Public domain

A physicist recruited to the Manhattan Project at nineteen, Hall passed detailed descriptions of the plutonium "Fat Man" bomb and its purification methods to Soviet intelligence during World War II, one of a small number of scientists who deliberately gave the Soviet Union the technical basis for its own atomic weapons. He was never prosecuted — the evidence against him came from decrypted Soviet cables the U.S. could not use in court without exposing its codebreaking — and he spent his later career as a working biophysicist in Britain.