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The German physicist, Werner Heisenberg, sends a secret report to the Army Weapons Bureau "On the Possibility of Technical Energy Production from Uranium Splitting. II."

Germany occupied Norway, seizing the world's only heavy-water production plant. 

Frisch and Rudolf Peierls submit a memorandum to the British government estimating the critical mass of uranium 235 needed for an atomic bomb.

The University of California begins building a giant cyclotron under the direction of Ernest O. Lawrence. Edwin McMillan bombards uranium with neutrons and produces the first "transuranic" element, neptunium (Atomic Number 93). This leads to the eventual development of plutonium (Atomic Number 94).

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