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McMillan, Edwin Mattison

neptunium nuclear element uranium

(1907–91) US physicist: discoverer of neptunium.

Educated at Caltech and Princeton, McMillan joined the University of California at Berkeley in 1935 and was there for the rest of his career. In the late 1930s he was mainly concerned with nuclear reactions and the design of cyclotrons.

In 1940 with P Abelson (1913–) he showed that, when uranium is bombarded with neutrons, one nuclear reaction that occurs leads to formation of a new element, the first discovered to be heavier than uranium; it was named neptunium. The nuclear reactions are:

McMillan obtained evidence that the radioactive neptunium decayed to form a new element, plutonium (number 94), but in 1940 he moved to defence work on radar and sonar and the new transuranic elements were studied by , with continuing success. For their work on this Seaborg and McMillan shared the 1951 Nobel Prize for chemistry.

Cyclotron had met a limit to its performance in the early 1940s; particles accelerated in it above a certain speed increased in mass in accord with theory of relativity and this put them out of phase with the electric impulses. McMillan in 1945 devised a solution to this by use of a variable frequency for the impulses, adjusted to keep in phase with the particles. This machine, the synchrocyclotron, could be designed to give results up to 40 times more powerful than the best cyclotrons.

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