Fission of uranium and thorium has been observed to be produced by irradiation with $\gamma$-rays. The cross section for this photo-fission produced by the $\gamma$-ray from fluorine bombarded with protons has been measured and found to be: $\{{\sigma }_{\mathrm{U}},=3.5\mathrm{}\pm 1.0\times {10}^{-27\mathrm{}}{\mathrm{cm}}^2,\}\{{\sigma }_{\mathrm{Th}},=1.7\mathrm{}\pm 0.5\times {10}^{-27\mathrm{}}{\mathrm{cm}}^2.\}$

• Received 30 August 1940

DOI:https://doi.org/10.1103/PhysRev.59.57