Animation of the radioactive decay of the medical tracer fluorodeoxyglucose. This chemical has been designed to contain a radioactive 18-F fluorine atom in place of one of the normal hydroxide groups. This means that it cannot be metabolised by cells, so it remains in the tissues in which it was absorbed. 18-F is an artificial radioisotope of fluorine (green atom in the molecule), which decays with a half life of some 110 minutes. When it decays, it emits a positron (red), which quickly collides with an electron (blue), leading to the annihilation of both, and the emission of two gamma rays (yellow) in opposite directions. A PET scanner detects these gamma rays, and uses them to locate tissues with a high glucose uptake. The decay product of the radio-fluorine is oxygen-18, which converts the glucose back into a form in which it can be metabolised normally. See clip K004 4148 for the use of this tracer in a scanner.