Photonuclear Cross Sections in Aluminum and Magnesium

Cross-section curves have been measured as functions of photon energy for the reactions ${\mathrm{Al}}^{27}(\gamma ,n){\mathrm{Al}}^{26}$, ${\mathrm{Mg}}^{24}(\gamma ,n){\mathrm{Mg}}^{23}$, ${\mathrm{Mg}}^{25}(\gamma ,p){\mathrm{Na}}^{24}$, and ${\mathrm{Mg}}^{26}(\gamma ,p){\mathrm{Na}}^{25}$. These curves exhibit the peaked shape characteristic of photonuclear reactions, the maximum cross sections being 8.1, 9.8, 14.8, and 19.3 millibarns respectively. The ($\gamma ,p$) peak positions occur about 2 Mev higher than those of the ($\gamma ,n$) reactions, and their cross sections as indicated above are considerably larger. It is shown that these peaked shapes result from a peaking of the photonuclear absorption cross sections. The larger values of the ($\gamma ,p$) peak positions and cross sections may be explained as resulting from a direct interaction between high energy photons and nuclear protons.