Alfred O. C. Nier

Alfred Otto Carl Nier was an American physicist, born in 1911 and died in 1994, whose refinements of the mass spectrometer made possible the modern practice of radiometric dating. At the University of Minnesota beginning in the 1930s, he built increasingly precise mass spectrometers to measure the relative abundance of isotopes, work that let him isolate the rare uranium-235 isotope in 1940 and confirm it as the fissionable component of natural uranium, a result central to the Manhattan Project. His mass spectrometer designs and his precise measurements of lead and argon isotopes became the standard instrumentation later used across the earth sciences for uranium-lead and potassium-argon age dating of rocks and minerals.