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Nuclear weapons possess enormous destructive power derived from nuclear fission or combined fission and fusion reactions. Starting with scientific breakthroughs made during the 1930s, the United States, the United Kingdom and Canada collaborated during World War II in what was called the Manhattan Project to counter the suspected Nazi German atomic bomb project. In August 1945 two fission bombs were dropped on Japan ending the Pacific War. The Soviet Union started development shortly thereafter with their own atomic bomb project, and not long after that both countries developed even more powerful fusion weapons known as "hydrogen bombs." ==Physics and politics in the 1930s- and 1940s== In the first decades of the 20th century, physics was revolutionised with developments in the understanding of the nature of atoms. In 1898, Pierre and Marie Curie discovered that pitchblende, an ore of uranium, contained a substance—which they named radium—that emitted large amounts of radioactivity. Ernest Rutherford and Frederick Soddy identified that atoms were breaking down and turning into different elements. Hopes were raised among scientists and laymen that the elements around us could contain tremendous amounts of unseen energy, waiting to be harnessed. H. G. Wells was inspired to write about atomic weapons in a 1914 novel, ''The World Set Free'', which appeared shortly before the First World War. In a 1924 article, Winston Churchill speculated about the possible military implications: "Might not a bomb no bigger than an orange be found to possess a secret power to destroy a whole block of buildings—nay to concentrate the force of a thousand tons of cordite and blast a township at a stroke?" In January 1933, Adolf Hitler was appointed Chancellor of Germany and it quickly became unsafe for Jewish scientists to remain in the country. Leó Szilárd fled to London where he proposed, and in 1934 patented, the idea of a nuclear chain reaction via neutrons. The patent also introduced the term critical mass to describe the minimum amount of material required to sustain the chain reaction and its potential to cause an explosion. (British patent 630,726.) He subsequently assigned the patent to the British Admiralty so that it could be covered by the Official Secrets Act. In a very real sense, Szilárd was the father of the atomic bomb academically. Also in 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles; Enrico Fermi reported similar results when bombarding uranium with neutrons. In December 1938, Otto Hahn and Fritz Strassmann sent a manuscript to ''Naturwissenschaften'' reporting that they had detected the element barium after bombarding uranium with neutrons.〔O. Hahn and F. Strassmann ''Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle'' (''On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons''), ''Naturwissenschaften'' Volume 27, Number 1, 11-15 (1939). The authors were identified as being at the Kaiser-Wilhelm-Institut für Chemie, Berlin-Dahlem. Received December 22, 1938.〕 Lise Meitner and her nephew Otto Robert Frisch correctly interpreted these results as being due to the splitting of the uranium atom. (Frisch confirmed this experimentally on January 13, 1939.〔O. R. Frisch ''Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment'', ''Nature'', Volume 143, Number 3616, 276-276 ((18 February 1939) ). The paper is dated 17 January 1939. The experiment for this letter to the editor was conducted on 13 January 1939; see Richard Rhodes ''The Making of the Atomic Bomb'' 263 and 268 (Simon and Schuster, 1986).〕) They gave the process the name "fission" because of its similarity to the splitting of a cell into two new cells.〔Lise Meitner and O. R. Frisch ''Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction'', ''Nature'', Volume 143, Number 3615, 239-240 ((11 February 1939) ). The paper is dated January 16, 1939. Meitner is identified as being at the Physical Institute, Academy of Sciences, Stockholm. Frisch is identified as being at the Institute of Theoretical Physics, University of Copenhagen.〕 Even before it was published, news of Meitner’s and Frisch’s interpretation crossed the Atlantic.〔Richard Rhodes ''The Making of the Atomic Bomb'' 268 (Simon and Schuster, 1986).〕 Scientists at Columbia University decided to replicate the experiment and on January 25, 1939, conducted the first nuclear fission experiment in the United States〔H. L. Anderson, E. T. Booth, J. R. Dunning, E. Fermi, G. N. Glasoe, and F. G. Slack ''The Fission of Uranium'', ''Phys. Rev.'' Volume 55, Number 5, 511 - 512 (1839). Institutional citation: Pupin Physics Laboratories, Columbia University, New York, New York. Received February 16, 1939.〕 in the basement of Pupin Hall.〔Richard Rhodes ''The Making of the Atomic Bomb'' 267-270 (Simon and Schuster, 1886).〕 The following year, they identified the active component of uranium as being the rare isotope uranium-235.〔(【引用サイトリンク】 title=Early American Work on Fission )〕 Uranium appears in nature primarily in two isotopes: uranium-238 and uranium-235. When the nucleus of uranium-235 absorbs a neutron, it undergoes nuclear fission, releasing energy and, on average, 2.5 neutrons. Because uranium-235 releases more neutrons than it absorbs, it can support a chain reaction and so is described as fissile. Uranium-238, on the other hand, is not fissile as it does not normally undergo fission when it absorbs a neutron. By the time Nazi Germany invaded Poland in 1939, beginning World War II, many of Europe's top scientists had already fled the imminent conflict. Physicists on both sides were well aware of the possibility of utilizing nuclear fission as a weapon, but no one was quite sure how it could be done. In August 1939, concerned that Germany might have its own project to develop fission-based weapons, Albert Einstein signed a letter to U.S. President Franklin D. Roosevelt warning him of the threat. Roosevelt responded by setting up the Uranium Committee under Lyman James Briggs but, with little initial funding ($6,000), progress was slow. It was not until the Japanese attack on Pearl Harbor in December, 1941, that the U.S. decided to commit the necessary resources. Organized research first began in Britain as part of the Tube Alloys project. The Maud Committee was set up following the work of Frisch and Rudolf Peierls who calculated uranium-235's critical mass and found it to be much smaller than previously thought which meant that a deliverable bomb should be possible. In the February 1940 Frisch–Peierls memorandum they stated that: "The energy liberated in the explosion of such a super-bomb...will, for an instant, produce a temperature comparable to that of the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city." Edgar Sengier, a director of Shinkolobwe Mine which produced by far the highest quality uranium ore in the world, had become aware of uranium's possible use in a bomb. In late 1940, fearful of its seizure by the Germans, he shipped the mine's entire stockpile of ore to a warehouse on Staten Island. For 18 months British research outpaced the American but by mid-1942, it became apparent that the industrial effort required was beyond Britain's already stretched wartime economy.〔 In September 1942, General Leslie Groves was appointed to lead the U.S. project which became known as the Manhattan Project. Two of his first acts were to obtain authorization to assign the highest priority AAA rating on necessary procurements, and to put in train the purchase of all 1,250 tons of the Shinkolobwe ore.〔 The Tube Alloys project was quickly overtaken by the U.S. effort and after Roosevelt and Churchill signed the Quebec Agreement in 1943, it was relocated and amalgamated into the Manhattan Project. 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「History of nuclear weapons」の詳細全文を読む スポンサード リンク
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