1931: Harold C. Urey discovers deuterium (heavy hydrogen)
1931: Robert J. Van de Graaff develops the high-voltage electrostatic generator used in atomic research
1932: James Chadwick discovers the neutron enabling the utilization of nuclear energy.
1932: J. D. Cockroft and E. T. S. Walton were the first to split the atom
1932: Ernest O. Lawrence and his colleagues M. Stanley Livingston and Milton White successfully operated the first cyclotron. (A Cyclotron is a machine used as a circular particle accelerator for positively charged ions (usually protons, deuterons, and alpha particles) used to initiate nuclear transformations upon collision with a suitable target)
1934: Enrico Fermi produces fission, the splitting of an atomic nucleus resulting in the release of large amounts of energy. Szilard files a patent application for the atomic bomb describing the concept of using neutron induced chain reactions to create explosions.
1938: In December 1938 Otto Hahn and Fritz Strassmann discover the process of fission in uranium
1938: Lise Meitner and Otto Frisch provided the evidence for the Hahn-Strassmann and nuclear fission and communicated their findings to Niels Bohr
1939: January 26,1939: Niels Bohr who had made foundational contributions to understanding atomic structure reported on the Hahn-Strassman results at a meeting on theoretical physics in Washington, D. C.
1939: Walter Zinn, Enrico Fermi, Herbert L. Anderson , John R. Dunning and Leo Szilard work together at Pupin Physics Laboratories at Columbia University in New York City, investigating whether uranium-238 fissioned with slow neutrons, as Fermi believed, or only the uranium-235 isotope, as Niels Bohr contended
1939: Hungarian physicist Leo Szilard was the first man to realized that nuclear power could be used to build a bomb of terrifying proportions
1939: August 2, 1939: A letter signed by Leo Szilard and Albert Einstein was sent to Alexander Sachs, who had access to President Franklin D. Roosevelt, to inform him that "a nuclear chain reaction in a large mass of uranium" was undoubtedly possible, and could lead to the construction of "extremely powerful bombs of a new type". The letter warned that Germany might be developing such a weapon
1939: September 1, 1939: Germany invades Poland and this event delays the meeting between Alexander Sachs and the President
1939: October 11-12, 1939: Alexander Sachs discusses Einstein's letter with President Roosevelt. FDR decides to act and appoints Lyman J. Briggs head of the Advisory Committee on Uranium, a top-secret project to investigate the possibility of utilizing energy from the atomic fission of uranium.
1939: October 19, 1939: President Roosevelt wrote back to Einstein informing the physicist that he had setup a committee consisting of Alexander Sachs and representatives from the Army and Navy to study uranium.
1939: November 1, 1939: The Uranium Committee recommends that the government purchase uranium oxide and graphite for fission research.
1940: February 1940: Physicists Otto Frisch and Rudolf Peierls write a theoretical analysis of the possibility of fast fission in U-235.
1940: The University of California begins building a giant cyclotron under the direction of Ernest O. Lawrence.
1940: July 1940: The U.S. Army Intelligence office denies Albert Einstein the security clearance needed to work on the Manhattan Project
1940: March 1940: John R. Dunning and his team demonstrate that fission is more readily produced in the rare uranium-235 isotope, not the more plentiful uranium-238.
1940: June 1940: Vannevar Bush is named head of the National Defense Research Committee and the Uranium Committee becomes a scientific subcommittee of the organization.
1941: February 24, 1941: Glenn T. Seaborg’s research group, including and Arthur Wahl, working at the University of California in Berkeley discovers plutonium and the group demonstrates that plutonium is fissionable on March 28, 1941
1941: May 3, 1941: Glenn Seaborg proves plutonium is more fissionable than uranium-235.
1941: June 28, 1941: Vannevar Bush is named head of the Office of scientific Research and Development and James B. Conant replaces Bush at the National Defense Research Committee
1941: July 2, 1941: The British MAUD report concludes that an atomic bomb is feasible and the findings are sent to Bush and Conant of the Manhattan Project .
1941: October 9, 1941: Vannevar Bush briefs FDR on the progress of atomic bomb research. The President instructs Bush to find out if an atomic bomb can be built and at what cost.
1941: The Japanese attack Pearl Harbor on December 7, 1941 and the United States enters the war on December 8, 1941.
1942: January 19, 1942: FDR approves production of the atomic bomb
1942: May 12, 1942: President Roosevelt signs an order creating a secret project, called the Manhattan Project, to develop the nuclear weapon
1942: September 23, 1942: Leslie Groves is put in charge of the Manhattan Project and recruits J. Robert Oppenheimer as Scientific Director.
1942: October 15, 1942: Leslie Groves asks J. Robert Oppenheimer to head Project Y, the new planned central laboratory for weapon physics research and design at the Manhattan Project .
1942: November 16, 1942: Los Alamos, New Mexico was selected as the site for Project Y, for the main atomic bomb scientific laboratory
1942: December 2, 1942: Enrico Fermi and his Manhattan Project team produce the world's first controlled and self-sustained nuclear fission reaction in a squash court under Stagg Field at the University of Chicago
1943: January, 1943: Groves acquires the Hanford Engineer Works on the Columbia River in Washington for plutonium production reactors and separation plants for the Manhattan Project
1943: August 19, 1943: The Quebec Agreement, between the United States and Great Britain, was signed outlining the coordinated development of the nuclear energy and the weapons that employed nuclear energy. A large team of British and Canadian scientists moved to the US to work on the Manhattan Project.
1943: November 4, 1943: A large experimental graphite reactor (the X-10) was constructed at Oak Ridge nuclear facility in Tennessee to provide research quantities of plutonium
1943: November 29, 1943: The US Military begin remodeling the B-29 Superfortress bomber for the delivery of the Atomic bomb.
1943: Four different size casings were made for the Atomic bombs. "The gadget" was the code name given to the first bomb tested. The "Thin Man" bomb was long and thin representing Roosevelt and "Fat Boy" was round and fat representing Churchill. The Little Boy bomb was a development of the unsuccessful Thin Man nuclear bomb using uranium rather than plutonium.
1943: Project Alberta began to prepare for the delivery of atomic weapons during combat by conducting weapons delivery tests, modifying aircraft for carrying the atomic weapons developed by the Manhattan Project. The B-29 Superfortress bomber was remodeled to deliver the Atomic bomb and the organization and training of flight crews and field teams for weapons handling began.
1944: January 1944: Construction begins on Abelson's thermal diffusion plant at the Philadelphia Naval Yard.
1944: March 1944: Manhattan Project Bomb models are tested at Los Alamos.
1944: Oak Ridge and Hanford produce increasing amounts of fissionable material and Los Alamos makes significant progress in weapon design for the Manhattan Project.
1944: July 17, 1944: The plutonium gun bomb (Thin Man) is abandoned to be replaced with the Little Boy bomb
1944: September 1944: Colonel Paul Tibbets and the 393rd Bombardment Squadron begin test drops with dummy bombs called "Pumpkins".
1945: February 4-11, 1945: Roosevelt, Churchill, and Stalin meet at the Yalta Conference.
1945: April 12, 1945: President Franklin D. Roosevelt dies, and Harry S. Truman becomes President of the United States.
1945: April 25,1945: Secretary of War Henry Stimson and Groves brief President Harry Truman on the Manhattan Project.
1945: April 27, 1945: The first meeting of the Target Committee was held to select targets for atomic bombing.
1945: June 1945: Scientists issue the Franck Report, advocating international control of atomic research and proposing a demonstration of the atomic bomb prior to its use in combat.
1945: June 6, 1945: Henry Stimson informs President Harry Truman that the Interim Committee recommends keeping the atomic bomb a secret and using it as soon as possible without warning.
1945: June 14,1945: Leslie Groves, overall director of the Manhattan Project, submits the target selections to General George Marshall.
1945: July 16, 1945: Trinity was the code name of the first detonation of a nuclear weapon (The Gadget) conducted by the United States Army at Alamogordo, New Mexico. 'The Gadget' atomic bomb exploded with a 18,000 ton TNT equivalent
1945: July 26, 1945: President Truman, British Prime Minister Clement Atlee and Chinese President Chiang Kai-Shek, issue the Potsdam Proclamation, calling for Japan to surrender unconditionally. The Japanese reject the idea.
1945: August 6, 1945: The Little Boy, is dropped on Hiroshima. Casualties have been estimated between 90,000 and 160,000 in Hiroshima, half of victims died within 24 hours of the blast.
1945: August 9, 1945: Fat Man, the implosion model plutonium bomb is dropped on Nagasaki. Casualties have been estimated at least 60,000 in Nagasaki.
1945: 15 August, 1945: Japan surrenders bringing WW2 to an end
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