Energy dissipation and suppression of capture cross sections in heavy ion reactions

Background: At energies above the capture barrier, coupled-channels (CC) calculations with a standard nuclear potential diffuseness (0.65 fm) increasingly overestimate experimental capture cross section as the charge product (Z(1)Z(2)) of the colliding nuclei increases. It has been suggested this may be linked to energy dissipation outside the capture barrier. Purpose: We investigate quantitatively the role of energy dissipation in suppressing capture in heavy ion fusion reactions. Method: The yields of sequential fission, including that resulting from deep inelastic collisions, and of fission following capture were determined simultaneously for collisions of O-18, Si-30, S-34, and Ca-40 + Th-232 at a range of energies around the respective capture barriers. Results: The ratio of experimental to CC capture cross sections was found to decrease with increasing Z(1)Z(2). Conversely, the ratio of sequential fission to capture-fission increased with increasing Z(1)Z(2). The sum of sequential and capture fission agrees quite well with the CC cross sections. Conclusions: The experimental capture fission and sequential fission cross sections, and their comparison with CC calculations, give a consistent picture that the increase in density overlap at the capture barrier with increasing Z(1)Z(2) of the colliding nuclei is correlated with increasing energy dissipative processes. These compete increasingly strongly with capture as the Z(1)Z(2) of the reaction increases. For the Ca-40 reaction, the total fission yield exceeds expectations from capture model calculations, indicating that deep inelastic processes occur both from trajectories that would have led to capture and also from more peripheral trajectories.