Edwin McMillan employer University of California, Berkeley
Lawrence Berkeley Laboratory
In 1932, McMillan was awarded a National Research Council fellowship, allowing him to attend a university of his choice for postdoctoral study. With his PhD complete, although it was not formally accepted until January 12, 1933, he accepted an offer from Ernest Lawrence at the University of California, Berkeley, to join the Berkeley Radiation Laboratory, which Lawrence had founded the year before. McMillan's initial work there involved attempting to measure the magnetic moment of the proton, but Otto Stern and Immanuel Estermann were able to carry out these measurements first.The main focus of the Radiation laboratory at this time was the development of the cyclotron, and McMillan, who was appointed to the faculty at Berkeley as an instructor in 1935, soon became involved in the effort. His skill with instrumentation came to the fore, and he contributed improvements to the cyclotron. In particular, he helped develop the process of "shimming", adjusting the cyclotron to produce a homogeneous magnetic field. Working with M. Stanley Livingston, he discovered oxygen-15, an isotope of oxygen that emits positrons. To produce it, they bombarded nitrogen gas with deuterons. This was mixed with hydrogen and oxygen to produce water, which was then collected with hygroscopic calcium chloride. Radioactivity was found concentrated in it, proving that it was in the oxygen. This was followed by an investigation of the absorption of gamma rays produced by bombarding fluorine with protons.In 1935, McMillan, Lawrence and Robert Thornton carried out cyclotron experiments with deuteron beams that produced a series of unexpected results. Deuterons fused with a target nuclei, transmuting the target to a heavier isotope while ejecting a proton. Their experiments indicated a nuclear interaction at lower energies than would be expected from a simple calculation of the Coulomb barrier between a deuteron and a target nucleus. Berkeley theoretical physicist Robert Oppenheimer and his graduate student Melba Phillips developed the Oppenheimer–Phillips process to explain the phenomenon. McMillan became an assistant professor in 1936, and an associate professor in 1941. With Samuel Ruben, he also discovered the isotope beryllium-10 in 1940. This was both interesting and difficult to isolate due to its extraordinarily long half-life, about 1.39 million years.
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