Electric quadrupole photoneutron reactions in 16O

Photoneutron energy spectra from the 16O(γ,n0)15O reaction were measured as functions of laboratory angle over a range of excitation energies from 30 to 35 MeV. Angular distribution Legendre-polynomial coefficients were extracted up to third order as functions of excitation energy. Nonzero values of the coefficients a1 (+0.25 +- 0.02) and a3 (-0.2 +- 0.02) were observed over the energy region explored, indicating interference between states of opposite parity. These values can be accounted for in a simple model incorporating electric quadrupole absorption strength decaying via the ground-state photoneutron channel. When combined with the previously-determined amplitude ratio for the E1 p→s and p→d single-particle transitions, the present results suggest that about 4% of the isovector energy-weighted sum rule is found in the (γ,n0) channel in the energy range studied. The value of the (γ,n0) cross section was found to vary from 1.5 +- 0.1 mb at E/sub x/ = 30 MeV to 0.8 +- 0.1 mb at 35 MeV. The average magnitude of the E2 contribution to this cross section was estimated to be 0.05 +- 0.02 mb. This is in reasonable agreement with a recent continuum random-phase approximation shell model calculation, but is in disagreement with a previous measurement