|
| allegiance = | branch = U.S. Army Corps of Engineers | type = | role = | size = | command_structure = | garrison = Oak Ridge, Tennessee, U.S. | garrison_label = | patron = | motto = | colors = | colors_label = | march = | mascot = | equipment = | equipment_label = | battles = | anniversaries = 13 August 1942 | decorations = | battle_honours = | disbanded = 15 August 1947 | commander1 = | commander1_label = | commander2 = | commander2_label = | commander3 = | commander3_label = | notable_commanders = James C. Marshall Kenneth Nichols | identification_symbol = | identification_symbol_label = Shoulder patch that was adopted in 1945 for the Manhattan District | identification_symbol_2 = | identification_symbol_2_label = Manhattan Project emblem (unofficial) | identification_symbol_3 = | identification_symbol_3_label = | identification_symbol_4 = | identification_symbol_4_label = }} The Manhattan Project was a research and development project that produced the first nuclear weapons during World War II. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the U.S. Army Corps of Engineers; physicist J. Robert Oppenheimer was the director of the Los Alamos National Laboratory that designed the actual bombs. The Army component of the project was designated the Manhattan District; "Manhattan" gradually superseded the official codename, Development of Substitute Materials, for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion (about $ in dollars). Over 90% of the cost was for building factories and producing the fissile materials, with less than 10% for development and production of the weapons. Research and production took place at more than 30 sites across the United States, the United Kingdom and Canada. Two types of atomic bomb were developed during the war. A relatively simple gun-type fission weapon was made using uranium-235, an isotope that makes up only 0.7 percent of natural uranium. Since it is chemically identical to the most common isotope, uranium-238, and has almost the same mass, it proved difficult to separate. Three methods were employed for uranium enrichment: electromagnetic, gaseous and thermal. Most of this work was performed at Oak Ridge, Tennessee. In parallel with the work on uranium was an effort to produce plutonium. Reactors were constructed at Oak Ridge and Hanford, Washington, in which uranium was irradiated and transmuted into plutonium. The plutonium was then chemically separated from the uranium. The gun-type design proved impractical to use with plutonium so a more complex implosion-type weapon was developed in a concerted design and construction effort at the project's principal research and design laboratory in Los Alamos, New Mexico. The project was also charged with gathering intelligence on the German nuclear weapon project. Through Operation Alsos, Manhattan Project personnel served in Europe, sometimes behind enemy lines, where they gathered nuclear materials and documents, and rounded up German scientists. Despite the Manhattan Project's tight security, Soviet atomic spies still penetrated the program. The first nuclear device ever detonated was an implosion-type bomb at the Trinity test, conducted at New Mexico's Alamogordo Bombing and Gunnery Range on 16 July 1945. Little Boy, a gun-type weapon, and Fat Man, an implosion-type weapon, were used in the atomic bombings of Hiroshima and Nagasaki, respectively. In the immediate postwar years, the Manhattan Project conducted weapons testing at Bikini Atoll as part of Operation Crossroads, developed new weapons, promoted the development of the network of national laboratories, supported medical research into radiology and laid the foundations for the nuclear navy. It maintained control over American atomic weapons research and production until the formation of the United States Atomic Energy Commission in January 1947. == Origins == The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, made the development of an atomic bomb a theoretical possibility. There were fears that a German atomic bomb project would develop one first, especially among scientists who were refugees from Nazi Germany and other fascist countries. In August 1939, physicists Leó Szilárd and Eugene Wigner drafted the Einstein–Szilárd letter, which warned of the potential development of "extremely powerful bombs of a new type". It urged the United States to take steps to acquire stockpiles of uranium ore and accelerate the research of Enrico Fermi and others into nuclear chain reactions. They had it signed by Albert Einstein and delivered to President Franklin D. Roosevelt. Roosevelt called on Lyman Briggs of the National Bureau of Standards to head the Advisory Committee on Uranium to investigate the issues raised by the letter. Briggs held a meeting on 21 October 1939, which was attended by Szilárd, Wigner and Edward Teller. The committee reported back to Roosevelt in November that uranium "would provide a possible source of bombs with a destructiveness vastly greater than anything now known."〔.〕 Briggs proposed that the National Defense Research Committee (NDRC) spend $167,000 on research into uranium, particularly the uranium-235 isotope, and the recently discovered plutonium.〔 On 28 June 1941, Roosevelt signed Executive Order 8807, which created the Office of Scientific Research and Development (OSRD),〔(【引用サイトリンク】title=Executive Order 8807 Establishing the Office of Scientific Research and Development )〕 with Vannevar Bush as its director. The office was empowered to engage in large engineering projects in addition to research.〔.〕 The NDRC Committee on Uranium became the S-1 Uranium Committee of the OSRD; the word "uranium" was soon dropped for security reasons.〔.〕 In Britain, Otto Frisch and Rudolf Peierls at the University of Birmingham had made a breakthrough investigating the critical mass of uranium-235 in June 1939.〔.〕 Their calculations indicated that it was within an order of magnitude of , which was small enough to be carried by a bomber of the day.〔.〕 Their March 1940 Frisch–Peierls memorandum initiated the British atomic bomb project and its Maud Committee,〔.〕 which unanimously recommended pursuing the development of an atomic bomb.〔 In July 1940, Britain had offered to give the United States access to its scientific research, and the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as far advanced. As part of the scientific exchange, the Maud Committee's findings were conveyed to the United States. One of its members, the Australian physicist Mark Oliphant, flew to the United States in late August 1941 and discovered that data provided by the Maud Committee had not reached key American physicists. Oliphant then set out to find out why the committee's findings were apparently being ignored. He met with the Uranium Committee, and visited Berkeley, California, where he spoke persuasively to Ernest O. Lawrence. Lawrence was sufficiently impressed to commence his own research into uranium. He in turn spoke to James B. Conant, Arthur H. Compton and George B. Pegram. Oliphant's mission was therefore a success; key American physicists were now aware of the potential power of an atomic bomb.〔.〕〔.〕 At a meeting between President Roosevelt, Vannevar Bush, and Vice President Henry A. Wallace on 9 October 1941, the President approved the atomic program. To control it, he created a Top Policy Group consisting of himself—although he never attended a meeting—Wallace, Bush, Conant, Secretary of War Henry L. Stimson, and the Chief of Staff of the Army, General George C. Marshall. Roosevelt chose the Army to run the project rather than the Navy, as the Army had the most experience with management of large-scale construction projects. He also agreed to coordinate the effort with that of the British, and on 11 October he sent a message to Prime Minister Winston Churchill, suggesting that they correspond on atomic matters.〔.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Manhattan Project」の詳細全文を読む スポンサード リンク
|