Positron Collisions

Thepositron scattering positron is the antiparticle of the electron, having the same mass but opposite charge. Positrons undergo collisions with atomic and molecular systems in much the same way as electrons do. Thus, the standard scattering theory for electrons can also be applied to positron scattering. However, there are a number of important differences from electron scattering, which we outline below. Since the positron is a distinct particle from the atomic electrons, it cannot undergo an exchange process with the bound electrons during a collision, as is possible with electrons. Thus, the nonlocal exchange terms that arise in the description of electron scattering are not present for positrons. This leads to a simplification of the scattering equations from those for electrons. However, there are scattering channels available with positron scattering that do not exist with electrons. These are dealt with in Sect. 51.1. Historically, beams of low-energy positrons were difficult to obtain and, consequently, there is considerably less experimental data available for positrons than for electrons. This was particularly true for quantities that required large incident positron fluxes, such as differential scattering cross sections and coincidence parameters. The recent development of cold trap-based positron beams with high resolution and high brightness by the San Diego group 1 has enabled significant progress in experimental positron scattering, which is discussed in Sect. 51.3. Throughout this chapter we will employ atomic units unless otherwise noted.