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 6He
and 11Li on C and Pb. We find good agreement with the available
experimental results.Comment: 10 pages, 3 figures, Phys.Lett.B in pres