Photoneutron cross sections forO17

Photoneutron cross sections involving the emission of one and two neutrons from ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ have been measured over the energy interval 8.5 to 39.7 MeV using monoenergetic photons from positron in-flight annihilation. The 6-MeV wide giant dipole resonance is observed to be centered at 23 MeV and a pygmy resonance is seen at about 13 MeV. Such structure as is apparent in the cross sections is not as pronounced as for the cases of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. Comparison of the total photoneutron cross section with recent ground-state data indicates that much of the pygmy resonance decays to the ground or first excited state of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, but that the giant dipole resonance decays mainly to highly excited states in the daughter. Excellent agreement is observed between the present results and a recent two-particle, one-hole shell-model calculation of the isospin-split giant dipole resonance states for this nucleus. New photoneutron cross-section results for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ up to 39.7 MeV are reported as well.