Momentum distributions and reaction mechanisms for breakup of two--neutron halos

A theoretical model able to describe fragmentation reactions of three--body halo nuclei on different targets, from light to heavy, is used to compute neutron and core momentum distributions. Both Coulomb and nuclear interactions are simultaneously included. We specify the different reaction mechanisms related to various processes. The method is applied to fragmentation of $^6$He and $^{11}$Li on C and Pb. We find good agreement with the available experimental results.