  The neutron halo structure of $^{6}$He and of $^{8}$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 $^{6}$He and $^{8}$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 $\beta$ decay spectrum, which are sensitive to
the halo structure. A fair agreement with experiment is obtained.
