Positronium. Muonium, Mesonic Atoms

In 1953 it was observed that a positron and an electron combine to form a pseudo-atom, somewhat similar to the hydrogen atom. In the hydrogen atom the electron can be described as moving around an essentially stationary nucleus, the proton. In the pseudo-atom formed by a positron and an electron, which has been given the name positronium, the two particles have the same mass, so that they carry out similar motions about their center of mass, the point midway between the two. 

It was found by the American physicist Martin Deutsch that there are two kinds of positronium. The kind in which the spin of the positron is antiparallel to that of the electron is called parapositronium, and that in which the two spins are parallel is called orthopositronium. Parapositronium decomposes with destruction of the positron and the electron and production of two photons, its half-life being 0.9 x I0 s. Orthopositronium decomposes with production of three photons, and half-life 1.0 x 10 s. The existence of positronium was detected by the observation of a delay between its production (by decomposition of sodium 22, which emits positrons) and its annihilation. The time of delay was found to correspond to the sum of two first-order reactions, with the values of the half-life given above. 

Muoniiim, a pseudo-atom involving a negative muon moving about a proton, has also been observed. Other mesonic atoms, having structures similar to ordinary atoms but with a muon or other meson replacing one of the electrons, have also been observed. For example, muonic neon is a neon atom with a negative muon in place of an electron. 

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