Direct versus sequential fragmentation of neutron-rich nuclei

The dissociation of neutron-rich nuclei from secondary beams incident on several targets can be explained within two distinct models: (a) The weakly bound neutrons form clusters near the nuclear surface, and (b) all protons can vibrate against all neutrons in a soft mode. We show that the momentum widths of the projectile fragments, as well as the total cross sections for the dissociation, are consistent with both hypotheses. Consequently, the interpretation of almost all recent experimental studies with secondary radioactive beams is ambiguous.