Dissociation of He-8 - art. no. 064311

We have performed kinematically complete measurements of dissociation of He -8 into He-6 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 He -6+n system is consistent with the ground state of He-7, suggesting sequent ial decay via He-7 in He-8 dissociation. Further evidence of the sequential decay was found in neutron momentum distributions. The excitation energy d istributions of He-8 were reconstructed. For the Sn target, it shows apeak which is suggestive of the first excited state of He-8 (E-x= 3.6 MeV, Gamma = 0.5 MeV, and J(pi) = 2(+)). The distribution for the Pb target was found to have a broad structure, indicating the El continuum, as observed in mea surements of other halo nuclei. The photonuclear cross section sigma (E1)(E -x) and the dipole strength function dB(E1)/dE(d) were deduced for the firs t time. The integrated dipole strength up to 3 MeV decay energy is 0.091 +/ - 0.026 e(2)fm(2). Insignificant energy-weighted strength, compared to the cluster sum rule, indicates that He-8 does not have the He-6+2n structure. Agreement of the measured neutron momentum distributions with the COSMA mod el is in favor of the He-4+4n structure.