The photoneutron cross sections for carbon and aluminum have been measured up to 37 MeV, using nearly monochromatic photons obtained from the annihilation of positrons in flight. For carbon, resonances were observed at 22.0, 23.3, 25.5, 27.1, 28.3, 30.5, and 35.2 MeV, having approximate dipole strengths of 19, 33, 14, 6, 6, 11, and 11%, respectively. The integrated cross section is 46±4 MeV mb. Peaks in the aluminum cross section were observed at 13.8, 14.5, 15.4, 15.7, 16.4, 17.0, 17.7, 18.2, 20.0, 21.5, 22.0, 23.1, and 27.2 MeV. The integral for the cross section $\sigma (\gamma ,n)+\sigma (\gamma ,2n)+\sigma (\gamma ,np)$ is 166±12 MeV mb, while that for $\sigma (\gamma ,2n)$ is 6.3 MeV mb. Measurements on the neutron energies indicate that ${\mathrm{C}}^{12}$ emits neutrons mostly in ground-state transitions, and ${\mathrm{Al}}^{27}$ mostly by direct interactions.

• Received 4 November 1965

DOI:https://doi.org/10.1103/PhysRev.143.790