Formation ofn=2 positronium from untreated metal surfaces

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Abstract
The formation of n=2 positronium is investigated using a positron beam incident on a variety of untreated metal foils and crystals. The maximum measured formation fraction is much higher than any previously reported for such targets. Fully 2% of the incident positrons are observed to form n=2 positronium at low input energy with little dependence upon the metal substrate of the formation surface. The n=2 positronium spectra exhibit lifetime components characteristic of two distinct populations with different kinetic energies. The dependence of each of these components on the incident positron energy is studied and interpreted as being consistent with existing theories of positron interactions with metals. The production of n=2 positronium using positrons transmitted through metal meshes and thin foils is also observed and discussed briefly.