Pair Production in the Field of Orbital Electrons by a Total-Absorption Method at 319 Mev

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The attenuation of (319\ifmmode\pm\else\textpm\fi{}4)-Mev photons in liquid hydrocarbons has been measured and has been used to infer the cross section for pair production in the field of the orbital electrons (triplet production).Targets of benzene (${\mathrm{C}}_{6}$${\mathrm{H}}_{6}$) and cyclohexane (${\mathrm{C}}_{6}$${\mathrm{H}}_{12}$) were used in good geometry. A pair spectrometer with three fast counter channels was used to measure the intensity of the transmitted photon beam. The synchrotron was monitored with a modified pair spectrometer that was sensitive only to the bremsstrahlung spectrum above 300 Mev.The total absorption cross sections in hydrogen and carbon at 319 Mev have been found to be 18.0\ifmmode\pm\else\textpm\fi{}1.8 mb per hydrogen atom, and 321.1\ifmmode\pm\else\textpm\fi{}2.8 mb per carbon atom, respectively.Subtraction of the theoretical nuclear pair and electron Compton cross sections and the experimental photomeson cross sections in hydrogen from the total hydrogen absorption cross section gives 6.8\ifmmode\pm\else\textpm\fi{}1.8 mb for the experimental triplet cross section, as compared with the theoretical value of 7.68 mb. Similarly, when one subtracts the cross sections for competing processes in carbon, one gets for the triplet cross section at 319 Mev the value 46.6\ifmmode\pm\else\textpm\fi{}6.4 mb, compared with the theoretical value of 44.4 mb. The large uncertainty in the carbon triplet cross section is due to uncertainties in the cross sections for the competing processes.