Positron-atom collision processes

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

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). 

From a material-centered point of view, the thermalization of positrons is a process very similar to that of electrons. The initial stage of inelastic collisions and the electromagnetic slowing down process form the usual radiation products. In fluids and molecular or atomic solids, these are excited and ionized molecules, fi-ee radicals, and electrons. In metals, however, plasmon and phonon excitation also play significant roles in the thermalization of positrons. 

The process, which leads to the liberation of a target electron, following the impact of a charged particle on an atom or a molecule, is one of the most important and fundamental phenomena studied in the field of atomic collisions. The interest in the ionization process goes back many decades, and it has been studied intensively since then. The largest volume of experimental data has been obtained for impact of electrons and protons, but a great deal of information has also been collected for impact of heavier ions. Recently, the electron data have been surveyed by for example Tawara et al. [4.1] and the proton data by Rudd et al. [4.2]. Since then, it has become possible to measure ionization cross sections for antiproton and positron impact. Some of the experimental measurements of positron-impact ionization were reviewed by Charlton and Laricchia [2.12]. From comparisons between data obtained with equivelocity p", p , e", and e , a substantial amount of new information on the ionization process has become available. In this section, we will review the new results obtained for the single-ionization cross section,... 

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. 

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 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]. 

The APH method is applicable to any three particle rearrangement collision for which the potential is known. A good example of this is a problem from atomic physics, a positron scattering with a hydrogen atom. Positrons are antipartides, positively charged electrons. Besides the usual elastic and inelastic scattering processes a rearrangement process also... 

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