Fusion at the barrier with light radioactive ion beams

The experimental results recently obtained for fusion reactions at energies close to the Coulomb barrier with light radioactive (loosely bound) beams are reviewed and critically discussed. There have been two conflicting views on the effect of the loose binding of the projectile on the fusion cross section. On the one hand one expects an enhancement of the fusion cross section due to the loose binding while, on the other hand, the easy breakup of the projectile is expected to inhibit the fusion cross section. We critically discuss these two aspects of loose binding by comparing the experimental results for a number of radioactive beams. The data for 17 F (where the last neutron binding energy S n =0.601 MeV), neither show breakup effects nor enhancement when compared with the fusion of the nucleus 19 F. The data for a 6 He beam ( S 2n =0.975 MeV) show enhancement, very strong in one case, and the strong breakup (BU) + transfer cross section may be related to this. The fusion data obtained with the halo nucleus 11 Be ( S n =0.504 MeV) do not show enhancement below the barrier, compared with 9 Be, and show moderate effects above the barrier. Moreover, 9 Be ( S n =1.665 MeV) shows a very strong BU + transfer cross section as in the case of 6 He, but this apparently has no direct effect on fusion. The data with the neutron-rich 38 S, 29,31 Al radioactive beams do not differ much from data with the stable 32 S, 27 Al, respectively. In conclusion we observe: (i) effects of loose binding in neutron skin nuclei but not with other halo nuclei, (ii) a reduction of the fusion cross section above the barrier due to breakup effects. The various theoretical approaches developed up to now based on “simple” assumptions give opposite predictions for fusion, with some expecting enhancement of the cross section while others expect a hindrance of the cross section. However, most of these approaches do not correctly treat the transitions to the continuum which are expected to be induced easily in these weakly bound nuclei. It is necessary to develop a more complete and possibly “simple” theoretical treatment that incorporates the continuum coupling which will play an important role in subbarrier fusion.