Measurement of Pair-Production Cross Section near Threshold

A determination has been made of the total pair production cross section near threshold in germanium ($Z=32\mathrm{}$) with use of lithium-drifted germanium gamma-ray detectors. The experimental results show a systematic trend when compared with the theoretical cross sections of Bethe and Heitler. The ratio of the experimental cross section to the Bethe-Heitler theoretical value increases as the photon energy approaches threshold; however, the increase is more rapid than expected from the calculations of Jaeger and Hulme.