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Reference.org
Plutonium-244
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<p>Plutonium-244 (244Pu) is an <a href="/facts/Isotope_of_plutonium/ASACBN4x">isotope of plutonium</a> that has a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 81.3 million years. This is longer than any other isotope of plutonium and longer than any other known isotope of an element beyond bismuth, except for the three naturally abundant ones: <a href="/facts/Uranium-235/pe0RznDM">uranium-235</a> (704 million years), <a href="/facts/Uranium-238/QhcHm6q8">uranium-238</a> (4.468 billion years), and <a href="/facts/Thorium-232/lRb3opzg">thorium-232</a> (14.05 billion years). Given the half-life of 244Pu, an exceedingly small amount should still be present on Earth, making plutonium a likely but unproven candidate as the shortest-lived <a href="/facts/Primordial_element/lFAUqHDv">primordial element</a>. </p>