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 ${\mathrm{C}}^{12}(\gamma , n){\mathrm{C}}^{11}$ 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.