Influence of Dynamic Processes upon Complete Fusion of Heavy Nuclei at Subbarrier Energies

Excitation functions for complete fusion of ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$+${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$+${}_{}{}^{94}{}_{}{}^{}\mathrm{Zr}$ have been found to decrease gradually over a broad energy range below the nominal barrier. The effective barrier for ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$+${}_{}{}^{94}{}_{}{}^{}\mathrm{Zr}$ was 10.5 ± 3.0 MeV higher than the nominal barrier implying a critical effective fissility of 31.5 ± 0.5. Although deep-interpenetration processes appear to limit fusion at near-barrier energies, weak-contact dynamics enable these cold, heavy nuclei to fuse with comparative ease at lower energies.