Deformability as a Critical Factor in Initiating Fusion between Very Heavy Ions

Fusion reactions are initiated by capture of two incident nuclei to form a quasistable dinuclear complex. Excitation functions for such capture reactions have been measured in bombardments with ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ on targets of ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{50}{}_{}{}^{}\mathrm{Ti}$, ${}_{}{}^{52}{}_{}{}^{}\mathrm{Cr}$, and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Fe}$. Compared with standard fusion models, based on the interaction of rigid spheres, the experiments show major deviations. These are explained by a dynamical model that takes deformations induced upon contact into account.