GiantE1resonance inO16observed with the reactionN15(p,γ0)O16

The giant electric dipole resonance based on the ground state (${\mathrm{O}}^{+}$) of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ has been studied with the reaction ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}(p,{\gamma }_0){}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ from $E_p=8.6\mathrm{} \mathrm{to} 18.0$ MeV ($E_{\gamma }=20.2\mathrm{} \mathrm{to} 29.0$ MeV). The $E1\mathrm{}$ strength in ($\gamma ,p_0$) is concentrated between $E_{\gamma }=20\mathrm{} \mathrm{and} 29$ MeV where it exhausts about 15% of the $E1\mathrm{}$ sum rule. Major peaks are displayed at 22.2 and 25.0 MeV and secondary structure at 21.0, 22.9, and in the 24 MeV region. The angular distributions indicate predominantly dipole radiation and are fairly constant over the entire range, except in the region of the secondary peaks where the coefficient of $P_2\left(cos\theta \right)$ displays striking interference effects. This behavior supports the suggestion that these structures are due to interference between more complex states and the simple particle-hole configurations of the $E1\mathrm{}$ resonance. The value of the coefficient of $P_2\left(cos\theta \right)$ can be used to restrict greatly the allowed configurations of the $p_0$ channel—the solution is predominantly either $d_{\frac{3}{2}}$ wave or $s_{\frac{1}{2}}$ wave. The presence of terms in $P_1\left(cos\theta \right)$ and $P_3\left(cos\theta \right)$ indicates significant $E2\mathrm{}$ strength which increases above the $E1\mathrm{}$ resonance. Data are also presented on the reactions ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}(p,{\gamma }_1-{\gamma }_4){}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}(p,\alpha {\gamma }_{15.1}){}_{}{}^{12}{}_{}{}^{}\mathrm{C}$.