Photoneutron cross sections for 13C

The photoneutron cross sections for 13C 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 13C. 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