Total Compton and Pair Production Cross Sections at 19.5 Mev

Total cross sections of 19 elements were measured at 19.5 Mev by the photonuclear detector method. The penetration of betatron bremsstrahlung (maximum energy, 20.4 Mev) through the absorbers was determined by the C¹²(γ, n)C¹¹ reaction (threshold, 18.7 Mev) in polyethylene. Simultaneously operated monitor-detector Geiger units, which canceled timing and intensity fluctuation errors, recorded the carbon-11 activity. Percent standard deviations averaged 0.6 for most elements; the estimated systematic error was 0.4 percent. The narrow energy band width permitted an accurate calculation of the correction for the proportion of counts due to secondary radiation.From the absorption data a value of the Compton cross section was determined. The integrated Klein-Nishina result fell within the 2 percent experimental error calculated by adding linearly to the statistical errors, an estimated 50 percent uncertainty in the published photonuclear cross sections employed in the determination. The cross sections for pair production found from the experiment σₑ, are related to the theoretical values σₜ, as derived by the Born approximation (Bethe-Heitler formula including screening), by ${({{σ}}ₜ{-}{{σ}}ₑ)}{{{σ}}ₜ}=(1.55±{}0.1)×{}{10}^{{-}5}{Z}²$, to a first approximation.