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. A strong increase of the fusion probability has been observed for heavy deformed nuclei. In light exotic nuclei such as 6He, 11Li and 11Be (termed 'halo' nuclei), 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, but alternatively the weak binding energy of the nuclei could inhibit the process. 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 6He with a 238U 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.
Erratum: The 11 B( p, α 0 ) 8 Be reaction at sub-Coulomb energies via the Trojan-horse method [Phys. Rev. C 69, 055806 (2004)] PACS number(s): 26.20.+f, 25.70.Hi, 99.10.CdIn the above article the energy range where our indirect data were normalized to the directly measured ones was mistakenly reported to be 800-900 keV. The energy range actually used in the normalization procedure was instead 400-900 keV. Moreover, the parametrization used for S(E) is not that shown in formula (11). With the correct parameters, this formula reads S b (E) = 0.30 + 1.97E − 0.67E 2 + 4.91 exp −0.5 E − 0.164 0.052 2 .
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