July 2: Francis Simon and Uranium-235

A German Jewish scientist who fled Hitler’s regime to Great Britain, where he devised the method of separating the Uranium-235 isotope, critical to the United States’ creation of the atomic bomb, Francis Simon was born in Berlin on this date in 1893. He was a World War I veteran and winner of the Iron Cross, which he left with his other military medals in Germany in 1933. A specialist in low-temperature physics, Simon was the first to produce the liquid helium by using magnetic cooling. He spent the latter part of World War II working in Los Alamos on the Manhattan Project before returning to Oxford in 1945. His “work in low-temperature physics reached a low of 20 millionths of a degree above absolute zero.... Simon worked on lowering temperatures below the point previously possible by the Joule-Thomson effect. His method was to withdraw heat by lining up paramagnetic molecules at very low temperatures and then allow their orientation to randomize, abstracting further heat from the surroundings and lowering the temperature still further. He came closer to absolute zero, though with more difficulty, by doing the same with nuclear spins.” —Today in Science