Three-body breakup of 6He and its halo structure

Abstract The Borromean halo nucleus 6He has been studied by a kinematically complete measurement of Coulomb and nuclear breakup into α + 2n on Pb and C targets at 70 MeV/nucleon. Fully quantum-mechanical four-body breakup calculations reproduce the energy and angular differential cross sections below E rel ∼ 1 MeV for both targets. The model used here reproduces the 6He ground-state properties as well as α-n and n-n scattering data and predicts an average opening angle 〈 θ n n 〉 of 68∘ between the two halo neutrons. However, the model underestimates the breakup cross sections for higher E rel , indicating a possible contribution from the inelastic breakup. Alternatively, we examine the empirically modified calculations that reproduce the energy-differential cross sections for a wide range of scattering angles for both targets. The extracted B(E1) peaks at E rel ∼ 1.4 MeV and amounts to 1.6(2) e2 fm2 for E rel ≤ 20 MeV, resulting 〈 θ n n 〉 = 56 − 10 + 9 degrees. In either interpretation, the current results show evidence of the dineutron spatial correlation in 6He.