Photoneutron cross sections forN15

Photoneutron cross sections involving the emission of one and two neutrons from ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ have been measured over the excitation energy interval from threshold (10.8 MeV) to 38 MeV using monoenergetic photons from the annihilation in flight of fast positrons. A very broad giant dipole resonance extending from about 16 to 30 MeV was observed, with a maximum ($\gamma , n_{\mathrm{tot}}$) cross section of about 11 mb at 23.5 MeV. The magnitude of the measured average photoneutron energies shows that most of the strength below 15 MeV decays to the ground state of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, whereas most of the strength in the giant resonance decays to excited states. Comparison with particle-capture cross-section data indicates that multiparticle-multihole interference effects probably account for some of the pronounced intermediate structure observed above 16 MeV in the ($\gamma , n$) cross section. Comparison with a recent shell-model calculation favors the use of a Tabakin potential over a $\delta$-function force with a Soper exchange mixture, in marked contrast with recent corresponding results for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$. Features of recently measured photoreaction cross sections for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$, and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ nuclei are compared as well.