Photoneutron cross sections forC13

The photoneutron cross sections for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ have been measured from near threshold to over 40 MeV using monoenergetic photons from positron in-flight annihilation. Several sharp features below the giant resonance were distinguished. The results both for this "pygmy-resonance" region and for the giant resonance near 24 MeV differ markedly from previously reported measurements and provide a much better quantitative comparison with recent theoretical calculations of the photoneutron reaction in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$. Comparison of the measured total photoneutron cross section with recent data on the ground-state photoreaction and with average photoneutron energies provides evidence for the isospin splitting of the giant resonance for this nucleus.