Projectile Energy and Atomic Number Dependence of Electron Capture from Pair Production in Relativistic Heavy Ion Collisions

We report the first measurement of the energy dependence of electron capture from electron-positron pair production in relativistic heavy ion collisions. For a La⁵⁷⁺ beam incident on Au, Ag, and Cu targets at energies of 0.405, 0.956, and 1.3 GeV/u we find that the cross sections for capture from pair production and the free pair production process increase with increasing collision energy at similar rates. Combining with uranium data reported previously gives a projectile atomic number dependence for 0.956 GeV/u ions on a Au target of Zₚ^6.54±0.65 for capture from pair production and Zₚ^1.53±0.80 for the free pair production process.