Photoneutron cross sections forO17

Abstract

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.NUCLEAR REACTIONS $^{17}\mathrm{O}(\ensuremath{\gamma},n)$, ${E}_{\ensuremath{\gamma}}=8.5\ensuremath{-}39.7$ MeV and $^{16}\mathrm{O}(\ensuremath{\gamma},n)$, ${E}_{\ensuremath{\gamma}}=15.9\ensuremath{-}39.7$ MeV; measured $4\ensuremath{\pi}$ neutron yield for monenergetic photons; $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},1n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},2n)$, integrated cross sections, isospin splitting of the giant resonance for $^{17}\mathrm{O}$.