Continuum spectroscopy of Borromean two-neutron halo nuclei

Energy and angular correlation distributions of the three fragments in He-6 breakup on Pb-208 at a collision energy of 240 MeV/nucleon are discussed within the microscopic four-body distorted wave model and compared with experimental data. The nuclear structure of the ground state and low-energy three-body continuum of He-6 is calculated by the method of hyperspherical harmonics within the three-body cluster model. Reflections of the fundamental permutation symmetry of the halo neutrons in angular and energy correlations are pointed out. The calculations describe the experimental data for fragment correlations near breakup threshold rather well, and the physics is contained in a few elementary modes; but with increasing excitation energy of He-6, some striking deviations from experimental distributions are encountered. Possible reasons for this are discussed.