formation in reactions between massive nuclei: Fusion barrier

The evaporation residue cross sections ${\mathrm{\sigma }}_{\mathrm{ER}}$ in reactions between massive nuclei have been analyzed within different models of complete fusion. The calculations in the framework of the optical model, the surface friction model, and the macroscopic dynamic model can give the results which are by few orders of magnitude different from experimental data. This takes place due to neglect of the competition between complete fusion and quasifission. A possible mechanism of compound nucleus formation in heavy-ion-induced reactions has been suggested. The analysis of the complete fusion of nuclei on the basis of dinuclear system approach has allowed one to reveal an important feature of the fusion process of massive nuclei, that is, the appearance of the fusion barrier during dinuclear system evolution to a compound nucleus. As a result, the competition between complete fusion and quasifission arises and strongly reduces the cross section of the compound nucleus formation. A model is proposed for calculation of this competition in a massive symmetric dinuclear system. This model is applied for collision energies above the Coulomb barrier. The ${\mathrm{\sigma }}_{\mathrm{ER}}$ values calculated in the framework of dinuclear system approach seem to be close to the experimental data. For illustration the reactions ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$${+}^{100}$Mo, ${}_{}{}^{110}{}_{}{}^{}\mathrm{Pd}$${+}^{110}$Pd, and ${}_{}{}^{124}{}_{}{}^{}\mathrm{Sn}$${+}^{96}$Zr have been considered.