Relative Pair Production Cross Sections in the Energy Region 50 to 300 Mev

A cloud-chamber survey has been made of pairs produced in Al, Ag, Au, and Th foils by an x-ray beam from the 300-Mev betatron. By comparing the pairs produced in two of the foils mounted to intercept the same x-ray beam, values of the relative pair production cross sections for Ag to Al, Au to Al, and Th to Al were found to be, respectively, (3.4±1.4), (7.9±1.5), and (11.7±1.7) percent low compared with the Bethe-Heitler theory predictions. The results are consistent with a correction factor to the theoretical pair cross section given by [$1-(1.4\pm 0.1){10}^{-5\mathrm{}}{Z}^2$]. The calculated number of Compton electrons between 50 and 200 Mev expected from the Al foil is 1201, from the Ag foil is 355, and from the Au foil is 113. The observed numbers with statistical errors indicated are, respectively, 1170±40, 354±30, and 70±18.