Is the inhibition/enhancement of fusion due to breakup still a puzzle?

Abstract

The effect of breakup in the fusion cross section in terms of suppression versus enhancement, discussed in a conflicting way in the literature, is addressed. Data and theoretical predictions available in the literature are compared. Excitation functions of the sub-and near-barrier fusion cross-sections for a wide variety of light and heavy systems are presented and interpreted. We have measured fusion excitation functions and breakup correlation functions for the medium weight systems 6 Li + 59 Co and 7 Li + 59 Co. These measurements help to establish the influence of the projectile breakup on the fusion process at near-barrier energies and contribute to the determination of how the mass of the target affects the breakup role. The results indicate a light fusion enhancement at sub-barrier energies and a geometry dominated cross section at barrier energies.Systematic experimental and theoretical studies on the effect of the coupling of collective degrees of freedom to the fusion process, which lead to a significant enhancement of sub-barrier fusion cross-section when compared to predictions of one dimensional barrier penetration models, have been, so far, widely reported in the literature (ref 1-3). The dynamical process leading to this enhancement seems to be well understood (3). However, in the case of reactions where at least one of the colliding nuclei has a very low binding energy, and the breakup process may become an important reaction channel, conflicting model predictions and experimental results are found (4-7).Many questions regarding this feature became more relevant due to the large impact they may have on the investigations made possible with the recent availability of radioactive beams and the renewed interest in superheavy elements. It is so far established that the coupling of collective additional degrees of freedom to the sub-barrier fusion channel systematically enhances the Complete Fusion (CF) cross section. However, the theoretically expected decrease of the survival probability of weakly bound nuclei at barrier energies suggests an inhibition of the CF cross section. Experimental results have shown that the nuclear breakup channel may be a major limiting process for the fusion probability above the barrier energy when light weakly bound nuclei (8,9) are involved. On the other hand, the Coulomb breakup may also be important when weakly bound nuclei interact with very massive heavy-ions systems (10)(11)(12). One open and interesting question that remains in the case involving medium weight nuclei is whether the breakup process hinders or enhances fusion cross sections in different energy regimes.Recently a systematic study of the fusion process in systems as light as 6,7 Li+ 12 C and 6,7 Li+ 9 Be has been (8,13,14) presented reporting, at energies E above the barrier V B , (E > 2V B ), a correlation between the participants binding energy, the size of the system and the degree of inhibition of the fusion probability. This study indicated that the degree of inhibition of the fusion probabilit...