Editing Positronium (section)

== Formation and decay in materials ==
After a radioactive atom in a material undergoes a [[beta decay|β<sup>+</sup> decay]] (positron emission), the resulting high-energy positron slows down by colliding with atoms, and eventually annihilates with one of the many electrons in the material. It may however first form positronium before the annihilation event. The understanding of this process is of some importance in [[positron emission tomography]]. Approximately:<ref name="Harpen2003">{{cite journal|last1=Harpen|first1=Michael D.|title=Positronium: Review of symmetry, conserved quantities and decay for the radiological physicist|journal=Medical Physics|volume=31|issue=1|year=2003|pages=57–61|issn=0094-2405|doi=10.1118/1.1630494|pmid=14761021}}</ref><ref name="pmid30641509">{{cite journal|vauthors=Moskal P, Kisielewska D, Curceanu C, Czerwiński E, Dulski K, Gajos A | display-authors=etal| title=Feasibility study of the positronium imaging with the J-PET tomograph. | journal=Phys Med Biol | year= 2019 | volume= 64 | issue= 5 | pages= 055017 | pmid=30641509 | doi=10.1088/1361-6560/aafe20 | arxiv=1805.11696| bibcode=2019PMB....64e5017M| doi-access=free }}</ref>
* ~60% of positrons will directly annihilate with an electron without forming positronium. The annihilation usually results in two gamma rays. In most cases this direct annihilation occurs only after the positron has lost its excess kinetic energy and has thermalized with the material.
* ~10% of positrons form ''para''-positronium, which then promptly (in ~0.12 ns) decays, usually into two gamma rays.
* ~30% of positrons form ''ortho''-positronium but then annihilate within a few nanoseconds by 'picking off' another nearby electron with opposing spin. This usually produces two gamma rays. During this time, the very lightweight positronium atom exhibits a strong zero-point motion, that exerts a pressure and is able to push out a tiny nanometer-sized bubble in the medium.
* Only ~0.5% of positrons form ''ortho''-positronium that self-decays (usually into ''three'' gamma rays). This natural decay rate of ''ortho''-positronium is relatively slow (~140 ns decay lifetime), compared to the aforementioned pick-off process, which is why the three-gamma decay rarely occurs.