Pair Production by Electrons

Pair production by electrons was observed by measuring the positrons emitted from thin copper targets irradiated directly by the synchrotron electron beam (230±{}20 Mev energy). The positrons were produced in either of two ways: either (1) directly by the electron beam, or (2) through a double process in which bremsstrahlung was first produced which then materialized in pairs in the same target. By measuring the positron rate as a function of target thickness a direct comparison of the two modes of production was obtained. The thickness of copper from which the positron yield was the same for both processes was 0.0043±{}0.001 inch (0.0073±{}0.0017 radiation lengths). The positrons had about 0.80 of the energy of the incident electron beam.The results of this experiment is shown to be in agreement with the Weizsaker-Williams approximation. In this approximation the formula for the number of virtual quanta associated with an incident electron contains a constant of the order of unity. Using the results of this experiment and the Bethe-Heitler formulas for the double process this constant was calculated to be 1.6±{}0.2.