No enhancement of fusion probability by the neutron halo of 6He

Quantum tunnelling through a potential barrier ( such as occurs in nuclear fusion) is very sensitive to the detailed structure of the system and its intrinsic degrees of freedom(1,2). A strong increase of the fusion probability has been observed for heavy deformed nuclei(3). In light exotic nuclei such as He-6, Li-11 and Be-11 ( termed 'halo' nuclei(4)), the neutron matter extends much further than the usual nuclear interaction scale. However, understanding the effect of the neutron halo on fusion has been controversial - it could induce a large enhancement of fusion(5), but alternatively the weak binding energy of the nuclei could inhibit the process(6). Other reaction channels known as direct processes ( usually negligible for ordinary nuclei) are also important: for example, a fragment of the halo nucleus could transfer to the target nucleus through a diminished potential barrier. Here we study the reactions of the halo nucleus He-6 with a U-238 target, at energies near the fusion barrier. Most of these reactions lead to fission of the system, which we use as an experimental signature to identify the contribution of the fusion and transfer channels to the total cross-section. At energies below the fusion barrier, we find no evidence for a substantial enhancement of fusion. Rather, the ( large) fission yield is due to a two-neutron transfer reaction, with other direct processes possibly also involved.