The total photoneutron cross sections of natural carbon and magnesium were measured from thresholds to 30 MeV. The neutron yield curves, determined in 0.5-MeV intervals using a bremsstrahlung beam, were analyzed by the Penfold-Leiss method to obtain two independent sets of cross sections of 1-MeV interval. The carbon cross section exhibits major structure at 18.8, 22.0, and 25.5 MeV, the integrated cross section up to 28 MeV being 39.2 MeV mb. The magnesium cross section indicates levels at 17.0, 19.5, 21.5, 23.5, and 25.5 MeV with the integrated cross section up to 28 MeV, 90.7 MeV mb. The magnesium cross section also shows a small structure at 14.7 MeV due to ${\mathrm{Mg}}^{25}$ and ${\mathrm{Mg}}^{26}$. The experimental results are compared with the available particle-hole-model calculations of $E1\mathrm{}$ levels.

• Received 3 August 1964

DOI:https://doi.org/10.1103/PhysRev.137.B301