Several important anniversaries in nuclear history are celebrated this month. Much of the credit for these events goes to a single man: Hungarian physicist Leo Szilard.
On July 4, 1934, Szilard filed the first patent application for a method of producing a nuclear chain reaction. The patent, filed in Britain, described the use of a neutron-induced chain reaction to create an explosion. Szilard also realized that such a reaction would require a certain "critical mass"; i.e., a minimum quantity of fissile material needed to sustain a chain reaction.
Szilard had been working at the University of Berlin until he was forced to flee Nazi persecution in 1933. Well aware of Nazi Germany's increasing aggression, and realizing the devastating potential of atomic weapons, Szilard assigned the patent to the British government in 1936 so the idea could be classified and protected.
On July 3, 1939, Szilard wrote to fellow physicist Enrico Fermi and described the use of a uranium lattice in carbon to create the chain reaction described in his patent. Three years later, in late 1942, Fermi and Szilard brought the idea to life when a fission chain reaction was sustained in atomic pile CP-1.
The successful operation of CP-1 was a crucial milestone of the Manhattan Project, which was started as a result of the famous letter written by Szilard and Albert Einstein to Franklin D. Roosevelt in late 1939. The letter alerted the President to the possibilities of nuclear weapons and warned him of Nazi research into atomic power.
On July 16, 1945, the idea that a neutron-induced chain reaction could create an explosion was dramatically proven at Trinity, New Mexico.