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.

NUCLEAR REACTIONS: ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$($\gamma$,$n$), $E_{\gamma }=7.6-41.8\mathrm{}$ MeV; measured $4\pi$ neutron yield for monoenergetic photons; $\sigma (E_{\gamma },1n)$, $\sigma (E_{\gamma },2n)$, integrated cross sections, isospin splitting of the giant resonance.

• Received 24 July 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1684