Positronium formation fraction

Jacobsen (1984) gave a full discussion of the effect of thermalization and concluded that positronium formation by the spur mechanism is unlikely in atomic gases, since R > rc irrespective of density. This is not the case for molecular gases, where R can be of a similar order of magnitude to rc at high densities. Thus, the positronium formation fraction in molecular... 

In this section we review the results from positron annihilation experiments, predominantly those performed using the lifetime and positron trap techniques described in section 6.2. Comparisons are made with theory where possible. The discussion includes positron thermalization phenomena and equilibrium annihilation rates, and the associated values of (Zeff), over a wide range of gas densities and temperatures. Some studies of positron behaviour in gases under the influence of applied electric fields are also summarized, though the extraction of drift parameters (e.g. mobilities) is treated separately in section 6.4. Positronium formation fractions in dense media were described in section 4.8. 

The work of Jacobsen (1984, 1986) and Mogensen (1982), described in subsection 4.8.1 above, pointed to the potential importance of positronium formation as a consequence of spur processes in dense molecular species. Table 4.1 drew attention to the fact that the positronium fractions for many molecular gases have been found to be both density and temperature dependent. We will not attempt a detailed compilation of these data here, but examples of the density and temperature variations observed are shown in Figure 4.31 for SF6 and CO2 gases. The lines are fits to equation (4.41), and the reader is referred to the work of Jacobsen (1986) for a full discussion of the fitting procedures and assumptions. The fact that a reasonable fit to the data can be produced is strong supporting evidence for positronium formation in spurs. 

In summary, it is clear that the o-Ps lifetime determined via the PALS technique provides accurate information on the apparent mean size of the nanoholes, which comprise the free volume in amorphous polymers. It also seems well established (see Chapter 11) that provided that the noise level in the PALS spectrum is sufficiently reduced, the distribution of o-Ps lifetimes can be obtained, which generates information regarding nanohole-size distribution. Concerns have been raised about the utility of the o-Ps intensity, I3, to characterize the number density of nanoholes and hence the fractional free volume via Eq. (12.2), because the value of I3 can be influenced significantly by the presence of species that inhibit or enhance positronium formation. We feel that we can utilize I3 values to evaluate fl actional free volumes via Eq. (12.2), provided either that the sample is rejuvenated by heating above Tg prior to measurement, and/or experiment indicates that the value of I3 remains constant within experimental error, during the time of exposure to the positron source. 

The parameter of a positron experiment that most obviously carries information on the chemistry of the substance is the positronium formation intensity. This value informs about the relative fraction of positrons forming Ps. For example, by completing the list of reactions in O Eqs. (27.1) and (27.2) with equations containing solute molecules, a system is gained in which positronium formation sensitively depends on the solute concentration. Moreover, the rate constants connected with this equation system depend on the changes of the electron structure of the molecules very sensitively. Thus, also the Ps formation intensity reflects these changes. 

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