Nuclear Photon Absorption in Carbon and Oxygen

The importance of narrow, isolated, resonances in the photonuclear absorption cross sections of C and O was investigated by means of an absorption experiment performed with 30.7-Mev bremsstrahlung. The experiment yielded values for the cross sections averaged over the ($\gamma , n$) cross sections for ${\mathrm{C}}^{12}$ and ${\mathrm{O}}^{16}$. The results for C were the same as those expected for a cross section without narrow resonances, while for O a definite resonance effect was demonstrated. The O photonuclear absorption cross section, averaged over the ${\mathrm{O}}^{16}(\gamma , n){\mathrm{O}}^{15}$ cross section, was found to be 26.7±3.5 millibarns, 16±4 millibarns higher than the value expected if narrow resonances contribute insignificantly. Estimated values for the parameters of the O resonances near 22 Mev are: average peak height, 106±14 mb; ratio of the radiative width (to ground state) to total width, (6.6±0.9)×${10}^{-3\mathrm{}}$; total width about 30 kev. The experimental results are insensitive to uncertainties in the non-nuclear x-ray attenuation cross sections for C and O.