Modeling the electric dipole strength of neutron-rich 8He: Prediction of multineutron collective excitations
Abstract
We investigate the multineutron excitations of neutron-rich 8He in the electric dipole strength using a 4He+n+n+n+n five-body model. Many-body unbound states of 8He are obtained in the complex scaling method. We found that the different excitation modes coexist in the dipole strength below the excitation energy of 20 MeV. The strength below 10 MeV comes from the 7He+n channel, indicating the sequential breakup of 8He via the 7He resonance to 6He+n+n. Above 10 MeV, the strength is obtained from many-body continuum states with strong configuration mixings, suggesting a new collective motion of four neutrons with the dipole oscillation against 4He.
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