Free The First War of Physics by Jim Baggott

Oppenheimer is standing in the centre of the back row. At the extreme right of the first and second rows are two Rad Lab physicists who were now busy at work on the uranium problem – Edwin McMillan and Philip Abelson.
    McMillan, a native Californian, had worked on Lawrence’s cyclotrons for many years, and when the discovery of fission had been announced he had devised some simple experiments to confirm the phenomenon. He had now become intrigued by some of the discovery’s more subtle aspects. Bombarding uranium with neutrons produced a radioactive substancewhich decayed in a characteristic time of about 23 minutes. Like Hahn, Strassman and Meitner, McMillan surmised that this substance was U-239, formed by the resonant capture of a neutron by the predominant isotope, U-238. But there was another radioactive substance produced, which had a characteristic decay time of about two days.
    He believed this second substance to be a new element, formed by emission of a beta particle from U-239, in the process turning a neutron into a proton. Just as Weizsäcker had done, sitting on the Berlin underground railway, so McMillan had reasoned that this was element 93, perhaps the first in a series of transuranic elements. And, just as Hahn had done, McMillan further surmised that element 93 might behave somewhat like the element rhenium.
    With the help of a Berkeley research associate, Emilio Segrè, who had worked previously with Fermi in Rome, McMillan tried to obtain evidence of rhenium-like chemical properties. But they could find nothing of the sort. It seemed that, after all, the transuranics would continue to remain elusive. Segrè published the results in Physical Review , as an ‘unsuccessful search for transuranic elements’.
    McMillan had now refined the measurement of the decay time of this second mysterious substance to 2.3 days and became determined to identify precisely what it was. In the spring of 1940 he used the 60-inch cyclotron to investigate it further, and was joined in the quest by Abelson, who had by this time moved to the Carnegie Institution in Washington but had returned to Berkeley in April for a working vacation. Abelson had studied chemistry as well as physics and turned his attention to the chemical identification of the mysterious substance.
    It turned out to have properties not so very different from uranium itself. Bohr had in fact already suggested some time before that the transuranics – if they existed – might behave chemically more like uranium. Further work demonstrated unambiguously that the substance with the 2.3-day decay time was formed directly from U-239, with its characteristic 23-minute decay time. There was only one conclusion: the second substance was element 93.
    McMillan had already devised a name for the new substance – neptunium – though he chose to withhold it for the time being. Just as element 93 is one step further along the periodic table from uranium, so Neptune is one planet further along in the solar system from Uranus. Unaware of any reasons for secrecy, on 27 May McMillan and Abelson submitted a paper describing the results of their work to the American journal Physical Review. The paper was published on 15 June, and read with great interest by Weizsäcker when the journal reached him in Berlin in July. 3
    Of course, this work raised a further question. If element 93 was radioactive, with a characteristic decay time of 2.3 days, what was it decaying into? McMillan had his suspicions. He thought that element 93 might decay through a further emission of a beta particle, turning another neutron into a proton and so forming element 94. He immediately began work to find evidence for it.
    Wild enough speculation
    Szilard was probably unaware of McMillan and Abelson’s paper until it was published. The physicists had not thought to send it to him to seek his advice on the safety or otherwise of its publication in the open literature. But, by pure coincidence, on the