Three-body breakup of ⁶He and its halo structure

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_nn〉 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_nn〉 = 56₋₁₀⁺⁹ degrees. In either interpretation, the current results show evidence of the dineutron spatial correlation in ⁶He.