Positronium interaction

The leading term in the interaction between positronium and a charged particle is the usual 1/R4 polarization potential, where R is the coordinate of the centre of mass of the positronium relative to the other particle. If, however, positronium interacts with a neutral atom or another positronium, the interaction is dominated by the van der Waals potential arising from the induced dipole-dipole and quadrupole-dipole terms, which has the long-range form... 

In this chapter we consider the physics of the positronium atom and what is known, both theoretically and experimentally, of its interactions with other atomic and molecular species. The basic properties of positronium have been briefly mentioned in subsection 1.2.2 and will not be repeated here. Similarly, positronium production in the collisions of positrons with gases, and within and at the surface of solids, has been reviewed in section 1.5 and in Chapter 4. Some of the experimental methods, e.g. lifetime spectroscopy and angular correlation studies of the annihilation radiation, which are used to derive information on positronium interactions, have also been described previously. These will be of most relevance to the discussion in sections 7.3-7.5 on annihilation, slowing down and bound states. Techniques for the production of beams of positronium atoms were introduced in section 1.5. We describe here in more detail the method which has allowed measurements of positronium scattering cross sections to be made over a range of kinetic energies, typically from a few eV up to 100-200 eV, and the first such studies are summarized in section 7.6. 

Most studies of positronium interactions have depended upon monitoring the annihilation process after positronium has been formed by f3+ particles stopping in relatively dense media (e.g. sections 7.3 and 7.4). Fortunately, as introduced in subsection 1.5.3 and described in more detail below, the availability of positron beams has made it possible to create variable energy positronium atoms under controlled conditions in vacuum. In this section we discuss the development of such beams, in which the positronium atom is considered as a swift atomic projectile. 

The ultrarelativistic A2e allows one to observe the new effects concerning the positronium interactions with matter and the final value of the formation time of an atom from an e+e -pair. 

Our knowledge of positron and positronium interactions with atomic hydrogen and the guasi-one-electron atoms is summarized in Table 2.3. For hydrogen, the first calculation that established the chemical stability of PsH was done in 1951 [3l]. Since then this system has been much studied, and its properties are now well understood. Quite recently, this molecule was found to bind a second positron as well [30]. 

Another important type of positronium interactions is ortho-para conversion. This can take place if the medium includes paramagnetic species containing unpaired electrons. On collision with such a species, the direction of one of the parallel spins in the orrfco-positronium is reversed, and at the same time the direction of the spin of the unpaired electron in the colliding molecule also reverses. In accordance with its lifetime, the resulting para-positroniunr is then rapidly annihilated. Hence, this effect also leads to a decrease in the lifetime of the positronium. 

PALS is based on the injection of positrons into investigated sample and measurement of their lifetimes before annihilation with the electrons in the sample. After entering the sample, positron thermalizes in very short time, approx. 10"12 s, and in process of diffusion it can either directly annihilate with an electron in the sample or form positronium (para-positronium, p-Ps or orto-positronium, o-Ps, with vacuum lifetimes of 125 ps and 142 ns, respectively) if available space permits. In the porous materials, such as zeolites or their gel precursors, ort/zo-positronium can be localized in the pore and have interactions with the electrons on the pore surface leading to annihilation in two gamma rays in pick-off process, with the lifetime which depends on the pore size. In the simple quantum mechanical model of spherical holes, developed by Tao and Eldrup [18,19], these pick-off lifetimes, up to approx. 10 ns, can be connected with the hole size by the relation ... 

Perhaps of more general applicability for the study of the properties of positronium is its production by the desorption of surface-trapped positrons and by the interaction of positrons with powder samples. According to equation (1.15) it is energetically feasible for positrons which have diffused to, and become trapped at, the surface of a metal to be thermally desorbed as positronium. The probability that this will occur can be deduced (Lynn, 1980 Mills, 1979) from an Arrhenius plot of the positronium fraction versus the sample temperature, which can approach unity at sufficiently high temperatures. The fraction of thermally desorbed positronium has been found to vary as... 

The physical basis of the interactions of positrons and positronium with atoms and molecules... 

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... 

Although, as described in section 1.5, positronium can be formed when positrons interact with many different media, in this chapter we are mainly concerned with the reaction... 

Before the advent of low energy beams, the only means of investigating positron interactions with atoms and molecules was to study their annihilation. Information could thereby be obtained directly on the annihilation cross section but only indirectly for other processes such as elastic scattering. In this chapter we consider the annihilation of so-called free positrons in gases. The fate of positrons which have formed positronium prior to annihilation is treated in Chapter 7. 

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