Editing Neptunium (section)

===Isotopes===
{{Main|Isotopes of neptunium}}
[[Image:Decay Chain(4n+1, Neptunium Series).svg|thumb|The 4''n'' + 1 [[decay chain]] of neptunium-237, commonly called the "neptunium series"]]
Twenty-four neptunium [[radioisotope]]s have been characterized, with the most stable being <sup>237</sup>Np with a [[half-life]] of 2.14 million years, <sup>236</sup>Np with a half-life of 154,000 years, and <sup>235</sup>Np with a half-life of 396.1 days. All of the remaining [[radioactive]] isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has at least four [[meta state]]s, with the most stable being <sup>236m</sup>Np with a half-life of 22.5 hours.<ref name="unc">{{cite web |url=http://www.nucleonica.net/unc.aspx |title=Universal Nuclide Chart |author=Nucleonica |date=2007–2013 |website=Nucleonica: Web Driven Nuclear Science |access-date=2013-10-15}} {{registration required}}.</ref>

The isotopes of neptunium range in [[atomic weight]] from 219.032 [[atomic mass unit|u]] (<sup>219</sup>Np) to 244.068 u (<sup>244</sup>Np), though <sup>221</sup>Np has not yet been reported.{{NUBASE2020|ref}} Most of the isotopes that are lighter than the most stable one, <sup>237</sup>Np, [[radioactive decay|decay]] primarily by [[electron capture]] although a sizable number, most notably <sup>229</sup>Np and <sup>230</sup>Np, also exhibit various levels of decay via [[alpha emission]] to become [[protactinium]]. <sup>237</sup>Np itself, being the [[beta-decay stable isobars|beta-stable isobar]] of mass number 237, decays almost exclusively by alpha emission into <sup>233</sup>[[Isotopes of protactinium|Pa]], with very rare (occurring only about once in trillions of decays) [[spontaneous fission]] and [[cluster decay]] (emission of <sup>30</sup>Mg to form <sup>207</sup>Tl). All of the known isotopes except one that are heavier than this decay exclusively via [[beta emission]].<ref name="unc" /><ref name="Yoshida702" /> The lone exception, <sup>240m</sup>Np, exhibits a rare (>0.12%) decay by [[isomeric transition]] in addition to beta emission.<ref name="unc" /> <sup>237</sup>Np eventually decays to form [[bismuth]]-209 and [[thallium]]-205, unlike most other common heavy nuclei which decay into [[isotopes of lead]]. This [[decay chain]] is known as the [[neptunium series]].<ref name="lanl">{{cite web| url=http://periodic.lanl.gov/93.shtml| title=Periodic Table Of Elements: LANL - Neptunium| publisher=Los Alamos National Laboratory| access-date=2013-10-13}}</ref><ref>{{cite book|author=C. M. Lederer|author2=J. M. Hollander|author3=I. Perlman|date=1968|title=Table of Isotopes|edition=6th|location=New York|publisher=[[John Wiley & Sons]]}}</ref> This decay chain had long been extinct on Earth due to the short half-lives of all of its isotopes above bismuth-209, but is now being resurrected thanks to artificial production of neptunium on the tonne scale.<ref>{{cite book|last1=Koch|first1=Lothar|title=Transuranium Elements, in Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a27_167|chapter=Transuranium Elements|isbn=978-3527306732}}</ref>

[[Image:Np sphere.jpg|thumb|This nickel-clad neptunium sphere was used to experimentally determine the critical mass of Np at Los Alamos National Lab.]]
The isotopes neptunium-235, -236, and -237 are predicted to be [[fissile]];<ref name="critical" /> only neptunium-237's fissionability has been experimentally shown, with the [[critical mass]] being about 60&nbsp;kg, only about 10&nbsp;kg more than that of the commonly used [[uranium-235]].<ref name="Weiss">{{cite journal |last=Weiss |first=Peter |date=2 July 2009 |title=Neptunium nukes?: Little-studied metal goes critical |journal=Science News |volume=162 |issue=17 |pages=259 |doi=10.2307/4014034 |jstor=4014034 }}</ref> Calculated values of the critical masses of neptunium-235, -236, and -237 respectively are 66.2&nbsp;kg, 6.79&nbsp;kg, and 63.6&nbsp;kg: the neptunium-236 value is even lower than that of [[plutonium-239]]. In particular, <sup>236</sup>Np also has a low neutron [[cross section (physics)|cross section]].<ref name="critical" /> Despite this, a neptunium [[atomic bomb]] has never been built:<ref name="Weiss" /> uranium and plutonium have lower critical masses than <sup>235</sup>Np and <sup>237</sup>Np, and <sup>236</sup>Np is difficult to purify as it is not found in quantity in [[spent nuclear fuel]]<ref name="Yoshida702" /> and is nearly impossible to separate in any significant quantities from <sup>237</sup>Np.<ref name="Jukka">{{cite book |author=Jukka Lehto |author2=Xiaolin Hou |date=2011|chapter=15.15: Neptunium |title=Chemistry and Analysis of Radionuclides |page=231 |no-pp=yes |edition=1st |publisher=[[John Wiley & Sons]] |isbn=978-3527633029}}</ref>