Positron collision processes

Most positron collision processes can be treated with high precision by considering only the Coulomb interactions with no absorption potentials taken into account. Then, the same kind of QBSs as explained in Section 1.2 for electronic systems are possible in positron collisions too. 

Recent progress in the study of positron scattering and positron annihilation processes is reviewed in Refs. [11,170-172]. Experimentally, the positron sources from radioisotopes and from electron accelerators are quite weak and have a broad spectrum of energies as compared with electron sources, making precise measurements of positron collision processes extremely difficult. Such measurements, however, have become possible recently due to the... 

Studies of positron collisions with atoms and molecules are of interest not only for their own sake but also because comparisons with the results obtained using other projectiles, such as electrons, protons and antiprotons, provide information about the effects on the scattering process of different masses and charges. 

The threshold for positronium formation in collisions of positrons with atoms and molecules is an example of a general class of thresholds in collision processes where there is no residual long-range Coulomb interaction between the constituent subsystems in either the initial or final states. Since the original work of Wigner (1948), there has been much discussion of the effect of the opening of a new channel on those already... 

Fig. 4.6. The two double-binary collision processes resulting in positronium formation following positron impact at high energies. The positron collides with an atomic electron on the left the electron then scatters off the residual ion into the same direction as the positron, whilst on the right the process is shown in which the positron scatters off the residual ion.

The HSCC equations have been solved for various Coulomb three-body processes, such as photoionization and photodetachment of two-electron systems and positronium negative ions [51, 105-111], electron or positron collisions [52, 112-115], ion-atom collisions [116-119], and muon-involving collision systems [103, 114, 120-125]. Figures 4.6, 4.7, 4.8, 4.9, and 4.10 are all due to HSCC calculations. Figure 4.12 illustrates the good agreement between the results of HSCC calculations [51] and the high-resolution photoionization experiment on helium [126]. See Ref. [127] for further detailed account of the comparison between the theory and experiment on QBSs of helium up to the threshold of He+(n = 9). 

Improvements in current, established technologies and the introduction of new ways to test materials, nondestmctively are expected to continue apac. One promising method is positron annihilation. The positron is the antiparticle of the electron thus apositron/electron pair is unstable and will annihilate. In this process, two gamma rays at approximately 180 to one another are emitted from the center of the mass of the pair. A very slight departure from 180° is directly proportional to the transverse component of the momentum, of the pair. The momenta of the electrons involved in such collisions can be calculated from the geometry and intensity of the gamma rays. The dynamics of the clcctron/positron system underlie the use of the technique for the study of defects in materials,... 

Additional aspects of positron annihilation, with particular emphasis on the processes of relevance to atomic collisions at low energies, are described in Chapter 6. 

Once reliable data for electron capture by positrons became available it was natural to compare the behaviour of the cross sections for this process with those for the analogous capture process in heavy positive particle impact, in particular, the cross section for the formation of atomic hydrogen in collisions of protons with various atoms and molecules, reaction (4.3). It is also pertinent to note that comparisons between the behaviour of protons and positrons are usually made at equal projectile speeds v rather than at equal energies. 

In this chapter we consider inelastic collisions of positrons with an atomic or molecular target X which result in electronic excitation or ionization (without positronium formation) of the target system. These processes can be summarized as... 

The first reported study of the behaviour of double differential cross sections for positron impact ionization was that of Moxom et al. (1992) these workers conducted a search for electron capture to the continuum (ECC) in positron-argon collisions. In this experiment electrons ejected over a restricted angular range around 0° were energy-analysed to search for evidence of a cusp similar to that found in heavy-particle collisions (e.g. Rodbro and Andersen, 1979 Briggs, 1989, and references therein), which would be the signature of the ECC process. 

Use of the charge-exchange mechanism, reaction (8.13), to produce antihydrogen was first proposed by Deutch et al. (1986), and subsequently it was shown that the cross section for this process could be obtained by applying the charge conjugation and time reversal operators to the process of positronium formation in positron-hydrogen collisions (Humberston et al., 1987, and see section 4.2). Under time reversal, the positronium formation process equation (4.5) becomes... 

Schultz, D.R. and Olson, R.E. (1988). Single-electron-removal processes in collisions of positrons and protons with helium at intermediate velocities. Phys. Rev. A 38 1866-1876. 

The purpose of the detector surrounding the antihydrogen trap is to discriminate between signals due to the annihilation of antihydrogen and those due to the trapped clouds of antiprotons and positrons. For this, one needs to provide temporal and spatial coincident detection of the annihilation of an antiproton and a positron. It should also allow reconstruction of the annihilation vertex with sufficient resolution to discriminate between annihilations on the wall of the charged particle traps and those resulting from possible collisions with residual gas atoms. In addition it must have a sufficiently high rate capability to allow the study of the time evolution of the recombination process. 

Initially the focus will be upon producing cold antihydrogen atoms. The rate for spontaneous radiative recombination of antiprotons and positrons is rather low because the emission of photons is a slow process on the time scale of collisions. Laser-stimulated recombination can increase the antihydrogen formation rate by orders of magnitude [14]. Other avenues towards antihydrogen production at low energies are pulsed-field recombination [15] or collisions of antiprotons with positronium [16]. 

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