Evidence for a nuclear halo from [sup 11]Li elastic scattering measured at 637 MeV incident energy on a [sup 12]C target

An optical model analysis of the elastic scattering of [sup 11]C and [sup 11]Li on [sup 12]C target nuclei has been performed, for measurements at 620 and 637 MeV laboratory incident energy, respectively. The data are from the group of the University of Notre Dame. It has turned out, in the case of the [sup 11]Li projectile, that it was necessary to use a real and imaginary surface potential peaked very far outside of the nucleus core, in order to reproduce the elastic scattering angular distribution at very forward angles. The extracted optical model reaction cross section leads to a radius of 3.7 fm for [sup 11]Li in agreement with what is already known in the literature from interaction cross section measurements. On the contrary, the extracted radius for the [sup 11]C nucleus is 2.5 fm in this experiment. Furthermore, at this high incident energy, a strong far-side component on [sup 11]Li is evidenced by a Fuller analysis. This large radius for the [sup 11]Li nucleus, added to the fact that strong refractive effects are observed in the elastic scattering, is a clear signature of a nuclear halo for the [sup 11]Li nucleus.