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title="Potassi 40 – Catalan" lang="ca" hreflang="ca" data-title="Potassi 40" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Drasl%C3%ADk-40" title="Draslík-40 – Czech" lang="cs" hreflang="cs" data-title="Draslík-40" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Potasio_40" title="Potasio 40 – Spanish" lang="es" hreflang="es" data-title="Potasio 40" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D9%BE%D8%AA%D8%A7%D8%B3%DB%8C%D9%85-%DB%B4%DB%B0" title="پتاسیم-۴۰ – Persian" lang="fa" hreflang="fa" data-title="پتاسیم-۴۰" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Potassium_40" title="Potassium 40 – French" lang="fr" hreflang="fr" data-title="Potassium 40" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko badge-Q70893996 mw-list-item" title=""><a href="https://ko.wikipedia.org/wiki/%EC%B9%BC%EB%A5%A8-40" title="칼륨-40 – Korean" lang="ko" hreflang="ko" data-title="칼륨-40" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-mk mw-list-item"><a href="https://mk.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BB%D0%B8%D1%83%D0%BC-40" title="Калиум-40 – Macedonian" lang="mk" hreflang="mk" data-title="Калиум-40" data-language-autonym="Македонски" data-language-local-name="Macedonian" class="interlanguage-link-target"><span>Македонски</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Kalium-40" title="Kalium-40 – Malay" lang="ms" hreflang="ms" data-title="Kalium-40" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Kalium-40" title="Kalium-40 – Dutch" lang="nl" hreflang="nl" data-title="Kalium-40" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E3%82%AB%E3%83%AA%E3%82%A6%E3%83%A040" title="カリウム40 – Japanese" lang="ja" hreflang="ja" data-title="カリウム40" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Pot%C3%A1ssio-40" title="Potássio-40 – Portuguese" lang="pt" hreflang="pt" data-title="Potássio-40" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BB%D0%B8%D0%B9-40" title="Калий-40 – Russian" lang="ru" hreflang="ru" data-title="Калий-40" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/Kalium-40" title="Kalium-40 – Finnish" lang="fi" hreflang="fi" data-title="Kalium-40" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Kalium-40" title="Kalium-40 – Swedish" lang="sv" hreflang="sv" data-title="Kalium-40" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Potasyum-40" title="Potasyum-40 – Turkish" lang="tr" hreflang="tr" data-title="Potasyum-40" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BB%D1%96%D0%B9-40" title="Калій-40 – Ukrainian" lang="uk" hreflang="uk" data-title="Калій-40" 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.infobox-subbox{padding:0;border:none;margin:-3px;width:auto;min-width:100%;font-size:100%;clear:none;float:none;background-color:transparent}.mw-parser-output .infobox-3cols-child{margin:auto}.mw-parser-output .infobox .navbar{font-size:100%}@media screen{html.skin-theme-clientpref-night .mw-parser-output .infobox-full-data:not(.notheme)>div:not(.notheme)[style]{background:#1f1f23!important;color:#f8f9fa}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .infobox-full-data:not(.notheme) div:not(.notheme){background:#1f1f23!important;color:#f8f9fa}}@media(min-width:640px){body.skin--responsive .mw-parser-output .infobox-table{display:table!important}body.skin--responsive .mw-parser-output .infobox-table>caption{display:table-caption!important}body.skin--responsive .mw-parser-output .infobox-table>tbody{display:table-row-group}body.skin--responsive .mw-parser-output .infobox-table tr{display:table-row!important}body.skin--responsive .mw-parser-output .infobox-table th,body.skin--responsive .mw-parser-output .infobox-table td{padding-left:inherit;padding-right:inherit}}</style><table class="infobox"><caption class="infobox-title">Potassium-40, <sup>40</sup>K</caption><tbody><tr><td colspan="2" class="infobox-image"><span class="mw-default-size" typeof="mw:File/Frameless"><a href="/wiki/File:Potassium-40.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/0/0a/Potassium-40.svg/220px-Potassium-40.svg.png" decoding="async" width="220" height="206" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/0a/Potassium-40.svg/330px-Potassium-40.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/0a/Potassium-40.svg/440px-Potassium-40.svg.png 2x" data-file-width="730" data-file-height="685" /></a></span></td></tr><tr><th colspan="2" class="infobox-header" style="background:#eee">General</th></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Table_of_nuclides" title="Table of nuclides">Symbol</a></th><td class="infobox-data"><sup>40</sup>K</td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/List_of_nuclides" title="List of nuclides">Names</a></th><td class="infobox-data">potassium-40, 40K, K-40</td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Atomic_number" title="Atomic number">Protons</a> <style data-mw-deduplicate="TemplateStyles:r886047488">.mw-parser-output .nobold{font-weight:normal}</style><span class="nobold">(<i>Z</i>)</span></th><td class="infobox-data">19</td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Neutron_number" title="Neutron number">Neutrons</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886047488"><span class="nobold">(<i>N</i>)</span></th><td class="infobox-data">21</td></tr><tr><th colspan="2" class="infobox-header" style="background:#eee"><a href="/wiki/Table_of_nuclides" title="Table of nuclides">Nuclide data</a></th></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Natural_abundance" title="Natural abundance">Natural abundance</a></th><td class="infobox-data"><span class="nowrap"><span data-sort-value="6996117000000000000♠"></span>0.0117(1)%</span></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Half-life" title="Half-life">Half-life</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r886047488"><span class="nobold">(<i>t</i><sub>1/2</sub>)</span></th><td class="infobox-data"><span class="nowrap"><span data-sort-value="7016394785576000000♠"></span>1.251(3)<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>9</sup> y</span></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Atomic_mass" title="Atomic mass">Isotope mass</a></th><td class="infobox-data">39.96399848(21) <a href="/wiki/Dalton_(unit)" title="Dalton (unit)">Da</a></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Spin_(particle_physics)" class="mw-redirect" title="Spin (particle physics)">Spin</a></th><td class="infobox-data">4<sup>−</sup></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Excess_energy" class="mw-redirect" title="Excess energy">Excess energy</a></th><td class="infobox-data"><span class="nowrap"><span data-sort-value="2995664950000000000♠"></span>−33<span style="margin-left:.25em;">505</span></span> <a href="/wiki/Electronvolt" title="Electronvolt">keV</a></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Binding_energy" title="Binding energy">Binding energy</a></th><td class="infobox-data"><span class="nowrap"><span data-sort-value="7005341523000000000♠"></span>341<span style="margin-left:.25em;">523</span></span> keV</td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Parent_isotope" class="mw-redirect" title="Parent isotope">Parent isotopes</a></th><td class="infobox-data"><a href="/wiki/Primordial_nuclide" title="Primordial nuclide"><sup></sup>Primordial</a></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/Decay_product" title="Decay product">Decay products</a></th><td class="infobox-data"><a href="/wiki/Calcium-40" class="mw-redirect" title="Calcium-40"><sup>40</sup>Ca (β<sup>−</sup>)</a><br /><a href="/wiki/Argon-40" class="mw-redirect" title="Argon-40"><sup>40</sup>Ar (EC, γ; β<sup>+</sup>)</a></td></tr><tr><th colspan="2" class="infobox-header" style="background:#eee">Decay modes</th></tr><tr style="font-weight:bold"><th scope="row" class="infobox-label"><a href="/wiki/Decay_mode" class="mw-redirect" title="Decay mode">Decay mode</a></th><td class="infobox-data"><a href="/wiki/Decay_energy" title="Decay energy">Decay energy</a> (<a href="/wiki/Electronvolt" title="Electronvolt">MeV</a>)</td></tr><tr><th scope="row" class="infobox-label"><span style="font-weight:normal">β<sup>−</sup></span></th><td class="infobox-data">1.31109</td></tr><tr><th scope="row" class="infobox-label"><span style="font-weight:normal"><a href="/wiki/Electron_capture" title="Electron capture">EC</a>, γ</span></th><td class="infobox-data">1.5049</td></tr><tr><td colspan="2" class="infobox-below" style="background:#eee"><a href="/wiki/Isotopes_of_potassium" title="Isotopes of potassium">Isotopes of potassium</a> <br /><a href="/wiki/Table_of_nuclides" title="Table of nuclides">Complete table of nuclides</a></td></tr></tbody></table> <p><b>Potassium-40</b> (<b><sup>40</sup>K</b>) is a <a href="/wiki/Radioactive_isotope" class="mw-redirect" title="Radioactive isotope">radioactive isotope</a> of <a href="/wiki/Potassium" title="Potassium">potassium</a> which has a long <a href="/wiki/Half-life" title="Half-life">half-life</a> of 1.25 billion years. It makes up about 0.012% (120 <a href="/wiki/Parts-per_notation" title="Parts-per notation">ppm</a>) of the total amount of potassium found in nature. </p><p>Potassium-40 undergoes three types of <a href="/wiki/Radioactive_decay" title="Radioactive decay">radioactive decay</a>. In about 89.28% of events, it decays to <a href="/wiki/Calcium-40" class="mw-redirect" title="Calcium-40">calcium-40</a> (<sup>40</sup>Ca) with emission of a <a href="/wiki/Beta_particle" title="Beta particle">beta particle</a> (β<sup>−</sup>, an <a href="/wiki/Electron" title="Electron">electron</a>) with a maximum energy of 1.31 <a href="/wiki/Electronvolt" title="Electronvolt">MeV</a> and an <a href="/wiki/Antineutrino" class="mw-redirect" title="Antineutrino">antineutrino</a>. In about 10.72% of events, it decays to <a href="/wiki/Argon-40" class="mw-redirect" title="Argon-40">argon-40</a> (<sup>40</sup>Ar) by <a href="/wiki/Electron_capture" title="Electron capture">electron capture</a> (EC), with the emission of a <a href="/wiki/Neutrino" title="Neutrino">neutrino</a> and then a 1.460 MeV <a href="/wiki/Gamma_ray" title="Gamma ray">gamma ray</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>Note 1<span class="cite-bracket">]</span></a></sup> The radioactive decay of this particular isotope explains the large abundance of argon (nearly 1%) in the <a href="/wiki/Earth%27s_atmosphere" class="mw-redirect" title="Earth's atmosphere">Earth's atmosphere</a>, as well as prevalence of <sup>40</sup>Ar over <a href="/wiki/Isotopes_of_argon" title="Isotopes of argon">other isotopes</a>. Very rarely (0.001% of events), it decays to <sup>40</sup>Ar by emitting a <a href="/wiki/Positron" title="Positron">positron</a> (β<sup>+</sup>) and a neutrino.<sup id="cite_ref-Engelkemeir_2-0" class="reference"><a href="#cite_note-Engelkemeir-2"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Potassium–argon_dating"><span id="Potassium.E2.80.93argon_dating"></span>Potassium–argon dating</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Potassium-40&action=edit&section=1" title="Edit section: Potassium–argon dating"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Potassium-40-decay-scheme.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Potassium-40-decay-scheme.svg/260px-Potassium-40-decay-scheme.svg.png" decoding="async" width="260" height="75" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Potassium-40-decay-scheme.svg/390px-Potassium-40-decay-scheme.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Potassium-40-decay-scheme.svg/520px-Potassium-40-decay-scheme.svg.png 2x" data-file-width="862" data-file-height="248" /></a><figcaption>Decay scheme</figcaption></figure> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/K%E2%80%93Ar_dating" title="K–Ar dating">K–Ar dating</a></div> <p>Potassium-40 is especially important in potassium–argon (K–Ar) dating. Argon is a gas that does not ordinarily combine with other elements. So, when a <a href="/wiki/Mineral" title="Mineral">mineral</a> forms – whether from molten <a href="/wiki/Rock_(geology)" title="Rock (geology)">rock</a>, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid. However, if the mineral contains any potassium, then decay of the <sup>40</sup>K isotope present will create fresh argon-40 that will remain locked up in the mineral. Since the rate at which this conversion occurs is known, it is possible to determine the elapsed time since the mineral formed by measuring the ratio of <sup>40</sup>K and <sup>40</sup>Ar atoms contained in it. </p><p>The argon found in Earth's atmosphere is 99.6% <sup>40</sup>Ar; whereas the argon in the Sun – and presumably in the primordial material that condensed into the planets – is mostly <a href="/wiki/Argon-36" class="mw-redirect" title="Argon-36"><sup>36</sup>Ar</a>, with less than 15% of <sup>38</sup>Ar. It follows that most of the terrestrial argon derives from potassium-40 that decayed into argon-40, which eventually escaped to the atmosphere. </p> <div class="mw-heading mw-heading2"><h2 id="Contribution_to_natural_radioactivity">Contribution to natural radioactivity</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Potassium-40&action=edit&section=2" title="Edit section: Contribution to natural radioactivity"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-left" typeof="mw:File/Thumb"><a href="/wiki/File:Evolution_of_Earth%27s_radiogenic_heat.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/b/b9/Evolution_of_Earth%27s_radiogenic_heat.svg/290px-Evolution_of_Earth%27s_radiogenic_heat.svg.png" decoding="async" width="290" height="211" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/b9/Evolution_of_Earth%27s_radiogenic_heat.svg/435px-Evolution_of_Earth%27s_radiogenic_heat.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/b9/Evolution_of_Earth%27s_radiogenic_heat.svg/580px-Evolution_of_Earth%27s_radiogenic_heat.svg.png 2x" data-file-width="899" data-file-height="653" /></a><figcaption>The evolution of Earth's mantle radiogenic heat flow over time: contribution from <sup>40</sup>K in yellow.</figcaption></figure> <p>The radioactive decay of <sup>40</sup>K in the Earth's mantle ranks third, after <a href="/wiki/Thorium-232" title="Thorium-232"><sup>232</sup>Th</a> and <a href="/wiki/Uranium-238" title="Uranium-238"><sup>238</sup>U</a>, as the source of <a href="/wiki/Radiogenic_heat" class="mw-redirect" title="Radiogenic heat">radiogenic heat</a>. The core also likely contains radiogenic sources, although how much is uncertain. It has been proposed that significant core radioactivity (1–2 TW) may be caused by high levels of U, Th, and K.<sup id="cite_ref-WohlersWood2015_3-0" class="reference"><a href="#cite_note-WohlersWood2015-3"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-MurthyWestrenenFei2003_4-0" class="reference"><a href="#cite_note-MurthyWestrenenFei2003-4"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p><p>Potassium-40 is the largest source of natural radioactivity in animals including humans. A 70 kg human body contains about 140 g of potassium, hence about <span class="nowrap">140g × 0.0117% ≈ 16.4 mg</span> of <sup>40</sup>K;<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> whose decay produces about 3850<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> to 4300 disintegrations per second (<a href="/wiki/Becquerel" title="Becquerel">becquerel</a>) continuously throughout the life of the person.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>Note 2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Samat_8-0" class="reference"><a href="#cite_note-Samat-8"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Banana_equivalent_dose">Banana equivalent dose</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Potassium-40&action=edit&section=3" title="Edit section: Banana equivalent dose"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Potassium-40 is famous for its usage in the <a href="/wiki/Banana_equivalent_dose" title="Banana equivalent dose">banana equivalent dose</a>, an informal unit of measurement, primarily used in general educational settings, to compare radioactive dosages to the amount received by consuming one <a href="/wiki/Banana" title="Banana">banana</a>. The radioactive dosage from consuming one banana is generally agreed to be 10<sup>−7</sup> <a href="/wiki/Sievert" title="Sievert">sievert</a>, or 0.1 microsievert, which is 1% of the average American's average daily exposure to radiation.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Potassium-40&action=edit&section=4" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Background_radiation" title="Background radiation">Background radiation</a></li> <li><a href="/wiki/Isotopes_of_potassium" title="Isotopes of potassium">Isotopes of potassium</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Potassium-40&action=edit&section=5" title="Edit section: Notes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">This photon would be called an <a href="/wiki/X-ray" title="X-ray">x-ray</a> if emitted from an electron. In nuclear physics, it is common to name photons according to their origin rather than their energy, high energy photons produced by electrical transitions are called "x-rays" while those emitted from atomic nuclei are called "<a href="/wiki/Gamma_rays" class="mw-redirect" title="Gamma rays">gamma rays</a>" irrespective of their energy.</span> </li> <li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text">The number of radioactive decays per second in a given mass of <sup>40</sup>K is the number of atoms in that mass, divided by the average lifetime of a <sup>40</sup>K atom in seconds. The number of atoms in one gram of <sup>40</sup>K is the <a href="/wiki/Avogadro_constant" title="Avogadro constant">Avogadro constant</a> <span class="nowrap"><span data-sort-value="7023602200000000000♠"></span>6.022<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>23</sup> mol<sup>−1</sup></span> divided by the <a href="/wiki/Atomic_weight" class="mw-redirect" title="Atomic weight">atomic weight</a> of potassium-40 (39.96 g/mol), which is about <span class="nowrap"><span data-sort-value="7022150700000000000♠"></span>0.1507<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>23</sup></span> per gram. As in any <a href="/wiki/Exponential_decay" title="Exponential decay">exponential decay</a>, the average lifetime is the half-life divided by the <a href="/wiki/Natural_logarithm" title="Natural logarithm">natural logarithm</a> of 2, or about <span class="nowrap"><span data-sort-value="7016568200000000000♠"></span>56.82<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>15</sup></span> seconds.</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Potassium-40&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239543626"><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-Engelkemeir-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-Engelkemeir_2-0">^</a></b></span> <span class="reference-text"> <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFEngelkemeirFlynnGlendenin1962" class="citation journal cs1">Engelkemeir, D. 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font-size:small;clear:both;"> <tbody><tr style="text-align: center;"> <td style="width:35%;">Lighter: <br /> <b><a href="/wiki/Potassium-39" class="mw-redirect" title="Potassium-39">potassium-39</a> </b> </td> <td style="width:30%;">Potassium-40 is an <br /> <b><a href="/wiki/Isotopes_of_potassium" title="Isotopes of potassium">isotope</a> of <a href="/wiki/Potassium" title="Potassium">potassium</a></b> </td> <td style="width:35%;">Heavier: <br /> <b><a href="/wiki/Potassium-41" class="mw-redirect" title="Potassium-41">potassium-41</a> </b> </td></tr> <tr style="text-align: center;"> <td style="width:35%;"><a href="/wiki/Decay_product" title="Decay product">Decay product</a> of: <br /> <b>— </b> </td> <td style="width:30%;"><b><a href="/wiki/Decay_chain" title="Decay chain">Decay chain</a></b> <br /> of potassium-40 </td> <td style="width:35%;"><a href="/wiki/Radioactive_decay" title="Radioactive decay">Decays</a> to: <br /> <b><a href="/wiki/Argon-40" class="mw-redirect" title="Argon-40">argon-40</a>, <a href="/wiki/Calcium-40" class="mw-redirect" title="Calcium-40">calcium-40</a>, Stable </b> </td></tr></tbody></table>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; 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