Giant dipole resonance in 17O observed with the ( gamma,p) reaction.

Abstract

The giant dipole resonance (GDR) in $^{17}\mathrm{O}$ has been studied with the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},p${)}^{16}$N from ${\mathit{E}}_{\ensuremath{\gamma}}$=13.50 to 43.15 MeV using quasimonoenergetic photons. The measured cross section shows major peaks at 15.1, 18.1, 19.3, 20.3, 22.2, 23.1, 24.4, and \ensuremath{\sim}26.5 MeV. The intermediate structure in the main GDR region is remarkably similar to that observed in $^{16}\mathrm{O}$, indicating that the valence neutron outside the doubly magic $^{16}\mathrm{O}$ core perturbs the core-excited states minimally, in support of the weak-coupling hypothesis. We correlate the trends in GDR structure of $^{16,17,18}\mathrm{O}$ with changes in ground-state properties related to static deformation. The (\ensuremath{\gamma},p) reaction selects strength predominantly from two-particle--one-hole configurations formed via E1 transitions from the 1${\mathit{p}}_{1/2}$ subshell; comparison with other reactions (photoneutron and radiative capture) provides information on the microscopic structure of E1 states. The peak observed near threshold at 15.1 MeV is remarkably strong; we infer that it originates from photoexcitation of a few narrow T=3/2 states and that M1 transitions contribute to the measured strength. The total absorption cross section is approximated by summing the (\ensuremath{\gamma},p) cross section and the previously published photoneutron cross section; comparison with particle-hole shell-model calculations shows that the main cross-section features, including isospin distribution, are well predicted. Evidence is found for isospin splitting in $^{17}\mathrm{O}$. Systematics of the integrated cross sections for the carbon, nitrogen, and oxygen isotopes are delineated.