Continuum excitations in 6He

The three-body breakup He-6 --> He-4 + n + n is studied experimentally, using a secondary He-6 ion beam of 240 MeV/nucleon incident on carbon and lead targets. integrated cross sections for one- and two-neutron knockout and differential cross sections d sigma/dE* and d sigma/d theta for inelastic nuclear or electromagnetic excitations into the He-6 continuum are presented. The E1-strength distribution is deduced from electromagnetic cross sections and is found to exhaust (10 +/- 2)% of the energy-weighted Thomas-Reichr-Kuhn sum rule or (40 +/- 8)% of the cluster sum rule for excitation energies below 5 MeV. Both the energy-weighted and non-energy-weighted dipole cluster sum rules are almost exhausted integrating the strength up to 10 MeV, a fact from which the root-mean-square distance between the a core and the two valence neutrons of r(alpha - 2n) = (3.36 +/- 0.39) fm is derived. The known I-pi = 2(+) (1.80 MeV) resonance in He-6 is observed in nuclear inelastic scattering; model-dependent values of the quadrupole deformation parameter delta(2) = (1.7 +/- 0.3) fm or B (E2.0(+) --> 2(+)) = (3.2 +/- 0.6)e(2) fm(4) are derived. No dear signature could be obtained for predicted higher-lying 2(+) resonances, but low-lying continuum strength of multipolarity other than dipole, likely of monopole and quadrupole multipolarity, is indicated by the data. Two-body correlations in the decaying He-4 + n + n system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process He-4(2n, gamma) He-6 is briefly discussed. [S0556-2813(99)05903-8].