Absolute pair-production cross-section measurements in targets of Z = 26, 29, 50, 82, and 92 with 1.119-MeV photons

Absolute cross sections for the production of electron-positron pairs by 1.119-MeV photons have been measured with targets of iron, copper, tin, lead, and uranium. An improved internal-source spectrometer was used which utilized two large intrinsic Ge detectors. The results are in excellent agreement with the exact numerical calculations of Tseng and Pratt for Z = 26, 29, and 50, and show a systematic departure of 13 and 19 % above the theoretical values for Z = 82 and 92, respectively. The present cross section at Z = 92 is lower than and slightly outside of the error limits from the results obtained with our older spectrometer. The present method has the definite advantage that the absolute activities of the radioactive sources need not be known. It also has the obvious advantages attributable to the high-energy resolution of the two intrinsic Ge detectors. A preliminary measurement in a lead target, using 1.0766-MeV γ rays, clearly shows that this technique can be extended to lower photon energies with resulting errors comparable to those at 1.119 MeV