He became a professor at Rome University, where in 1934 he discovered how to produce slow (thermal) neutrons. He used these to create new radioisotopes, for which he was awarded the 1938 Nobel Prize. In 1938 he and his Jewish wife emigrated to the USA. In 1942 he led the team that built the first atomic pile (nuclear reactor) in Chicago. Fermi was an influential theoretical physicist who, independently of Paul Dirac, discovered Fermi–Dirac statistics. He also proposed the first proper theory of weak interactions in 1933.