Editing Neptunium (section)

===Discovery===
[[File:Berkeley 60-inch cyclotron.jpg|thumb|The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939|alt=Black-and-white picture of heavy machinery with two operators sitting aside.]]
As research on nuclear fission progressed in early 1939, [[Edwin McMillan]] at the [[Berkeley Radiation Laboratory]] of the [[University of California, Berkeley]] decided to run an experiment bombarding uranium using the powerful 60-inch (1.52 m) [[cyclotron]] that had recently been built at the university. The purpose was to separate the various fission products produced by the bombardment by exploiting the enormous force that the fragments gain from their mutual electrical repulsion after fissioning. Although he did not discover anything of note from this, McMillan did observe two new beta decay half-lives in the uranium trioxide target itself, which meant that whatever was producing the radioactivity had not violently repelled each other like normal fission products. He quickly realized that one of the half-lives closely matched the known 23-minute decay period of uranium-239, but the other half-life of 2.3 days was unknown. McMillan took the results of his experiment to chemist and fellow Berkeley professor [[Emilio Segrè]] to attempt to isolate the source of the radioactivity. Both scientists began their work using the prevailing theory that element 93 would have similar chemistry to rhenium, but Segrè rapidly determined that McMillan's sample was not at all similar to rhenium. Instead, when he reacted it with [[hydrogen fluoride]] (HF) with a strong [[oxidizing agent]] present, it behaved much like members of the [[rare earths]]. Since these elements comprise a large percentage of fission products, Segrè and McMillan decided that the half-life must have been simply another fission product, titling the paper "An Unsuccessful Search for Transuranium Elements".<ref>{{cite journal| last1=Segrè| first1=Emilio| title=An Unsuccessful Search for Transuranium Elements| date=1939| pages=1104–5| journal=Physical Review|volume=55| issue=11|bibcode = 1939PhRv...55.1104S |doi = 10.1103/PhysRev.55.1104 }}</ref><ref>Rhodes, pp. 346–350.</ref><ref>Yoshida et al., pp. 699–700.</ref>
 
However, as more information about fission became available, the possibility that the fragments of nuclear fission could still have been present in the target became more remote. McMillan and several scientists, including [[Philip H. Abelson]], attempted again to determine what was producing the unknown half-life. In early 1940, McMillan realized that his 1939 experiment with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor. In a new experiment, McMillan tried subjecting the unknown substance to HF in the presence of a [[reducing agent]], something he had not done before. This reaction resulted in the sample [[Precipitation (chemistry)|precipitating]] with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare-earth metal. 
Shortly after this, Abelson, who had received his [[Doctor of Science|graduate degree]] from the university, visited Berkeley for a short vacation and McMillan asked the more able chemist to assist with the separation of the experiment's results. Abelson very quickly observed that whatever was producing the 2.3-day half-life did not have chemistry like any known element and was actually more similar to uranium than a rare-earth metal. This discovery finally allowed the source to be isolated and later, in 1945, led to the classification of the [[actinide series]]. As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from <sup>239</sup>U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction:<ref name="EL93" />
:<chem>{^{238}_{92}U} + {^{1}_{0}n} -> {^{239}_{92}U} ->[\beta^-][23\ \ce{min}] {^{239}_{93}Np} ->[\beta^-][2.355\ \ce{days}] {^{239}_{94}Pu}</chem> <small>''(The times are [[half-life|half-lives]].)''</small>

This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered. McMillan and Abelson published their results in a paper entitled ''Radioactive Element 93'' in the ''[[Physical Review]]'' on May 27, 1940.<ref name="EL93">{{cite journal| doi =10.1103/PhysRev.57.1185.2| title =Radioactive Element 93| date =1940| author =Mcmillan, Edwin| journal =Physical Review| volume =57| pages =1185–1186| last2 =Abelson| first2 =Philip| issue =12|bibcode = 1940PhRv...57.1185M | doi-access =free}}</ref> They did not propose a name for the element in the paper, but they soon decided on the name ''neptunium'' since [[Neptune]] is the next planet beyond [[Uranus]] in our solar system, which uranium is named after.<ref name="lanl" /><ref>{{cite book|title=Handbook on the Physics and Chemistry of Rare Earths|volume=18 – Lanthanides/Actinides: Chemistry|editor=K. A. Gschneidner, Jr.|editor2=L, Eyring|editor3=G. R. Choppin|editor4=G. H. Landet|date=1994 |publisher=Elsevier|chapter= Ch. 118. Origin of the actinide concept|author=Seaborg, G. T.|pages=4–6, 10–14}}</ref><ref>Rhodes, pp. 348–350.</ref><ref>Yoshida et al., p. 700.</ref><ref>The name ''neptunium'' was used by R.Hermann in 1877 for a chemical element which, in his opinion, could be separated from a mineral tantalite; actually, this was a misidentification. See {{cite journal| title =Chemical Notes : The New Metals Ilmenium and Neptunium| date =1877| url=https://www.nature.com/articles/015520a0| journal =Nature| volume =15| pages =520–521| doi =10.1038/015520a0}}</ref> McMillan and Abelson's success compared to Nishina and Kimura's near miss can be attributed to the favorable half-life of <sup>239</sup>Np for radiochemical analysis and quick decay of <sup>239</sup>U, in contrast to the slower decay of <sup>237</sup>U and extremely long half-life of <sup>237</sup>Np.<ref name="Ikeda" />