Photodisintegration ofH3andHe3

The photoneutron cross sections for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been measured from threshold to ∼25 MeV with monoenergetic photons from the annihilation in flight of fast positrons. These reactions include the two-body breakup of ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and the three-body breakup of both ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$; these measurements for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ are the first to span the energy region across the peaks of the cross sections. An efficient B${\mathrm{F}}_3$-tube-and-paraffin neutron detector and high-pressure gaseous samples were employed in these measurements. The results, when compared with each other and with results for the two-body breakup cross section for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ from the literature, show that: (a) the two-body breakup cross sections for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have nearly the same shape, but the one for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ lies lower in magnitude; (b) the three-body breakup cross section for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ lies higher in magnitude, broader in the peak region, and also rises less sharply from threshold than that for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$; and (c) these differences between the cross sections for the breakup modes largely compensate in their sum, so that the total photon absorption cross sections for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ are nearly the same in both size and shape at energies near and above their peaks. Theoretical results from the literature disagree with the experimental results to a certain extent over the entire photon-energy region for which the photoneutron cross sections were measured. Sum rule predictions also fail to reproduce the experimental results. These discrepancies constitute a challenge to the principle of charge symmetry of the nuclear force, but more complete theoretical calculations are needed to ascertain whether these discrepancies can be ascribed entirely to electromagnetic effects.