Production of Electron Pairs by 550-Mev Electrons

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Abstract
The mean free path of high-energy electrons for direct electron pair production has been measured in Ilford G-5 nuclear emulsions. Electrons were produced by directing the 850-Mev bremsstrahlung beam from the Cornell synchrotron upon a Pb target and analyzing the negative members of the resultant electron pairs in a magnetic momentum analyzer. Electrons in the range 400 to 800 Mev were intercepted by glass-mounted emulsions 400 μ thick and, after processing, the resultant tracks were followed a distance of 1 cm in a search for electron pairs. Separation of bremsstrahlung-produced pairs from the real tridents was achieved by measuring the spatial distribution of the former about the primary tracks. This distribution, after being corrected for scanning efficiency and fitted with the theoretical distribution of Koshiba and Kaplon, indicated that for the conditions of this experiment 72% of the events classified initially as tridents were in reality bremsstrahlung pairs. The mean free path for the direct pair production process was found to be 143±47 cm for a primary electron energy of 536±94 Mev. The theoretical prediction of Racah for this energy is 148 cm and is in good agreement with this experiment.

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