Dipole Strength Distribution of 8 He and Decay Characteristics
The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus 8 He is the most neutron-rich bound nucleus with a mass-to-charge ratio of 𝐴/𝑍 = 4. We measure the dipole response of 8 He, including for the first time the four-neutron decay channel. A total dipole strength of ∑𝐵(𝐸1)(𝐸*< 15 MeV) = 0.95(16)𝑒 2 fm 2 and a dipole polarizability of 𝛼 𝐷 = 0.61(1) fm 3 are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the 4𝑛 decay threshold, by two-neutron emission, pointing to a 6 He + 2𝑛 structure of the excited dipole mode. No indication was found for a 4𝑛 final-state correlation, while pronounced 𝑛𝑛 and 6 He−𝑛 final-state correlations are apparent.