Edwin McMillan. Portrait of Edwin Mattison McMillan (1907-91), American physicist and the discoverer of neptunium, the first element heavier than uranium. At the University of California at Berkeley, McMillan worked with apparatus (cyclo- trons) designed to propel subatomic particles at high speeds. In 1940 he used a cyclotron to bombard uranium with neutrons and discovered that a new element had been formed. He later worked on the development of radar and improved the cyclotron to create the synchro-cyclotron or synchrotron. This allowed much higher particle speeds to be achieved. McMillan was jointly awarded the 1951 Nobel prize for chemistry.