atomic bomb or A-bomb, weapon deriving its explosive force from the release of atomic energy through the fission (splitting) of heavy nuclei (see nuclear energy nuclear 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
..... Click the link for more information. ). 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 Project Manhattan 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.
..... Click the link for more information. , led by Dr. Robert Oppenheimer Oppenheimer, J. Robert (ŏp`ənhī'mər), 1904–67, American physicist, b. New York City, grad. Harvard (B.A., 1925), Ph.
..... Click the link for more information. . It began in 1940, two years after the German scientists Otto Hahn and Fritz Strassman discovered nuclear fission. On Aug. 6, 1945, an atomic bomb was dropped on Hiroshima Hiroshima (hĭr'ōshē`mə, hērō`shmä), city (1990 pop.
..... Click the link for more information. with an estimated equivalent explosive force of 12,500 tons of TNT, followed three days later by a second, more powerful, bomb on Nagasaki Nagasaki (näg'äsä`kē), city (1990 pop. 444,599), capital of Nagasaki prefecture, W Kyushu, Japan, on Nagasaki Bay.
..... Click the link for more information. . 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 reaction chain 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
..... Click the link for more information. . 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 fallout fallout, 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.
..... Click the link for more information. far from the site of the explosion.

See disarmament, nuclear disarmament, 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
..... Click the link for more information. ; hydrogen bomb neutron bomb, which would have a minimum trigger and a nonfissionable tamper; there would be blast effects and a hail of lethal neutrons but almost no radioactive fallout; this theoretically would cause minimal physical damage to buildings and equipment but kill most living things.
..... Click the link for more information. ; nuclear strategy nuclear 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.
..... Click the link for more information. ; and nuclear weapons nuclear 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.
..... Click the link for more information. ; see also nuclear energy.

Bibliography

See G. Herken, The Winning Weapon (1988) and Brotherhood of the Bomb (2002); R. Rhodes, The Making of the Atomic Bomb (1986); 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).

atomic bomb

First atomic bomb test, near Alamogordo, New Mexico, July 16, 1945.

(credit: Courtesy of Los Alamos National Laboratory, New Mexico)

Weapon whose great explosive power results from the sudden release of energy upon the splitting, or fission, of the nuclei of heavy elements such as plutonium or uranium (see nuclear fission). With only 11–33 lb (5–15 kg) of highly enriched uranium, a modern atomic bomb could generate a 15-kiloton explosion, creating a huge fireball, a large shock wave, and lethal radioactive fallout. The first atomic bomb, developed by the Manhattan Project during World War II, was set off on July 16, 1945, in the New Mexico desert. The only atomic bombs used in war were dropped by the U.S. on Hiroshima on Aug. 6, 1945, and on Nagasaki three days later. In 1949 the Soviet Union tested its first atomic bomb, followed by Britain (1952), France (1960), China (1964), India (1974), and Pakistan (1998). Israel and South Africa were suspected of testing atomic weapons in 1979. See also hydrogen bomb; Nuclear Non-proliferation Treaty; nuclear weapon.

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

atomic bomb

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

See : Destruction