Abstract
Differential cross sections have been measured for the reaction (γ,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 coefficients are observed indicating the absorption of non-E1 radiation. The measured cross section and 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.
- Received 30 January 1987
DOI:https://doi.org/10.1103/PhysRevC.36.1243
©1987 American Physical Society