1920 
                                 Rutherford speculates on the existence of the neutron at the Royal Society. 

                          1931 
                                 November-Urey discovers deuterium. 

                          1932 
                                 February-Chadwick discovers the neutron. 

                          1933 
                                 April-Born, Courant, Franck and many other scientists are compelled to leave the University of
                                 Gšttingen because of their "Jewish physics"
                                 October-Szilard recollects that "It occurred to me in October, 1933 that a chain reaction might be
                                 set up if an element could be found that would emit two neutrons when it swallowed one
                                 neutron."This idea became a classified British patent in 1935 before the fission was discovered. 

                          1934 
                                 Artificial radioactivity discovered by Curie/Joliot (bombardment with alpha particles) and by
                                 Fermi (bombardment with neutrons). 

                          1938 
                                 December-Fermi receives the Nobel prize for the discovery of transuranic elements (actually
                                 fission of uranium) and departs for the "new world"
                                 December 22, Hahn and Strassmann (later Meiter and Frisch) conclude that the identification of
                                 barium implies that the uranium nucleus has been fissioned by neutrons. 

                          1939 
                                 January to May-Many experiments on uranium fission.
                                 August 2-Szilard, Wigner, and Teller obtain a letter from Einstein on the possibility of a uranium
                                 weapon; Roosevelt receives the letter on October 11, 1939 from Sachs.