Electric quadrupole photoneutron reactions inO16

Abstract

Photoneutron energy spectra from the $^{16}\mathrm{O}(\ensuremath{\gamma},{\mathrm{n}}_{0})^{15}\mathrm{O}$ 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 ${a}_{1} (+0.25\ifmmode\pm\else\textpm\fi{}0.02)$ and ${a}_{3} (\ensuremath{-}0.2\ifmmode\pm\else\textpm\fi{}0.02)$ 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 $E1$ $p\ensuremath{\rightarrow}s$ and $p\ensuremath{\rightarrow}d$ single-particle transitions, the present results suggest that about 4% of the isovector energy-weighted sum rule is found in the ($\ensuremath{\gamma}$,${\mathrm{n}}_{0}$) channel in the energy range studied. The value of the ($\ensuremath{\gamma}$,${\mathrm{n}}_{0}$) cross section was found to vary from 1.5\ifmmode\pm\else\textpm\fi{}0.1 mb at ${E}_{x}=30$ MeV to 0.8\ifmmode\pm\else\textpm\fi{}0.1 mb at 35 MeV. The average magnitude of the $E2$ contribution to this cross section was estimated to be 0.05\ifmmode\pm\else\textpm\fi{}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.