Microscopic multicluster description of the neutron-rich helium isotopes

The neutron halo structure of ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ and of ${}_{}{}^8{}_{}{}^{}\mathrm{He}$ is studied in a microscopic three-body and five-body model, respectively. Various cluster arrangements are included to embody a variety of correlations between the clusters. The intercluster wave function is determined with the stochastic variational method. The ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^8{}_{}{}^{}\mathrm{He}$ energies are reproduced with the same effective force very well. The radii and densities are compared with the results of empirical analyses. The calculated difference of the neutron and proton radii exceeds 0.8 fm, confirming the thick neutron skin. The two-neutron removal spectroscopic amplitudes are calculated and used for evaluating the momentum distribution and the β decay spectrum, which are sensitive to the halo structure. A fair agreement with experiment is obtained.