Competition between fusion-fission and quasi-fission at near-barrier energies

摘要

Fission fragment angular distributions and cross-sections have been measured for the reactions of (sup 16) O + (sup 238) U and (sup 28)Si + (sup 208) Pb at energies around the Coulomb barrier. For the former reaction, fission following full momentum transfer was selected using the folding angle technique. The resulting fission fragment anisotropies rise rapidly as the beam energy decreases through the fusion barrier region. This is interpreted as showing that collisions with the tips of the deformed target nuclei lead to quasi-fission, while collisions with the sides lead to fusion-fission. For the (sup 28) Si + (sup 208) Pb reaction the fusion barrier distribution shows a large influence due to the (sup 28) Si, seemingly not inconsistent with an oblate deformation. However, the fission fragment anisotropy data drop monotonically as the beam energy decreases through the fusion barrier region, indicating that (sup 28) Si does not carry the effect of deformation into the dinuclear regime, unlike (sup 238) U. An explanation for this in terms of the process of dinucleus formation is proposed. 24 refs., 2 figs. (Atomindex citation 27:060968)