The dipole strength distribution of ${}^8$He and decay characteristics

Abstract

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 $\sum B(E1)(E^{\ast }<15$~MeV$)=0.95(16)e^2$fm${}^2$ and a dipole polarizability of ${\alpha }_D=0.61(1)$~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.