Mechanism of fusion reactions for superheavy elements

In heavy-ion fusion reactions at energies around the Coulomb barrier, excitations of low-lying collective motions play an important role. In order to take into account those excitations during fusion reactions, the coupled-channels approach has been a standard tool. The channel coupling effect leads to a distribution of fusion barriers, which has a responsibility to enhance capture cross sections below the Coulomb barrier as compared to a prediction of a simple potential model. The fusion barrier distribution can be actually extracted from measured fusion cross sections, taking the second energy derivative of the product of the incident energy and fusion cross sections, which has been performed experimentally for many systems. The extracted fusion barrier distributions have revealed that the barrier distribution is sensitive to the channel couplings, thus providing a finger print of the underlying dynamics of fusion reactions.

Recently, this technique has been applied to a hot fusion system relevant to superheavy nuclei, that is, the ⁴⁸Ca+²⁴⁸Cm system. The coupled-channels analysis for the measured barrier distribution, which takes into account the deformation of the target nucleus ²⁴⁸Cm, has clearly shown that the maximum of the evaporation residue cross sections for this system appears at an energy slightly above the barrier height for the side collision, in good agreement with the notion of compactness.

In this contribution, we will present the analysis of barrier distribution for the ⁴⁸Ca+²⁴⁸Cm system using the coupled-channels approach. We will also discuss the reaction dynamics of fusion reactions of a deformed nucleus in general, in particular, the role of deformation of the target nucleus in the synthesis of superheavy elements, by using the extended version of fusion-by-diffusion model.