Photoneutron energy spectra from the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}(\gamma ,{\mathrm{n}}_0){}_{}{}^{15}{}_{}{}^{}\mathrm{O}$ 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 $a_1 (+0.25\mathrm{}\pm 0.02)$ and $a_3 (-0.2\mathrm{}\pm 0.02)$ 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 $E1$ $p\to s$ and $p\to d$ single-particle transitions, the present results suggest that about 4% of the isovector energy-weighted sum rule is found in the ($\gamma$,${\mathrm{n}}_0$) channel in the energy range studied. The value of the ($\gamma$,${\mathrm{n}}_0$) cross section was found to vary from 1.5±0.1 mb at $E_x=30\mathrm{}$ MeV to 0.8±0.1 mb at 35 MeV. The average magnitude of the $E2\mathrm{}$ contribution to this cross section was estimated to be 0.05±0.02 mb. This is in reasonable agreement with a recent continuum randomphase approximation shell model calculation, but is in disagreement with a previous measurement.

• Received 6 February 1984

DOI:https://doi.org/10.1103/PhysRevC.30.1789