Absorption of 1-Bev Photons

The total cross section for the attenuation of high-energy gamma rays has been measured in various elements. The variation with atomic number, $Z$, has been observed by measuring the absorption of 1-Bev gamma rays in 12 different elements ranging from hydrogen to uranium. Additional measurements were made in copper at 400 and 700 Mev to show the energy dependence of the absorption processes. These results are then combined with those of other investigators at lower energies and show that the theory of pair production in the nuclear field correctly predicts the cross section as a function of atomic number and photon energy. The measurements in low-$Z$ elements give information on pair production in the field of the electron. The results are in closer agreement to the calculations of Wheeler and Lamb than to the estimate of Joseph and Rohrlich. In addition a short experiment was performed to measure the symmetry of the energy distribution between the electron and positron members of the pair. The results show that the energy-sharing curve is symmetrical as predicted by theory.