Angular correlations in peripheral fragmentation of halo nuclei

The angular correlation between momenta of He-5 and its decay products has been calculated for the peripheral fragmentation of He-6 using a two-step model in a sudden approximation: (1) One neutron knock-out leading to the unbound nucleus He-5. (2) Subsequent decay of He-5 undisturbed by the target in an alpha-particle and a neutron. Angular correlations measured in a recent experiment dealing with the peripheral fragmentation of 240 MeV/u He-6 on a carbon target can be well described by a dominant He-5 3/2(-) ground state configuration plus a small admixture of approximate to 7% of the first excited 1/2(-) state. The influence of an s-state admixture as well as a contribution of the momentum transfer to the correlation function are investigated. The results obtained for He-6 are then used as a touch stone for a discussion of the methods to investigate peripheral fragmentation of other Borromean halo nuclei like Li-11 and Be-14. It is shown that the evidence on the possible existence of low-lying s-states in Li-10 and Be-13 can be derived from the corresponding angular correlations.