atomic bomb

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atomic bomb

or

A-bomb,

weapon deriving its explosive force from the release of nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
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 through the fission (splitting) of heavy atomic nuclei. The first atomic bomb was produced at the Los Alamos, N.Mex., laboratory and successfully tested on July 16, 1945. This was the culmination of a large U.S. army program that was part of the Manhattan ProjectManhattan Project,
the wartime effort to design and build the first nuclear weapons (atomic bombs). With the discovery of fission in 1939, it became clear to scientists that certain radioactive materials could be used to make a bomb of unprecented power. U.S.
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, led by Dr. Robert OppenheimerOppenheimer, J. Robert
, 1904–67, American physicist, b. New York City, grad. Harvard (B.A., 1925), Ph.D. Univ. of Göttingen, 1927. He taught at the Univ. of California and the California Institute of Technology from 1929 (as professor from 1936) until his appointment
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. It began in 1940, two years after the German scientists Otto HahnHahn, Otto
, 1879–1968, German chemist and physicist. His important contributions in the field of radioactivity include the discovery of several radioactive substances, the development of methods of separating radioactive particles and of studying chemical problems by the
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 and Fritz Strassman discovered nuclear fission. On Aug. 6, 1945, an atomic bomb was dropped on HiroshimaHiroshima
, city (1990 pop. 1,085,705), capital of Hiroshima prefecture, SW Honshu, Japan, on Hiroshima Bay. It is an important commercial and industrial center manufacturing trucks, ships, automobiles, steel, rubber, furniture, and canned foods.
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 with an estimated equivalent explosive force of 12,500 tons of TNT, followed three days later by a second, more powerful, bomb on NagasakiNagasaki
, city (1990 pop. 444,599), capital of Nagasaki prefecture, W Kyushu, Japan, on Nagasaki Bay. It is one of Japan's leading ports. Shipbuilding is the chief industry; machinery and electronics manufacturing and fishing are also important.
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. Both bombs caused widespread death, injury, and destruction, and there is still considerable debate about the need to have used them.

Atomic bombs were subsequently developed by the USSR (1949; now Russia), Great Britain (1952), France (1960), and China (1964). A number of other nations, particularly India, Pakistan, Israel, and North Korea now have atomic bombs or the capability to produce them; South Africa formerly possessed a small arsenal. The three smaller Soviet successor states that inherited nuclear arsenals (Ukraine, Kazakhstan, and Belarus) relinquished all nuclear warheads, which have been removed to Russia.

Atomic bombs have been designed by students, but their actual construction is a complex industrial process. Practical fissionable nuclei for atomic bombs are the isotopes uranium-235 and plutonium-239, which are capable of undergoing chain reactionchain reaction,
self-sustaining reaction that, once started, continues without further outside influence. Proper conditions for a chain reaction depend not only on various external factors, such as temperature, but also on the quantity and shape of the substance undergoing the
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. If the mass of the fissionable material exceeds the critical mass (a few pounds), the chain reaction multiplies rapidly into an uncontrollable release of energy. An atomic bomb is detonated by bringing together very rapidly (e.g., by means of a chemical explosive) two subcritical masses of fissionable material, the combined mass exceeding the critical mass. An atomic bomb explosion produces, in addition to the shock wave accompanying any explosion, intense neutron and gamma radiation, both of which are very damaging to living tissue. The neighborhood of the explosion becomes contaminated with radioactive fission products. Some radioactive products are borne into the upper atmosphere as dust or gas and may subsequently be deposited partially decayed as radioactive falloutfallout,
minute particles of radioactive material produced by nuclear explosions (see atomic bomb; hydrogen bomb; Chernobyl) or by discharge from nuclear-power or atomic installations and scattered throughout the earth's atmosphere by winds and convection currents.
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 far from the site of the explosion.

See disarmament, nucleardisarmament, nuclear,
the reduction and limitation of the various nuclear weapons in the military forces of the world's nations. The atomic bombs dropped (1945) on Japan by the United States in World War II demonstrated the overwhelming destructive potential of nuclear weapons
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; hydrogen bombhydrogen bomb
or H-bomb,
weapon deriving a large portion of its energy from the nuclear fusion of hydrogen isotopes. In an atomic bomb, uranium or plutonium is split into lighter elements that together weigh less than the original atoms, the remainder of the mass
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; nuclear strategynuclear strategy,
a policy for the use of nuclear weapons. The first atomic bombs were used in the context of the Allies' World War II policy of strategic bombing. Early in the cold war, U.S.
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; and nuclear weaponsnuclear weapons,
weapons of mass destruction powered by atomic, rather than chemical, processes. Nuclear weapons produce large explosions and hazardous radioactive byproducts by means of either nuclear fission or nuclear fusion.
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; see also nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
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.

Bibliography

See G. Herken, The Winning Weapon (1988) and Brotherhood of the Bomb (2002); R. Rhodes, The Making of the Atomic Bomb (1986, repr. 1995); R. Serber, The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb (1992); R. Fermi et al., Picturing the Bomb: Photographs from the Secret World of the Manhattan Project (1995); P. B. Hales, Atomic Spaces: Living on the Manhattan Project (1997); J. Baggott, The First War of Physics: The Secret History of the Atom Bomb, 1939–1949 (2010); G. Farmelo, Churchill's Bomb: How the United States Overtook Britain in the First Nuclear Arms Race (2013).

atomic bomb

[ə′täm·ik ′bäm]
(ordnance)
Also known as A bomb.
A device for suddenly producing an explosively rapid neutron chain reaction in a fissile material such as uranium-235 or plutonium-239. Also known as fission bomb.
Any explosive device which derives its energy from nuclear reactions, including a fusion bomb. Also known as nuclear bomb.

atomic bomb

(A-bomb) fission device of enormous destructive power. [Am. Sci.: EB, I: 628]

atomic bomb

, atom bomb
a type of bomb in which the energy is provided by nuclear fission. Uranium-235 and plutonium-239 are the isotopes most commonly used in atomic bombs