Radiative electron capture and the photoelectric effect at high energies

In high-energy atomic collisions between bare high-Z projectiles and low-Z target atoms, an electron may be captured radiatively into the projectile [radiative electron capture (REC)]. The photon angular distributions can be very well represented by radiative recombination (RR) of the projectile with free electrons. This process is the inverse of the photoelectric effect. In this paper, we present exact differential RR cross sections for Au⁷⁹⁺ and U⁹²⁺ at projectile energies of 0.2, 0.5, 1, 2, 5, and 10 GeV/u. We also show the differential cross sections for the photoelectric effect at x-ray energies corresponding to the former projectile energies. It is seen that because of the dramatic forward-peaking of the cross sections at high energies, measurements of REC from low-Z targets are the most practical way to study the photoelectric effect. In one particular example, 10.8 GeV/u Au⁷⁹⁺ on Au targets, we show that the RR cross section multiplied with the number of target electrons is very close to the REC cross section calculated within the impulse approximation. © 1996 The American Physical Society.