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.

• Received 16 June 2000

DOI:https://doi.org/10.1103/PhysRevC.62.064311