The dissociation reaction ${}^6\mathrm{He}\to \alpha +2n$ was studied with ${}^6\mathrm{He}$ projectiles at 23.9 MeV/nucleon in targets of C, Al, Cu, Sn, Pb, and U. Relative to Al, the $2n$ removal cross section ${\sigma }_{-2n}$ with each of the other targets was determined. With U, the Coulomb part accounts for 2/3 of ${\sigma }_{-2n}.$ The widths of the $\alpha$ particle and neutron parallel momentum distributions increase with target Z. For the $\alpha$ particle, ${}^6\mathrm{He}$ dissociation on C gave width $\sigma =40.2\mathrm{}\pm 2.3\hspace{0.5em}\mathrm{MeV}/c,$ corresponding to an rms radius of the halo neutrons of (2.95±0.17) fm, which is (0.35±0.17) fm larger than the rms radius of all four neutrons. The relation between the widths of the $\alpha$ particle and neutron distributions indicates only a small correlation between the two neutrons. This conclusion is supported by the distribution function for the angle between the two neutrons, which was obtained in kinematically complete measurements. The latter measurements with the U target also yielded an $E1\mathrm{}$ strength function, and it agreed with one determined in an experiment at ten times our beam energy.

We have performed kinematically complete measurements of dissociation of ${}^8\mathrm{He}$ into ${}^6\mathrm{He}$ and two neutrons at $E/A=24\mathrm{}$ MeV. The targets were Sn and Pb. A prominent peak around 0.4 MeV in the relative energy distribution of the ${}^6\mathrm{He}+n$ system is consistent with the ground state of ${}^7\mathrm{He},$ suggesting sequential decay via ${}^7\mathrm{He}$ in ${}^8\mathrm{He}$ dissociation. Further evidence of the sequential decay was found in neutron momentum distributions. The excitation energy distributions of ${}^8\mathrm{He}$ were reconstructed. For the Sn target, it shows a peak which is suggestive of the first excited state of ${}^8\mathrm{He}$ ${(E}_x=3.6\mathrm{}$ MeV, $\Gamma =0.5\mathrm{}$ MeV, and $J^{\pi }{=2\mathrm{}}^{+}).\mathrm{}$ The distribution for the Pb target was found to have a broad structure, indicating the $E1\mathrm{}$ continuum, as observed in measurements of other halo nuclei. The photonuclear cross section ${\sigma }_{E1\mathrm{}}{(E}_x)$ and the dipole strength function ${\mathrm{dB}\left(E1\mathrm{}\right)/dE}_d$ were deduced for the first time. The integrated dipole strength up to 3 MeV decay energy is $0.091\pm 0.026$ $e^2{\mathrm{fm}}^2.$ Insignificant energy-weighted strength, compared to the cluster sum rule, indicates that ${}^8\mathrm{He}$ does not have the ${}^6\mathrm{He}+2n$ structure. Agreement of the measured neutron momentum distributions with the COSMA model is in favor of the ${}^4\mathrm{He}+4n$ structure.