Continuum excitations in6He

Abstract

The three-body breakup 6He→4He+n+n is studied experimentally, using a secondary 6He 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 dσ/dE* and dσ/dϑ for inelastic nuclear or electromagnetic excitations into the 6He 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-Reiche-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 α core and the two valence neutrons of rα−2n=(3.36±0.39) fm is derived. The known Iπ=2+ (1.80 MeV) resonance in 6He is observed in nuclear inelastic scattering; model-dependent values of the quadrupole deformation parameter δ2=(1.7±0.3) fm or B(E2,0+→2+)=(3.2±0.6)e2 fm4 are derived. No clear 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 4He+n+n system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process 4He(2n,γ)6He is briefly discussed.Received 27 October 1998DOI:https://doi.org/10.1103/PhysRevC.59.1252©1999 American Physical Society