Editing Radioactive tracer (section)

===Carbon===
[[Carbon-11|<sup>11</sup>C]] decays by [[positron emission]] with a half-life of ca. 20 min. <sup>11</sup>C is one of the isotopes often used in [[positron emission tomography]].<ref name=fowler>Fowler J. S. and Wolf A. P. (1982) The synthesis of carbon-11, fluorine-18 and nitrogen-13 labeled radiotracers for biomedical applications. Nucl. Sci. Ser. Natl Acad. Sci. Natl Res. Council Monogr. 1982.</ref>

[[Carbon-14|<sup>14</sup>C]] decays by [[beta decay]], with a half-life of 5730 years. It is continuously produced in the upper atmosphere of the earth, so it occurs at a trace level in the environment. However, it is not practical to use naturally-occurring <sup>14</sup>C for tracer studies. Instead it is made by neutron irradiation of the isotope [[carbon-13|<sup>13</sup>C]] which occurs naturally in carbon at about the 1.1% level. <sup>14</sup>C has been used extensively to trace the progress of organic molecules through metabolic pathways.<ref>{{cite journal | vauthors = Kim SH, Kelly PB, Clifford AJ | title = Calculating radiation exposures during use of (14)C-labeled nutrients, food components, and biopharmaceuticals to quantify metabolic behavior in humans | journal = Journal of Agricultural and Food Chemistry | volume = 58 | issue = 8 | pages = 4632–7 | date = April 2010 | pmid = 20349979 | pmc = 2857889 | doi = 10.1021/jf100113c }}</ref>