Near barrier scattering of 8 He from heavy targets

Résumé

The objective of this thesis is the study of the elastic scattering of 8He from 208Pb at energies around the Coulomb barrier. This work is an extension of the investigations performed by the collaboration, in which the Grupo de Estructura de la Materia of the University of Huelva takes part, on 6He reactions at near-barrier energies. The direct comparison of the experimental data from the 6He+208Pb and 8He+208Pb experiments will allow for studying the subtle differences in the dynamics of halo and skin nuclei. To achieve this objective, the experiment E587S has been performed at GAÑIL (Caen, France) in 2010. The angular distributions of the elastic channel, and the 6He and 4He yields from the reactions induced by 8He on the 208Pb target have been measured at two different energies, 16 and 22 MeV. As part of this work, a new compact silicon array named GLORIA has been developed. Its design allows for the measurement of reaction fragments in a continuous angular range from 15Q to 165Q. GLORIA has been used for the first time during the experiment E587S and has provided continuous angular distributions of elastic and reaction cross sections in a wide angular range. The interpretation of the elastic scattering angular distributions have been performed by means of the Optical Model. A first comparison with previous phenomenological potentials for5Li and 7Li has revealed the differences between 8He and 6,7Li on the scattering from a heavy target at near-barrier energies. The main difference arises from the fact that the potentials for6-7Li generate the so-called Coulomb-nuclear rainbow, characteristic of light stable nuclei. This effect is absent in the 8He experimental data. In order to better adjust the experimental data, larger values are needed for the diffuseness of the imaginary part, responsible for the absorption of flux from the elastic channel. This effect has been already observed for exotic nuclei as 6He. This suggests the existence of long range mechanisms which produce the vanishing of the Coulomb-nuclear rainbow. However, the value of the imaginary diffuseness is still smaller than in the case of halo nuclei. As a consequence, the behaviour of 6He differs from pure Rutherford scattering at larger distances than 8He. A CRC calculation taking into consideration the coupling to the one-neutron transfer channel have been also presented. It has been found that the elastic data can be properly adjusted with no real Dipole Polarization Potential (DPP). Compared to 6He, the results suggest that the dipole effects are much smaller for 8He. For fitting the elastic data at 16 MeV, it has been found that the depth of the imaginary part increases. This same effect was already observed for 6Li suggesting that the breakup effects were still present at sub-barrier energies. For 8He it could be also due to strong breakup mechanisms which hold up well below the barrier. For the interpretation of the experimental 6He/8He ratio two DWBA calculations have been performed: In and 2n transfer calculations. For the latter, a simplified model has been considered in which both neutrons are transferred together as a di-neutron. From the one-neutron transfer calculation a lower limit to the production of 6He has been obtained. The two-neutron transfer calculations have been summed up with the previous ones, properly normalised. As a result, for 22 MeVr the ratio 6He/8He can be reproduced just by means of transfer mechanisms at angles larger than 75Q. For smaller angles, the breakup process gets more important, becoming dominant at very forward angles. For 16 MeV, the transfer mechanisms are enough to reproduce the experimental data for larger angles, around 100Q. As expected, transfer mechanisms are less probable as the energy decreases since the colliding nuclei should be close enough. Moreover, for the sub-barrier energy, the 6He yield is still large indicating an important role of breakup. For the interpretation of the experimental 4He/8He ratio the two-neutron transfer leading to 6He in an excited state has been considered. It has been found that, at both energies, the calculations underestimate the data by a factor of around 10.000. However, multiplying by this factor, the shape of the experimental distributions can be reproduced. For 22 MeV, from 75Q the production of 4He is due to transfer while for smaller angles the breakup processes are relevant. In the case of 16 MeV, the 4He yield can be explained, to a large extent, by means of breakup. In general, a preliminary interpretation of the experimental data through simple models have been presented. Further studies will be performed in the near future, including CRC and CDCC calculations.