Positronium

Positronium is a bound state of an electron and a positron. It decays mainly by annihilation, with its lifetime and decay products determined by its spin state.

Positronium annihilation: The conversion of a positronium electron–positron pair into photons or other particles. Annihilation is the dominant decay process that ends positronium’s existence.
Positron: The positively charged antiparticle of the electron, with the same mass and spin. It is positronium’s antimatter constituent and annihilates with the electron.
Positron emission tomography: A medical imaging technique that reconstructs tracer distribution from pairs of photons produced after positron emission. Positronium formation and decay can alter photon timing and provide additional tissue information.
Carl Anderson: The American physicist who discovered the positron in cosmic-ray tracks in 1932. His discovery supplied the antiparticle needed for positronium’s existence.
Hydrogen atom: The simplest ordinary atom, consisting of a proton bound to an electron. Both systems are Coulomb-bound, but positronium has equal-mass constituents and annihilates.
Para-positronium: The positronium state with antiparallel electron and positron spins and total spin zero. Its two-photon decay gives it a shorter vacuum lifetime than ortho-positronium.
Electron: A stable elementary particle with negative electric charge and spin one-half. It supplies the ordinary-matter constituent of the bound pair.
Positron annihilation lifetime spectroscopy: A materials-analysis method that infers nanoscale defects from positron annihilation lifetimes. Positronium lifetimes can reveal free volume and pore structure in some materials.
John Wheeler: The American theoretical physicist who introduced the name positronium in 1946. He proposed treating the electron–positron bound state as a hydrogen-like system.
Muonium: An exotic atom consisting of an electron bound to an antimuon. Muonium has an antiparticle constituent but lacks positronium’s direct electron–positron annihilation.

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