Frisch–Peierls memorandum

Document revealing, for the first time, how small the critical mass (the minimum quantity of substance required for a nuclear chain reaction to begin) of uranium needed to be if the isotope uranium-235 was separated from naturally occurring uranium; the memo thus implied the feasibility of using this isotope to make an atom bomb. It was written by Otto Frisch and Rudolf Peierls at the University of Birmingham in 1940.