Arthur Jeffrey Dempster

Arthur Jeffrey Dempster was a Canadian-American physicist who built the first modern mass spectrometer and used it to find the fissile isotope of uranium. Born in Toronto in 1886, he took his first degrees at the University of Toronto, studied in Germany at Gottingen, Munich and Wurzburg, and completed a doctorate at the University of Chicago, where he joined the physics faculty in 1916 and stayed until his death. In 1918 he developed an instrument more than a hundred times as accurate as earlier ones, and in 1935 he used a spectrometer of his own design to identify uranium-235, the lighter isotope that makes up only about 0.7 percent of natural uranium and that later served as the fuel of early reactors and weapons. During the Second World War he was chief physicist of the Metallurgical Laboratory at Chicago, and from 1946 he directed a division at Argonne National Laboratory. He died in Florida in 1950. He is included here as the discoverer of the isotope on which fission power first depended.