Positronium experiment

They enable us to study very small numbers of atoms or molecules. Indeed, in the positronium experiments, the average number of atoms present in the apparatus is much less than one. The experiments have become possible because the small numbers of atoms have been made available in bunches. We can now also observe states in atoms or molecules with very small populations, or extremely rare isotopes produced in small numbers by accelerators or even by neutrino interactions. 

Fig. 4.14. Layout of the positron-atomic hydrogen scattering experiment developed by the Bielefeld-Brookhaven collaboration. Reprinted from Physical Review Letters 68, Sperber et al., Measurement of positronium formation in positron collisions with hydrogen atoms, 3690-3693, copyright 1992 by the American Physical Society.

Fig. 4.17. Total positronium formation cross sections for positron-helium scattering. Experiment , Overton, Mills and Coleman (1993) o, Fornari, Diana and Coleman (1983) and Diana et al. (1986b) , Fromme et al. (1986). The error...

Fig. 4.20. Positronium formation in positron-hydrogen scattering. Experiment ...

The formation of excited states of positronium, usually termed Ps, through reaction 4.2, can make significant contributions to erPs. This has already been discussed briefly in the theoretical section 4.2 and in subsection 4.4.3, where we saw that excited state positronium is thought to dominate alkali metals. Here we describe the only experiment to date which has directly detected excited state positronium formation in gases, namely that of Laricchia et al. (1985). 

Measurements of daPs/dQ for positron-argon scattering have also been made by Finch et al. (1996a, b) and Falke et al. (1995, 1997) and for positron-krypton scattering by Falke et al. (1997). The principle of the experiments, involving the detection of the positronium in coincidence with an atomic ion, is illustrated schematically in Figure 4.28. More details of the system used by Finch et al. (1996a), which has also been used to study the differential ionization cross section, can be found in section 5.6. 

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. 

Experiments on these two gases, reported by Griffith and Heyland (1978), showed that a fast component, with a density-dependent decay rate, was present in the lifetime spectra, and this was tentatively linked to the dearth of long-lived ortho-positronium. Furthermore, it was found for mixtures of krypton with helium that the maximum value of F, which was observed at a concentration of around 0.01% of krypton, was in excess of the sum of the individual F-values for the two gases when pure. 

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