Positronium formation

Mills and Pfeiffer, 1979 Lynn, 1979 Poulsen et al, 1991 see also the discussion in subsection 1.5.3 below) that heating the metal surface can thermally activate positronium formation, the activation energy Ea being given by... 

The final method which is proving of value is the gas-cell technique, in which use is made of the natural peaking of the positronium formation cross section in the direction of the incident positrons (see Chapter 4 for further discussion of this feature) for the reaction described by equation (1.12). This method was pioneered independently by Brown (1985, 1986), and by Laricchia and Charlton and coworkers (Laricchia et al., 1986, 1987b, 1988), who have shown that a tunable positronium beam with narrow energy width can be produced by the capture reaction in gases. Further discussion of this technique, and some applications in atomic physics, can be found in section 7.6. 

The total positron scattering cross section, erT, is the sum of the partial cross sections for all the scattering channels available to the projectile, which may include elastic scattering, positronium formation, excitation, ionization and positron-electron annihilation. Elastic scattering and annihilation are always possible, but the cross section for the latter process is typically 10-2O-10-22 cm2, so that its contribution to erT is negligible except in the limit of zero positron energy. All these processes are discussed in greater detail in Chapters 3-6. 

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. 

Fig. 2.14. Compendium of total cross section data for positron-noble gas and electron-noble gas scattering. The arrows refer to thresholds for (in order of increasing energy) positronium formation (positrons only), excitation and ionization. (From Kauppila and Stein, 1982.)...

S-5P excitation — —, 5S-4D excitation uneven solid curve, at bottom of figure, sum of 5S-6S and 5S-6P excitation cross sections positronium formation. 

Campeanu et al. (1987) also discussed the behaviour of the ionization cross sections for positrons and electrons near to the ionization threshold, but our treatment of this topic is deferred until subsection 5.4.5. Furthermore, in subtracting first excitation threshold of the helium atom. Their derived cross section appeared to contain a cusp or threshold anomaly around EPs, but more recent experimentation and theoretical analysis has cast some doubt on the existence of a feature of this size in helium. Further discussion of these interesting phenomena is given in Chapters 3 and 4. 

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