Dynamics of the fusion process

The binary reaction products from the interaction of a ²⁰⁸Pb beam with targets of ²⁶Mg, ²⁷Al, ⁴⁸Ca, ⁵⁰Ti, ⁵²Cr, ⁵⁸Fe and ⁶⁴Ni have been studied with the aid of a large position-sensitive ring counter, operated in a two-particle coincidence mode. The intensity of γ-rays and X-rays per event was also recorded. Within a broad range around mass symmetry, the center of mass angular distributions, γ-ray multiplicities, total kinetic energy distributions, and absolute mass yields have been determined as a function of the bombarding energy, ranging from 1.0 to 1.8 times the interaction barrier. When analysed on the basis of fusion models, the cross section for the mass equilibration reaction demonstrates that deformations induced at contact influence the fusion of these heavy systems significantly and characteristically. The γ-ray multiplicities appear to be strongly influenced by statistical angular momentum components that remain with the fragments after separation. Their magnitudes indicate that during the reaction, the collision complex becomes at least as compact as the liquid-drop saddle shape appropriate to a non-rotating nucleus with the same total mass and charge. Finally, some angular distributions show that the entire process of coalescence and reseparation can occur before the system has made one-half revolution; i.e. within a time of 5–10 × 10^?21 s. No X-ray emission from the combined system ²⁰⁸Pb + ⁴⁸Ca is observed.