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.

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The elastic scattering of 6He on 208Pb has been measured at laboratory energies of 14, 16, 18 and 22 MeV. These data were analyzed using phenomenological Woods- Saxon form factors and optical model calculations. A semiclassical polarization po- tential was used to study the e ect of the Coulomb dipole polarizability. Evidence for long range absorption, partially arising from Coulomb dipole polarizability, is reported. The energy variation of the optical potential was found to be consistent with the dispersion relations which connect the real and imaginary parts of the potential

The structure of the three-body Borromean nucleus 6 He is approximated by a two-body di-neutron cluster model. The binding energy of the 2n-α system is determined to obtain a correct description of the 2n-α coordinate, as given by a realistic three-body model calculation. The model is applied to describe the breakup effects in elastic scattering of 6 He on several targets, for which experimental data exist. We show that an adequate description of the di-neutron-core degree of freedom permits a fairly accurate description of the elastic scattering of 6 He on different targets.A. M. MORO et al. PHYSICAL REVIEW C 75, 064607 (2007)

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Abstract. Accurate elastic scattering angular distribution data measured at bombarding energies just above the Coulomb barrier have shapes that can markedly differ from or be the same as the expected classical Fresnel scattering pattern depending on the structure of the projectile, the target or both. Examples are given such as 18 O + 184 W and 16 O + 148,152 Sm, where the expected rise above Rutherford scattering due to Coulomb-nuclear interference is damped by coupling to the target excited states, and the extreme case of 11 Li scattering, where coupling to the 9 Li + n + n continuum leads to an elastic scattering shape that cannot be reproduced by any standard optical model parameter set. An early indication that the projectile structure can modify the elastic scattering angular distribution was the large vector analyzing powers observed in polarised 6 Li scattering. The recent availability of high-quality 6 He, 11 Li and 11 Be data provides further examples of the influence that coupling effects can have on elastic scattering. Conditions for strong projectile-target coupling effects are presented with special emphasis on the importance of the beam-target charge combination being large enough to bring about the strong coupling effects. Several measurements are proposed that can lead to further understanding of strong coupling effects by both inelastic excitation and nucleon transfer on near-barrier elastic scattering. A final note on the anomalous nature of 8 B elastic scattering is presented as it possesses a more or less normal Fresnel scattering shape whereas one would a priori not expect this due to the very low breakup threshold of 8 B. The special nature of 11 Li is presented as it is predicted that no matter how far above the Coulomb barrier the elastic scattering is measured, its shape will not appear as Fresnel like whereas the elastic scattering of all other loosely bound nuclei studied to date should eventually do so as the incident energy is increased, making both 8 B and 11 Li truly "exotic".

Expérience GANIL/SPIRAL/MUST2/E525SThe low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p(8He, t) with the 8He beam delivered by the SPIRAL facility at 15.4 A MeV. The light charged particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2+ state, two new resonances were observed: at E∗ = 2.6±0.3 MeV (width Γ = 1.6±0.4 MeV) and at 5.3±0.3 MeV with Γ = 2 ± 1 MeV. Through the analysis of the angular distributions, they correspond to a 2+ state and to an L = 1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects

The 7 Li+ 28 Si elastic scattering was studied at near-barrier energies, namely, 8, 8.5, 9, 10, 11, 13, 15, and 16 MeV, with the aim to map the real and imaginary part of the optical potential and therefore probe the threshold anomaly. Angular distributions were measured over a wide angular range of ( lab = 25°to 150°) for the lower energies and of ( lab = 10°to 100°) for the higher energies. The present data, together with previous ones on heavier targets ( 138 Ba and 208 Pb) at near barrier energies, were analyzed by using optical potentials obtained in a double-folding framework. The results were compared with previous measurements of 6 Li on the same targets. It was found that a striking difference occurs between the imaginary potentials of 6 Li and 7 Li, which, respectively, present an increasing and decreasing behavior approaching the barrier from higher to lower energies. On the other hand, this energy variation is not fully reflected to the real part of the potential, as it is described by dispersion relations. The strength of the real potential remains almost constant with a weak declining and uprising trend for the 6 Li and 7 Li, respectively. For a better understanding of our results, continuum-discretized-coupled-channel calculations were also performed and are discussed.