Electron Triplet Production by High-Energy Photons in Hydrogen

The 323-Mev hardened bremsstrahlung beam from the Berkeley synchrotron was used to produce electron-positron pairs and triplets in a 4-in.-diam liquid hydrogen bubble chamber. It was found that the experimental triplet cross sections for detectable recoils (momentum greater than 0.27 Mev/c) and for recoils with momentum greater than $\mathrm{mc}$ rise logarithmically with photon energy to 100 Mev, then level off at approximately 2.8 mb and 1.5 mb, respectively. The total triplet cross section agrees with that of Borsellino above 20-Mev photon energy. No contribution due to exchange terms was found. The positron energy distribution agrees with that of Wheeler and Lamb. The recoil momentum distribution agrees substantially with that of Suh and Bethe. Approximately one event due to multiple pair production was expected. None was found.