Neutron angular distributions from the core break-up reactions of the $^{11}$Be and $^{11}$Li halo nuclei

International audienceThe halo nuclei 11Be and 11Li have been studied through core break-up reactions, where the halo neutrons are detected in anti-coincidence with the core of the halo nucleus. In this particular channel, the halo neutrons are not expected to participate in the reaction and should therefore show the same properties as when situated inside the halo nucleus. The widths of the halo neutron momentum distributions have been extracted in coincidence with He fragments, Γ = 32±4 MeV/c, and Li fragments, Γ = 42±4 MeV/c for 11Be and with He fragments, Γ = 42±6 MeV/c for 11Li. An experimental value of the shadow effect for 11Be when breaking up to Li and He fragments has been obtained to be 0.63. A simple theoretical calculation to reproduce this value is given