Effects of channel and potential radiative transitions in theO17(γ, n0)O16reaction

Abstract

The angular distribution for the $^{17}\mathrm{O}(\ensuremath{\gamma}, {n}_{0})^{16}\mathrm{O}$ reaction was observed throughout the excitation energy region 4.3-7 MeV and at angles of 90\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{}. The ground-state radiation widths for resonances in this energy region were extracted from the data. The value of the radiation width for the ${d}_{\frac{5}{2}}\ensuremath{\rightarrow}{d}_{\frac{3}{2}}$ spin-flip transition at 5.08 MeV was found to be approximately $\frac{1}{3}$ of the value expected for a pure single-particle transition. The implications that this result has for the nuclear structure of $^{17}\mathrm{O}$ is discussed. The effects of potential radiative capture were observed directly in a photoneutron reaction for the first time. At the location of the 5.38-MeV, $\frac{3}{{2}^{\ensuremath{-}}}$ resonance in $^{17}\mathrm{O}$, an anomalous symmetric dip was observed in the cross section at both reaction angles. The data were interpreted in terms of a general $R$-matrix reaction theory which includes the effects of internal, channel, and potential radiative capture in a self-consistent manner. The neutron channel was defined by incorporating an $R$-matrix analysis of the $^{16}\mathrm{O}(n, n)^{16}\mathrm{O}$ reaction into the present interpretation. The anomalous minimum at 5.38 MeV was found to be due to a unique feature of channel capture. The $R$-matrix prediction for the total cross section was extrapolated into the keV region and the significance that this cross section has for stellar nucleosynthesis is discussed.NUCLEAR REACTIONS $^{17}\mathrm{O}(\ensuremath{\gamma}, {n}_{0})^{16}\mathrm{O}$; observed angular distribution; ${E}_{\ensuremath{\gamma}}=4.3\ensuremath{-}7$ MeV; $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}, 135\ifmmode^\circ\else\textdegree\fi{}$; $R$-matrix analysis; measured ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$ for $E1$ and $M1$ resonances.