We have performed kinematically complete measurements of dissociation of 8He into 6He 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 6He+n system is consistent with the ground state of 7He, suggesting sequential decay via 7He in 8He dissociation. Further evidence of the sequential decay was found in neutron momentum distributions. The excitation energy distributions of 8He were reconstructed. For the Sn target, it shows a peak which is suggestive of the first excited state of 8He (Ex = 3.6 MeV, Γ = 0.5 MeV, and Jπ = 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 σE1(Ex) and the dipole strength function dB(E1)/dEd were deduced for the first time. The integrated dipole strength up to 3 MeV decay energy is 0.091 ± 0.026 e2fm2. Insignificant energy-weighted strength, compared to the cluster sum rule, indicates that 8He does not have the 6He+2n structure. Agreement of the measured neutron momentum distributions with the COSMA model is in favor of the 4He+4n structure.
|Number of pages||5788000|
|Journal||Physical Review C - Nuclear Physics|
|Publication status||Published - Dec 2000|
ASJC Scopus subject areas
- Nuclear and High Energy Physics