Absorption of 5.3-Mev, 10.3-Mev, and 17.6-Mev Gamma-Rays

Gamma-ray absorption cross sections have been measured for Cu, Sn, Pb, and U at each of the gamma-ray energies, 5.3-Mev, 10.3-Mev, and 17.6-Mev. Betatron radiation collimated to give "good" geometry is used for the transmission measurements. The energy sensitive detector used is a 180\ifmmode^\circ\else\textdegree\fi{} homogeneous magnetic field pair spectrometer with crystal-and-photomultiplier detectors. The measured cross sections in units of barns per atom are at 5.3 Mev, 3.244 (Cu), 7.05 (Sn), 15.07 (Pb), and 18.43 (U); at 10.3 Mev, 3.285 (Cu), 7.95 (Sn), 17.83 (Pb), and 21.73 (U); and at 17.6 Mev, 3.688 (Cu), 9.22 (Sn), 20.47 (Pb), and 25.32 (U). The statistical errors range from 0.5 percent to 1.5 percent.The pair cross sections are inferred by subtracting the best-known values of the cross sections for the Compton, nuclear, and photoelectric effects from the total cross section. The ratio of this pair cross section to the value calculated from the Bethe-Heitler theory is given as a function of energy. The present results give the transition of this ratio which is greater than unity for low energy gamma-rays to a value less than unity at extremely high gamma-ray energies.