21. The decay of positronium is easily observed by detecting the two 511 keV gamma ray photons emitted in the decay. 22. A positronium " atom " ( as physicists dub it ) consists of an electron and antielectron, orbiting each other. 23. The measurement of the splitting between the 2 3 S 1 and the 1 3 S 1 energy levels of positronium yields 24. The positronium created in such an excited state will quickly cascade down to the ground state, where annihilation will occur more quickly. 25. The classic onium is positronium , which consists of an electron and a positron bound together as a long-lived metastable state. 26. Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton antilepton particles such as positronium . 27. In 1951, Deutsch measured and confirmed the existence of positronium , a bound state of electrons and positrons whirling about each other. 28. Deutsch was philosophical about his failure to win a Nobel Prize, for which he was nominated in 1956 for his work on positronium . 29. The neutral pion has also been observed to decay into positronium with a branching fraction of the order of 10 & minus; 9. 30. Mohorovi i is often called " the father of positronium " because his most significant work is the prediction of the existence of positronium.
