Inelastic scattering cross sections, positronium

Fig. 2.1. Schematic illustration of the behaviour of the positron-helium and electron-helium total scattering cross sections. Notable are the large differences in magnitude of the cross sections at low energies, their merging at approximately 200 eV and the onset of inelastic processes at the positronium formation threshold EPS in the positron curve.

As the positron energy is raised above the positronium formation threshold, EPs, the total cross section undergoes a conspicuous increase. Subsequent experimentation (see Chapter 4) has confirmed that much of this increase can be attributed to positronium formation via the reaction (1.12). Significant contributions also arise from target excitation and, more importantly, ionization above the respective thresholds (see Chapter 5). In marked contrast to the structure in aT(e+) associated with the opening of inelastic channels, the electron total cross section has a much smoother energy dependence, which can be attributed to the dominance of the elastic scattering cross section for this projectile. 

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