Electron Trident and Pair Production at 500 MeV

The cross sections, differential in the recoil positron energy, for trident and quartet production and for bremsstrahlung followed by pair and triplet production were measured for beryllium, aluminum, and platinum targets at a nominal incident electron energy of $E_0=500\mathrm{}$ MeV and for recoil positron energies in the range above $0.80E_0$. The estimated experimental errors are about ±6% for the direct process and about ±3% for the double process at most points. The (pair+triplet) data for all three elements are in excellent agreement with theoretical predictions which contain detailed screening calculations and which take into account electron-electron bremsstrahlung and pair production in the electron field. A satisfactory fit (${\chi }^2$ probability 0.22) to the (trident+quartet) data was obtained with the no-screening calculations of Murota, Ueda, and Tanaka when the integration parameter ($\alpha$ in their notation) had a value 0.75±0.05 and when quartets were taken into account by the substitution $Z^2\to Z(Z+\eta )$, where $\eta$ is the ratio of the total cross sections for electron-electron and electron-nucleon bremsstrahlung. If the timelike photon contributions, which amount to roughly 35% of the net cross section, are left out, the integration parameter has an optimum value of 2.00±0.20. The corresponding ${\chi }^2$ probability is much less than 0.01, indicating the necessity of the timelike terms.