Editing Radioactive tracer (section)

==Tracer isotopes==

===Hydrogen===
[[Tritium]] (hydrogen-3) is produced by neutron irradiation of [[lithium|<sup>6</sup>Li]]:
:[[lithium|<sup>6</sup>Li]] + [[neutron|n]] → [[Helium|<sup>4</sup>He]] + [[tritium|<sup>3</sup>H]]
Tritium has a [[half-life]] {{val|4500|8}} days (approximately 12.32 years)<ref>{{cite journal | vauthors = Lucas LL, Unterweger MP | title = Comprehensive Review and Critical Evaluation of the Half-Life of Tritium | journal = Journal of Research of the National Institute of Standards and Technology | volume = 105 | issue = 4 | pages = 541–9 | year = 2000 | pmid = 27551621 | doi = 10.6028/jres.105.043 | url = http://nvl.nist.gov/pub/nistpubs/jres/105/4/j54luc2.pdf | archive-url = https://web.archive.org/web/20111017042101/http://nvl.nist.gov/pub/nistpubs/jres/105/4/j54luc2.pdf | url-status = dead | archive-date = 2011-10-17 | pmc=4877155}}</ref> and it decays by [[beta decay]]. The [[electron]]s produced have an average energy of 5.7&nbsp;keV. Because the emitted electrons have relatively low energy, the detection efficiency by scintillation counting is rather low. However, hydrogen atoms are present in all organic compounds, so tritium is frequently used as a tracer in [[biochemistry|biochemical]] studies.

===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>

=== Nitrogen ===
[[nitrogen|<sup>13</sup>N]] decays by [[positron emission]] with a half-life of 9.97 min. It is produced by the nuclear reaction
:[[proton|<sup>1</sup>H]] + [[oxygen|<sup>16</sup>O]] → [[nitrogen|<sup>13</sup>N]] + [[alpha particle|<sup>4</sup>He]]
[[nitrogen|<sup>13</sup>N]] is used in [[positron emission tomography]] (PET scan).

=== Oxygen ===
[[oxygen|<sup>15</sup>O]] decays by positron emission with a half-life of 122 seconds. It is used in positron emission tomography.

=== Fluorine ===
[[fluorine|<sup>18</sup>F]] decays predominantly by β emission, with a half-life of 109.8 min. It is made by proton bombardment of [[oxygen|<sup>18</sup>O]] in a cyclotron or [[linear particle accelerator]]. It is an important isotope in the [[Radiopharmacology|radiopharmaceutical]] industry. For example, it is used to make labeled [[fluorodeoxyglucose]] (FDG) for application in PET scans.<ref name=fowler/>

===Phosphorus===
[[phosphorus-32|<sup>32</sup>P]] is made by neutron bombardment of [[sulfur|<sup>32</sup>S]]
:[[sulfur|<sup>32</sup>S]] + [[neutron|n]] → [[phosphorus-32|<sup>32</sup>P]] + [[proton|p]]
It decays by beta decay with a half-life of 14.29 days. It is commonly used to study protein phosphorylation by [[kinases]] in biochemistry.

[[phosphorus-33|<sup>33</sup>P]] is made in relatively low yield by neutron bombardment of [[phosphorus|<sup>31</sup>P]]. It is also a beta-emitter, with a half-life of 25.4 days. Though more expensive than [[phosphorus-32|<sup>32</sup>P]], the emitted electrons are less energetic, permitting better resolution in, for example, DNA sequencing.

Both isotopes are useful for labeling [[nucleotide]]s and other species that contain a [[phosphate]] group.

===Sulfur===
[[Sulfur-35|<sup>35</sup>S]] is made by neutron bombardment of [[chlorine|<sup>35</sup>Cl]]
:[[chlorine-35|<sup>35</sup>Cl]] + [[neutron|n]] → [[Sulfur-35|<sup>35</sup>S]] + [[proton|p]]
It decays by beta-decay with a half-life of 87.51 days. It is used to label the sulfur-containing [[amino-acid]]s [[methionine]] and [[cysteine]]. When a sulfur atom replaces an oxygen atom in a [[phosphate]] group on a [[nucleotide]] a [[thiophosphate]] is produced, so <sup>35</sup>S can also be used to trace a phosphate group.

===Technetium===
{{main|technetium-99m}}
[[Technetium-99m|<sup>99m</sup>Tc]] is a very versatile radioisotope, and is the most commonly used radioisotope tracer in medicine. It is easy to produce in a [[technetium-99m generator]], by decay of [[molybdenum|<sup>99</sup>Mo]].
:<sup>99</sup>Mo → <sup>99m</sup>Tc + {{Subatomic particle|Electron-}} + {{Subatomic particle|Electron antineutrino}}
The molybdenum isotope has a half-life of approximately 66 hours (2.75 days), so the generator has a useful life of about two weeks. Most commercial <sup>99m</sup>Tc generators use [[column chromatography]], in which <sup>99</sup>Mo in the form of molybdate, MoO<sub>4</sub><sup>2−</sup> is adsorbed onto acid alumina (Al<sub>2</sub>O<sub>3</sub>). When the <sup>99</sup>Mo decays it forms [[pertechnetate]] TcO<sub>4</sub><sup>−</sup>, which because of its single charge is less tightly bound to the alumina. Pulling normal saline solution through the column of immobilized <sup>99</sup>Mo elutes the soluble <sup>99m</sup>Tc, resulting in a saline solution containing the <sup>99m</sup>Tc as the dissolved sodium salt of the pertechnetate. The pertechnetate is treated with a [[reducing agent]] such as [[tin|Sn<sup>2+</sup>]] and a [[ligand]]. Different ligands form [[coordination complex]]es which give the technetium enhanced affinity for particular sites in the human body.

<sup>99m</sup>Tc decays by gamma emission, with a half-life: 6.01 hours. The short half-life ensures that the body-concentration of the radioisotope falls effectively to zero in a few days.

===Iodine===
{{main|Isotopes of iodine}}
[[Isotopes of iodine|<sup>123</sup>I]] is produced by proton irradiation of <sup>124</sup>[[xenon|Xe]]. The [[caesium]] isotope produced is unstable and decays to <sup>123</sup>I. The isotope is usually supplied as the iodide and hypoiodate in dilute sodium hydroxide solution, at high isotopic purity.<ref>[http://www.mds.nordion.com/documents/products/I-123_Solu_Can.pdf I-123 fact sheet]{{dead link|date=April 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> <sup>123</sup>I has also been produced at Oak Ridge National Laboratories by proton bombardment of [[tellurium|<sup>123</sup>Te]].<ref>{{cite journal | vauthors = Hupf HB, Eldridge JS, Beaver JE | title = Production of iodine-123 for medical applications | journal = The International Journal of Applied Radiation and Isotopes | volume = 19 | issue = 4 | pages = 345–51 | date = April 1968 | pmid = 5650883 | doi = 10.1016/0020-708X(68)90178-6 }}</ref>

<sup>123</sup>I decays by [[electron capture]] with a half-life of 13.22 hours. The emitted 159&nbsp;[[keV]] [[gamma radiation|gamma ray]] is used in [[single-photon emission computed tomography]] (SPECT). A 127&nbsp;keV gamma ray is also emitted.

[[Isotopes of iodine|<sup>125</sup>I]] is frequently used in [[radioimmunoassay]]s because of its relatively long half-life (59 days) and ability to be detected with high sensitivity by gamma counters.<ref>{{cite journal | vauthors = Gilby ED, Jeffcoate SL, Edwards R | title = 125-Iodine tracers for steroid radioimmunoassay | journal = The Journal of Endocrinology | volume = 58 | issue = 1 | pages = xx | date = July 1973 | pmid = 4578967 }}</ref>

[[Isotopes of iodine|<sup>129</sup>I]] is present in the environment as a result of the testing of [[nuclear weapons]] in the atmosphere. It was also produced in the [[Chernobyl disaster|Chernobyl]] and [[Fukushima Daiichi nuclear disaster|Fukushima]] disasters. <sup>129</sup>I decays with a [[half-life]] of 15.7 million years, with low-energy [[beta particle|beta]] and [[gamma ray|gamma]] emissions. It is not used as a tracer, though its presence in living organisms, including human beings, can be characterized by measurement of the gamma rays.

=== Other isotopes ===
{{main|Radiopharmacology}}
Many other isotopes have been used in specialized radiopharmacological studies. The most widely used is [[gallium|<sup>67</sup>Ga]] for [[gallium scan]]s. <sup>67</sup>Ga is used because, like <sup>99m</sup>Tc, it is a gamma-ray emitter and various ligands can be attached to the Ga<sup>3+</sup> ion, forming a [[coordination complex]] which may have selective affinity for particular sites in the human body.

An extensive list of radioactive tracers used in hydraulic fracturing can be found below.