Observations of positronium Lyman-α radiation

The first observation of Lyman-α radiation from positronium is described. Positronium is formed in vacuum by a slow positron beam incident on a solid target. Single photons transmitted through an ≈ 20-\AA{}-wide interference filter peaked near the 2430-\AA{} Lyman-α (2P→1S) line of positronium are observed in coincidence with the annihilation γ's of ground state positronium. The observed intensity corresponds to a production of one positronium "atom" in the excited state for every 10⁴ incident positrons.