Compound nucleus formation effects in heavy-ion fusion cross sections

Heavy-ion-fusion reactions at near-barrier energies take place in two steps. In the first, the system overcomes the potential barrier. In the second, the collision partners merge to form a single body, losing their identities. Most theoretical models ignore the latter, assuming that barrier transmission guarantees fusion. However, it is well known that this assumption is not valid in collisions of very heavy systems. In that case, the collision partners may fuse or emerge from the inner region of the barrier, roughly keeping their original properties. The present paper investigates the influence of the second stage of the collision on the fusion cross section. For this aim, we compare the fusion data of many systems in different mass regions. To eliminate the influence of the height and radius of the Coulomb barrier, the data used in our comparisons are reduced by the classical fusion function method. We find that the evolution of the system is determined by the effective fissility parameter. Barrier transmission guarantees fusion for light and medium-heavy systems where the value of this parameter is less than 25. A different situation is encountered above this limit. The projectile may be captured by the target, or the collision partners may reseparate with their original identities or differ by exchanging a few nucleons. As a result, the capture cross section is suppressed compared to the predictions of the barrier penetration models.