Following the discovery of
nuclear fission in
uranium by
Otto Hahn and
Fritz Strassmann in 1939, McMillan began experimenting with uranium. He bombarded it with
neutrons produced in the Radiation Laboratory's cyclotron through bombarding
beryllium with deuterons. In addition to the
nuclear fission products reported by Hahn and Strassmann, they detected two unusual radioactive isotopes, one with a half-life of about 2.3 days, and the other with one of around 23 minutes. McMillan identified the short-lived isotope as
uranium-239, which had been reported by Hahn and Strassmann. McMillan suspected that the other was an isotope of a new, undiscovered element, with an
atomic number of 93. At the time it was believed that element 93 would have similar chemistry to
rhenium, so he began working with
Emilio Segrè, an expert on that element from his discovery of its
homolog technetium. Both scientists began their work using the prevailing theory, 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 like members of the
rare-earth elements. Since these comprise a large percentage of fission products, Segrè and McMillan decided that the half-life must have been simply another fission product, titling the article "An Unsuccessful Search for Transuranium Elements". McMillan realized that his 1939 work 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
precipitating with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare earth. In May 1940,
Philip Abelson from the
Carnegie Institute in
Washington, DC, who had independently also attempted to separate the isotope with the 2.3-day half-life, visited Berkeley for a short vacation, and they began to collaborate. Abelson observed that the isotope with the 2.3-day half-life did not have chemistry like any known element, but was more similar to uranium than a rare earth. This 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 239U 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: :{}^{238}_{92}U + {}^{1}_{0}n -> {}^{239}_{92}U ->[\beta^-] [23\ \text{min}] \overset{neptunium}{^{239}_{93}Np} ->[\beta^-] [2.355\ \text{days}] {}^{239}_{94}Pu 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 an article entitled
Radioactive Element 93 in the
Physical Review on May 27, 1940. They did not propose a name for the element in the article, but they soon decided on "neptunium", since uranium had been named after the planet
Uranus, and
Neptune is the next planet beyond in the
Solar System. McMillan suddenly departed for war-related work at this point, leaving
Glenn Seaborg to pursue this line of research and discover the second transuranium element,
plutonium. In 1951, McMillan shared the
Nobel Prize in Chemistry with Seaborg "for their discoveries in the chemistry of the transuranium elements". ==World War II==