Positron beam

The advent of intense positron beams opens up new possibilities for ACAR measurements small samples, surface and near-surface phenomena, and thin films are amongst the opportunities presented by the stationing the two-camera system at the end of a beam line [40]. 

Positron beams essentially separate the thermalisation of positrons implanted into a material from their eventual annihilation in another. While the field has been enlivened by a number of ingenious and exciting experiments— LEPD, PAES, etc. (reviews are to be found in 4, 35, 41, 42), in this section we shall concentrate on the basic elements of positron beam experimentation. 

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In 1946 Wheeler undertook a theoretical study of the stability of various systems of positrons and electrons, which he termed polyelectrons. He found, as expected, that positronium was bound, but that so too was its negative ion (e-e+e-). This entity, Ps-, was not observed until much later (Mills, 1981), after the development of positron beams. 

Table 1.1. Selected properties of some of the radioisotopes commonly used in the creation of low energy positron beams...

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

Fig. 1.10. The fully electrostatic high-brightness positron beam developed by the Brandeis group. The positron Soa gun is located near B. The beam is deflected at C using a cylindrical mirror analyser and focussed onto a remoderator in chamber D. The extracted beam is then focussed and remoderated at the lower left of D. The double brightness-enhanced beam is then transported into the target chamber, E. Reprinted from Nucl. Instrum. Methods B143, Charlton, Review of Positron Physics, 11-20, copyright 1998, with permission from Elsevier Science.

Almost since the earliest attempts to produce well-defined beams of low energy positrons, various types of accelerator have been used for this purpose, e.g. electron linear accelerators, microtrons and cyclotrons (see e.g. Dahm et al., 1988 Itoh et ai, 1995). Positron beams have also been developed at nuclear reactors (Lynn et ai, 1987). 

When using an electron accelerator, fast positrons are produced by pair production from bremsstrahlung gamma-rays generated as the high energy electrons from the accelerator slow down in matter, whereas with cyclotrons and reactors, very intense primary positron sources are produced directly. Slow positron beams are then produced and transported using similar techniques to those described previously in this section. 

Although there have been many technical advances in this area, no one facility has yet emerged as significantly superior to any other. A brief overview of such facilities around the world can be found in the Proceedings of the Sixth International Workshop on Slow Positron Beam... 

In what follows, we describe how positronium has been formed in gases and solids using low energy positron beams and also, in what is now regarded as the traditional way, using /3+ particles directly from radioactive sources. 

Positrons exhibit resonance phenomena in collisions with some atomic and molecular targets and, as with electrons, an infinite series of resonances is expected to be associated with each degenerate excitation threshold (Mittleman, 1966). For electrons, such thresholds can only arise with hydrogenic targets, but for positrons there are also degenerate thresholds in the excitation of positronium. Several of these resonances have been identified theoretically for a few simple target systems, but they are too narrow to be observed experimentally with the presently available energy resolution of positron beams. 

In this chapter we describe the elastic scattering of positrons by atoms and molecules over the kinetic energy range from zero to several keV, concentrating mainly on the angle-integrated cross section, crei- However, reference is also made to differential cross sections, dae /dQ, which have recently become amenable to experimental measurement using crossed gas and positron beams. 

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. 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.

More recent experiments by Overton, Mills and Coleman (1993) may have gone some way towards resolving this dispute in favour of theory. As mentioned above, this group used the Texas technique to measure erPS but paid particular attention to obtaining a well-controlled positron beam,... 

Before the advent of low energy positron beams the only method of studying positronium formation in gases was by allowing / + particles to stop in dense samples. (Typically gas densities were > 1025 m-3.) Such experiments were useful in elucidating the basic mechanisms by which positronium can be formed, and in this section we briefly review this body of work. 

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