Positron apparatus

Because the positron moderator and the Ps converter require well characterized metal surfaces, the entire positron apparatus is in an ultra high vacuum chamber ( 2x10 torr). 

Positron Imaging. Creating images of distributions of positron emitters requires a somewhat different type of apparatus. Positron cameras use many of the same technologies as do cameras for other isotopes, but there is a broader array of methods and physical arrangements. AH of these systems take advantage of the physical characteristics of positrons. 

Fig. 1.9. Apparatus for the production of a magnetically confined positron beam in operation at University College London.

Another system for measuring total cross sections, which has also been used for both positrons and electrons, is that developed by the Detroit group (see e.g. Stein, Kauppila and Roellig, 1974). Their apparatus and method embody many features not present in the widely used TOF technique. In addition, this group has made the most comprehensive survey of targets using both projectiles. 

Fig. 2.4. The apparatus used by the Detroit group for the measurement of total cross sections for the scattering of positrons and electrons from noble and molecular gases. Linking the two parts of the apparatus is the curved positron energy filter.

Fig. 2.5. Schematic illustration of the apparatus used by the Bielefeld group to measure total scattering cross sections. Reprinted from Journal of Physics B 13, Sinapius, Raith and Wilson, Low-energy positrons scattering from noble gas atoms, 4079-4090, copyright 1980, with permission from IOP Publishing.

The final apparatus we describe briefly here is that used by the Detroit group (Zhou et al., 1994a) for the first measurements of the total cross section for positron scattering by atomic hydrogen, and again it... 

The apparatus and technique used by Stein et al. (1985), Kwan et al. (1991), Parikh et al. (1993) and Kauppila et al. (1994) has already been discussed in section 2.3. Kwan et al. (1991) stated that their data for positron and electron scattering by potassium superseded those obtained by Stein et al. (1985), and consequently the earlier results are not described here. 

The atomic beam was formed by a multichannel capillary array, placed perpendicular to the positron beam, with a 2.5 mm2 effusing area and a length-to-diameter ratio of 25 1. The head pressure behind the array was kept at 9 torr (ss 103 Pa) in the initial measurements. An annealed tungsten moderator was used to provide a beam of more than 105 positrons per second at 200 eV. A much more intense beam of electrons could also be obtained by reversing the electrostatic potentials on the various elements which made up the transport system. Channel electron multipliers (CEM1 and CEM2 respectively) were used to monitor the incident and scattered beams. In later versions of the apparatus, a third... 

Fig. 3.15. Schematic illustration of the crossed-beam apparatus developed by Hyder et al. (1986) for the measurement of positron elastic differential scattering cross sections. Reprinted from Physical Review Letters 57, Hyder et al, Positron differential elastic scattering cross section measurements for argon, 2252-2255, copyright 1986 by the American Physical Society.

Fig. 4.12. Schematic of the apparatus developed by Fornari, Diana and Coleman (1983) for studies of positronium formation. Reprinted from Physical Review Letters 51, Fornari et al., Positronium formation in collisions of positrons with He, Ar and H2, 2276-2279, copyright by the American Physical Society.

The results obtained by the Texas group (Fornari, Diana and Coleman, 1983 Diana et al., 1986b) are discussed below along with those obtained by the Bielefeld group, whose apparatus is shown schematically in Figure 4.13. In this case the manner of detecting positronium depended upon method (iii) outlined above, i.e. detection of the ions formed without an accompanying free positron in the final state. 

Fig. 4.15. Illustration of the apparatus developed by Zhou et al. (1994b) for studies of positronium formation in positron-alkali metal collisions.

Fig. 4.16. Layout of the apparatus of Moxom, Laricchia and Charlton (1995a) for studies of positronium formation and ionization (not to scale). Reprinted from Applied Surface Science 85, Moxom et al, A gated positron beam incorporating a scattering cell and novel ion extractor, 118-123, copyright 1995, with permission from Elsevier Science.

Fig. 4.24. Apparatus of Laricchia et al. (1985) for the detection of excited state positronium formed in positron-gas collisions.

In addition to the work on atoms, the study of Katayama, Sueoka and Mori (1987) produced cross sections attributable to excitation of the O2 molecule by positron impact. The TOF apparatus and the method of analysis were similar to those described above. However, for O2 a secondary peak was found which, when allowances were made for the energy width of the beam and for positrons which had been scattered through large angles, was concentrated in an energy-loss interval AE ... 

The final system described here is that of Jones et al. (1993), which was developed for positron-hydrogen ionization studies and is illustrated in Figure 5.10. Similar apparatus has been used by Ashley, Moxom and Laricchia (1996) (see subsection 5.4.5 below), Kara et al. (1997a, b) and Kara (1999). Several of the basic features, including the pulsed ion extraction and ion transport systems, are similar to those developed by Knudsen et al. (1990). E x B plates were introduced by Jones et al. (1993) to remove the slow positrons from the fast / + particles, secondary electrons and gamma-ray flux emanating from the source. 

Fig. 5.10. Apparatus, not to scale, of Jones et al. (1993) for positron impact ionization of atomic hydrogen. Squares with crosses, Helmholtz coil shaded rectangles, stainless steel shielding black rectangles in beam line, lead shielding.

Lennard et al. (1988) studied L-shell ionization ratios for gold, where it was hoped that the Coulomb effect would be so reduced as to permit the observation of certain basic differences between positron-electron scattering (Bhabha, 1936) and electron-electron scattering (Mpller, 1932), for large energy transfers. Later Schneider, Tobehn and Hippier (1991, 1992), using the apparatus described above, measured the same quantities. As... 

Thermalization in N2 gas has also been studied using the positron-trap apparatus developed by Surko and coworkers and described in subsection 6.2.2. By storing positrons in the trap at a known pressure for various lengths of time before ejecting them and measuring their mean... 

Fig. 6.17. Schematic illustration of the positron-drift apparatus used by Paul and coworkers.

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