Isospin effects in the photodisintegration of light nuclei

Using the corpus of data on the (γ,p), (γ,n), (γ,2n), and (γ,${\mathit{n}}_0$) cross sections of nine light nuclei ${(}^{13}$C, ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$), the ${\mathit{T}}_{>}$ and ${\mathit{T}}_{<}$ isospin components of the giant dipole resonance have been separated. The relative strengths of these components have been extracted, together with the energy differences between the centroids of the components. The ratio of the ${\mathit{T}}_{>}$ energy-weighted integrated cross section to the total cross section is somewhat better represented by the simple geometric factor 1/(${\mathit{T}}_0$+1), where ${\mathit{T}}_0$ is the isospin of the ground state of the excited nucleus, than by a more complete expression which takes into account dynamical effects. If the energy difference between the centroids is represented by ${\mathit{U}}^{\mathrm{*}}$(${\mathit{T}}_0$+1)/A, the average value of ${\mathit{U}}^{\mathrm{*}}$ for the three p shell nuclei is found to be 57 MeV, while that for the six s-d shell nuclei is 93 MeV.