Inelastic scattering formation, ionization

The foundations of the Wannier treatment for positron impact are not as solid as for electron impact. The reason is that in the latter case, there are only two open channels near threshold elastic and inelastic scattering and ionization, whereas for positron impact, there is an additional, very strong channel, positronium formation. Furthermore, in the e impact case, the two light particles are much closer to one another in the final state, this casting doubt on the non-overlapping, classical orbits of the model. 

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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