National Museum of Nuclear Science & HistorySeptember 1, 1939
Nazi Germany invades Poland, beginning World War II.
1938 to 1939: Discovering Fission
1938 to 1939: Discovering Fission
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August 31, 1939
Niels Bohr and John A. Wheeler publish a theoretical analysis of fission. This theory implies that uranium-235 is more fissile than U-238, and that the isotope of the undiscovered element 94 with 239 nucleons is also very fissile. These implications are not immediately recognized.
August 2, 1939
Pres. Roosevelt receives "The Einstein Letter" warning about the prospect of an atomic bomb.
July 3, 1939
Leo Szilard writes to Fermi describing the idea of using a uranium lattice in carbon (graphite) to create a chain reaction.
June 1939
Enrico Fermi and Leo Szilard submit a paper to Physical Review describing subcritical neutron multiplication in a lattice of uranium oxide in water, but it is clear that natural uranium and water cannot make a self-sustaining reaction.
April 22, 1939
Frederic Joliot and his group publish their work on the secondary neutrons released in nuclear fission. This demonstrates that a chain reaction is indeed possible.
March 1939
Enrico Fermi and Herbert Anderson find that there are about two neutrons produced for every one consumed in fission.
February 5, 1939
Niels Bohr realizes that uranium-235 and uranium-238 must have different fission properties, uranium-238 could undergo fission by fast neutrons but not slow ones, and that uranium-235 accounted for observed slow fission in uranium.
January 29, 1939
J. Robert Oppenheimer hears about the discovery of fission. Within a few minutes, he realizes that excess neutrons must be emitted, and that it might be possible to build a bomb.
January 28, 1939
Physicists recreate fission experiment at the Carnegie Atomic Physics Observatory in Washington D.C.
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