Photonuclear cross sections forO18

All the major photonuclear cross sections for ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, including $\sigma (\gamma ,p)$, $\sigma \left[\right(\gamma ,n)+(\gamma ,np\left)\right]$, and $\sigma (\gamma ,2n)$, were measured as a function of photon energy from threshold to 42 MeV. The photon energy resolution was between 200 and 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for both single- and double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. The photoproton cross section was determined by counting the delayed neutrons from the $\beta$-decay of the residual ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ nuclei. Integrated cross sections were extracted from the data and compared with sum-rule predictions. Resonances in the pygmy-resonance region exhibit substantial decay branching to negative-parity states in ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$. Significant differences in the branching of resonances in the giant-resonance region to the various partial cross sections, together with the photoneutron-energy data, allow isospin assignments to be made for all the resonances, with the consequent delineation of the isospin strength distribution for ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. Evidence was found for two weak but broad resonances above the giant-resonance region. Finally, comparison with the corresponding cross sections for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ shows considerable differences between the two nuclei.