The yields of the two reactions ${\mathrm{U}}^{238}(\gamma , n){\mathrm{U}}^{237}$ and ${\mathrm{U}}^{238}(\gamma , \mathrm{fission})$ produced by bremsstrahlung have been measured as a function of the maximum energy of the x-ray spectrum. The ratio of the number of fissions to the number of ${\mathrm{U}}^{237}$ atoms produced decreased from 0.390 at 8 Mev to 0.275 at 12 Mev and then increased to 0.465 at 22 Mev. These data are interpreted to show that for maximum x-ray energies in excess of 12 Mev, a fraction of the ${\mathrm{U}}^{237}$ atoms formed by the ($\gamma , n$) reaction are left with sufficient excitation energy that fission or secondary neutron emission can occur. The cross sections for the reactions as a function of gamma-ray energy have been calculated from the yield data.

• Received 24 November 1952

DOI:https://doi.org/10.1103/PhysRev.89.1042