Ground-state photoneutron reactions in sup18O.

Abstract

Differential cross sections have been measured for the reaction $^{18}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{17}$O over the region of excitation energy from 14 to 26 MeV. The angle-integrated cross section for the ground-state transition reveals that this channel accounts for less than 20% of the total photoneutron cross section in the structured pygmy resonance region (near 14 MeV) and is a small fraction (10--15 %) of the cross section in the region of the giant resonance (near 25 MeV). The values of angular distribution coefficients fitted to the data are consistent with a description of this reaction in which electric dipole excitations dominate the cross section in the pygmy resonance. Narrow regions exist near 15.0, 16.0, and 20.0 MeV where nonzero ${a}_{1}$ coefficients are observed indicating the absorption of non-E1 radiation. The measured cross section and ${a}_{2}$ coefficients are compared to a direct-semidirect calculation which gives reasonable agreement and suggests that f-wave neutron emission dominates the ground-state channel and that there is little justification for the introduction of E2 amplitudes other than a pure direct E2 term.