$^{24}Mg+\,^{12}C$ fusion reaching the no-coupling limit far below the barrier.

The low-energy fusion experiment on $^{24}Mg+\,^{12}C$ showed that the cross-sections far below the Coulomb barrier, actually reach the no-coupling limit. The ratio of experimental cross-sections ($\sigma_{exp}$) to no-coupling calculations ($\sigma_{noc}$) $vs.$ energy distance from the Coulomb barrier has been obtained and compared with $^{30}Si+\,^{12}C$ and two heavier systems $^{48}Ca+\,^{48}Ca$ and $^{58}Ni+\,^{54}Fe$. In all these cases the no-coupling calculations are based on Woods-Saxon potentials. The hindrance effect has been observed in all four systems. As expected, the sub-barrier enhancement is larger for the heavier systems, with respect to the two lighter systems. This prevents observing the behaviour of the ratio $\sigma_{exp}/\sigma_{noc}$ below a certain limit, whereas it is clearly seen that for $^{30}Si+\,^{12}C$ and in particular for $^{24}Mg+\,^{12}C$ the ratio decreases to one (no enhancement) at the lowest energies. The question is then whether the cross-sections follow the no-coupling calculation or if, since the ion-ion potential effectively becomes a one-body potential after the touching point, the excitation function decreases even below that limit.