Alfred Nier (born 28.05.1911) is an American experimental physicist and a member of the National Academy of Sciences (1950). He was born in St Paul (Minnesota). He graduated from the University of Minnesota (1933), where he has worked since 1938 (professor from 1944).
His research concerned mass spectrometry and its applications in physics, chemistry, geology and medicine. He introduced many improvements in the mass spectrometer. He discovered rare isotopes of potassium, sulfur, carbon, osmium and others. He studied the isotopic composition of a number of minerals, in particular measuring the relative abundance of the isotopes of lead in uranyl minerals and of the isotopes of uranium itself, which made it possible to obtain the first accurate value of the decay constant of uranium-235. He worked on the problem of separating isotopes. In 1940, together with J. Dunning, he developed the gaseous diffusion method for separating uranium-235 and isolated it, showing that it is this isotope that is fissioned by slow neutrons. In 1939 he proposed a method of determining geological age with the help of a mass spectrometer. He proved that Ar40 is a product of the decay of K40, which was important for the development of the potassium-argon method of determining geological age. He made many accurate determinations of atomic masses with mass spectrometers.