Total gamma absorption cross sections were measured of ${\mathrm{C}}^{12}$ from 20.0 to 21.2 Mev, and of ${\mathrm{N}}^{14}$, ${\mathrm{O}}^{16}$, and ${\mathrm{Al}}^{27}$ from 20.0 to 20.5 Mev using monochromatic gamma rays. A direct absorption technique was used, utilizing ${\mathrm{T}}^3(p, \gamma ){\mathrm{He}}^4$ photons, varied in energy by changing the energy of the incident protons. The ${\mathrm{C}}^{12}$ cross section showed structure with possible resonances at 20.15 Mev, 20.46 Mev, and 20.92 Mev, with integrated cross sections of 1.1, 1.0, and 6.6 Mev millibarns, respectively. ${\mathrm{O}}^{16}$ showed a sharply rising cross section suggesting a strong resonance above about 20.3 Mev. The cross sections of ${\mathrm{N}}^{14}$ and ${\mathrm{Al}}^{27}$ were smooth over the energy interval investigated.

• Received 21 December 1959

DOI:https://doi.org/10.1103/PhysRev.118.1256