Positronium is an analogue of the hydrogen atom where the proton is replaced by a positron, the anti-particle of the electron. Positronium can be formed during the collision of a positron with an atomic system when one of the bound electrons is captured by the positron. Since the target is left in an ionized state, this is a form of ionization, although with a distinct signature from direct ionization where both the incident positron and the ejected electron are unbound. The electron-positron pair has a binding energy of 6.8 eV in the ground state and therefore the threshold for this reaction is I-6.8 where I is the ionization potential of the atom (in eV). For most atoms, positronium formation has the lowest threshold of all inelastic channels.
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