Dissociation of 8He

We have performed kinematically complete measurements of dissociation of ${}^{8}\mathrm{He}$ into ${}^{6}\mathrm{He}$ and two neutrons at $E/A=24$ 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 ${}^{8}\mathrm{He}$ ${(E}_{x}=3.6$ MeV, $\ensuremath{\Gamma}=0.5$ MeV, and ${J}^{\ensuremath{\pi}}{=2}^{+}).$ The distribution for the Pb target was found to have a broad structure, indicating the $E1$ continuum, as observed in measurements of other halo nuclei. The photonuclear cross section ${\ensuremath{\sigma}}_{E1}{(E}_{x})$ and the dipole strength function ${\mathrm{dB}(E1)/dE}_{d}$ were deduced for the first time. The integrated dipole strength up to 3 MeV decay energy is $0.091\ifmmode\pm\else\textpm\fi{}0.026$ ${e}^{2}{\mathrm{fm}}^{2}.$ 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.