Three-body photodisintegration ofHe3

The photoneutron cross section for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been measured from threshold to 30 MeV using monochromatic photons from positron annihilation in flight. The cross section reaches its maximum of about 0.95 mb at about 15 MeV. Several distinct structures are apparent, some correlated with observed fore-aft asymmetry of the photoneutrons. The integrated cross section is 13.0 MeV mb, and the $\frac{1}{E}$ integrated cross section is 0.77 mb. With reasonable extrapolations to higher energies this is consistent with sum-rule predictions, well within experimental uncertainties. Current theoretical calculations are capable of accounting for the over-all size and shape of the cross section, but not for the fine details of the present measurement.