Transition curves in lead of the photons responsible for various nuclear reactions have been obtained using 322-Mev bremsstrahlung from the Berkeley synchrotron. The area under these curves or "track length" determines an effective photon energy. The integrated cross sections of the reactions ${\mathrm{C}}^{12}(\gamma ,n){\mathrm{C}}^{11}$ and ${\mathrm{Cu}}^{63}(\gamma ,n){\mathrm{Cu}}^{62}$ are found to be 0.090 and 0.76 Mev-barn, respectively. Relative yields of various reactions from Zn have been measured, and relative cross sections have been calculated from these and the effective photon energies. A lower limit of 1.4 Mev-barns is obtained for the total cross section for photon absorption by the Zn nucleus, and 30 Mev is found for a lower limit to the average photon energy absorbed by the same nucleus. These results are discussed with respect to the proposed theories for nuclear photon absorption.

• Received 30 October 1950

DOI:https://doi.org/10.1103/PhysRev.81.973