Positronium
Positronium is a kind of atom (exotic atom) in which electrons and positrons are electrically bound and paired. It is written as Ps as an element symbol. In the classical mechanical atomic model, it means that electrons and positrons are turning around each other around a common center of gravity. When positrons are irradiated in a substance, electrons and positrons in the substance usually pair annihilate and emit γ rays, but positronium is formed as a metastable state in a considerable proportion in the insulator. ...