Proposal / Letter of Intent / Addendum ... ] to the ISOLDE and Neutron Time-of-Flight Committee / / Proposals for ISOLDE low-energy experiments Study of Interaction of 8 He with 209 Bi sub-barrier energy May 29 wednesday

Excitation functions for evaporation residues in the reaction 209 Bi( 8 He, xn) 217-xn At are proposed for measurement at the ISOLDE facility. A coincidence between neutron and alpha particles allows to get on break-upfusion and direct breakup phenomena of 8 He. Study of Interaction of 8 He with 209 Bi sub-barrier energy 1. Motivation It has been found long ago that the fusion of stable nuclei at energies close to the Coulomb barrier strongly depends on the coupling with other reaction channels, in particular with direct reactions [1, 2]. In the case of some light neutron-rich nuclei an extended distribution of nuclear matter is observed and the presence of valence neutrons can lead to the formation of a neutron halo or skin, characterized by small separation energy of the constituent nucleons, 6, 8 He are such examples. The reactions with halo nuclei have aroused much interest and continue to be a challenge both to experiment and theory. In particular, much effort has been devoted to studying near-barrier fusion of light weakly bound nuclei. What concerns the interaction of the neutron-rich nuclei 6,8 He with stable targets, one can expect increased fusion crosssection due to the reduction of the reaction Coulomb barrier because of the larger radius. On the other hand, the weak binding of the halo neutrons leads to a higher breakup probability of the nuclei which, in turn, reduces the fusion cross-section. If breakup occurs, consequent capture of the residual nucleus (the core) by the target nucleus or transfer of nucleons without any further interaction between the nuclei can take place. The exchange of one or several nucleons between the target and projectile, inelastic scattering, etc. are also probable. However, if the couplings between fusion and transfer reactions with positive Q-values are taken into account, it is expected that the fusion involving 6 He will be enhanced [3,4] similarly to the cases observed for stable beams [1, 2, 5]. In any case, the variety of processes makes it difficult to analyze the