Photoneutron Cross Sections for Aluminum and Silicon to 62 MeV

Photoneutron cross sections for ${\mathrm{Al}}^{27}$ and ${\mathrm{Si}}^{28}$ have been measured from threshold to 62 MeV using least-structure analysis of the bremsstrahlung activation curves. The aluminum yield was due to the reactions ${\mathrm{Al}}^{27}(\gamma ,n){\mathrm{Al}}^{26*}$ and ${\mathrm{Al}}^{27}(\gamma ,2n){\mathrm{Al}}^{25}$. Secondary maxima in the aluminum cross section, occurring at energies beyond the giant resonance, peak at 49.4±0.8 and 56.3±0.9 MeV. The cross section integrated to 62 MeV is 69±4 MeV mb. The silicon yield was due to the ${\mathrm{Si}}^{28}(\gamma ,n){\mathrm{Si}}^{27}$, ${\mathrm{Si}}^{28}(\gamma ,2n){\mathrm{Si}}^{26}$, and ${\mathrm{Si}}^{28}(\gamma ,pn){\mathrm{Al}}^{26*}$ reactions. Peak energies for secondary maxima are 40.1±0.9, 46.4±0.8, 52.5±0.8, and 58.2±1.0 MeV. The integrated cross section is 195±7 MeV mb.