positronium (redirected from Positonium)

positronium [‚päz·ə′trō·nē·əm]

(particle physics)

The bound state of an electron and a positron.

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Positronium

An atomic-like system consisting of an electron and positron. Just as in the hydrogen atom, the energy levels of positronium are quantized, with the deepest levels bound by about 6.8 eV. The electron and positron spins can be aligned in the same direction (singlet states) or in opposite directions (triplet states). Annihilation of the positron and electron destroys the lowest-energy singlet state (parapositronium) in about 10^-10 s, but the lowest triplet state (orthopositronium) survives longer, about 10^-7 s. This allows sufficient time for precise measurement of the energy levels of triplet states. Because of the absence of nuclei in positronium, these measurements provide an accurate test of theories of the electromagnetic force (quantum electrodynamics) without interference from the strong force. See Atomic structure and spectra, Electron, Fundamental interactions, Positron

Since the formation of positronium requires the close approach of a positron and an electron, beams of slow positrons can be used as probes of the electron density in gases, in insulating solids, or near surfaces. Since the singlet and triplet forms of positronium have very different lifetimes, and transitions between the two states can be induced by neighboring electrons, study of the decay of positronium can also provide information about electron densities on a microscopic scale. This is especially useful in the study of density fluctuations in gases near the critical point for condensation into liquids or solids. See Critical phenomena

Annihilation radiation from positronium forms a component of the gamma-ray spectrum observed by astronomers, in particular from the galactic center.