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بمب اتمی - Atomic Bomb

Atomic Bomb

Atomic Bomb, powerful explosive nuclear weapon fueled by the splitting, or fission, of the nuclei of specific isotopes of uranium or plutonium in a chain reaction. The strength of the explosion created by an atomic bomb is on the order of the strength of the explosion that would be created by thousands of tons of TNT (see Trinitrotoluene).

An atomic bomb must provide enough mass of plutonium or uranium to reach critical mass, the mass at which the nuclear reactions going on inside the material can make up for the neutrons leaving the material through its outside surface. Usually the plutonium or uranium in a bomb is separated into parts so that critical mass is not reached until the bomb is set to explode. At that point, a set of chemical explosives or some other mechanism drives all the different pieces of uranium or plutonium together to produce a critical mass. After this occurs, there are enough neutrons bouncing around in the material to create a chain reaction of fissions. In the fission reactions, collisions between neutrons and uranium or plutonium atoms cause the atoms to split into pairs of nuclear fragments, releasing energy and more neutrons. Once the reactions begin, the neutrons released by each reaction hit other atoms and create more fission reactions until all the fissile material is exhausted or scattered

This process of fission releases enormous energy in the form of extreme heat and a massive shock wave; this is the intense explosion. In addition to its nearly unimaginable destructive force, consisting of pressure waves, flash burns, and high winds, a nuclear explosion also produces deadly radiation in the form of gamma rays and neutrons. The radiation destroys living matter and contaminates soil and water. See also “Effects of the Atomic Bombs.”

Atomic bombs were the first nuclear weapons to be developed, tested, and used. In the late 1930s physicists in Europe and the United States realized that the fission of uranium could be used to create an extremely powerful explosive weapon. In August 1939 German American physicist Albert Einstein sent a letter to U.S. president Franklin D. Roosevelt that described this discovery and warned of its potential development by other nations. The U.S. government established the top secret Manhattan Project in 1942 to develop an atomic device. The leader of the Manhattan Project was U.S. Army Brigadier General Leslie R. Groves. His team, working in several locations but in large part at Los Alamos, New Mexico, under the direction of American physicist J. Robert Oppenheimer, designed and built the first atomic bombs.

The first atomic explosion was conducted, as a test, at Alamogordo, New Mexico, on July 16, 1945. The energy released from this explosion was equivalent to that released by the detonation of 20,000 tons of TNT. Near the end of World War II, on August 6, 1945, the United States dropped the first atomic bomb on the Japanese city of Hiroshima. It followed with a second bomb against the city of Nagasaki on August 9. According to U.S. estimates, 60,000 to 70,000 people were killed by the initial blast of the Hiroshima bomb, called “Little Boy,” and about 40,000 by the bomb dropped on Nagasaki, called “Fat Man.” Japan agreed to Allied terms of surrender on August 14th. These are the only times that a nuclear weapon has been used in a conflict between nations. See also the sidebar “Was It Necessary to Bomb Hiroshima?”

Fusion bombs, also called hydrogen or thermonuclear bombs, were developed and tested in the early 1950s, but these have never been used in warfare. A thermonuclear device depends on a fission reaction to produce extreme heat that causes hydrogen isotopes of deuterium and tritium to come together, or fuse, but the main energy source for thermonuclear devices comes from the fusion reaction, not the triggering fission reaction. For more information on this type of bomb, see Hydrogen Bomb.

Several nations have exploded nuclear devices in tests in the atmosphere, under the earth, and under the sea. Only the United States, Russia, the United Kingdom, France, China, India, Pakistan, and North Korea admit to possessing nuclear weapons. South Africa admitted to building and then dismantling a number of bombs. Israel is believed to have nuclear weapons but maintains an official silence about whether it possesses them. See also Arms Control; Nuclear Weapons Proliferation

Atomic Bomb

Fission Bomb

The first atomic bomb used in warfare was dropped by the United States on August 6, 1945. Called the Little Boy bomb, it produced an explosion that devastated the city of Hiroshima in Japan and killed tens of thousands of people in less than one minute. The Little Boy bomb was a type of bomb called a gun-type fission bomb, shown here, that generates a nuclear explosion by firing one piece of fissile material into another of the same type. In this case, the material is uranium. The bomb is gunlike in that a small wedge of uranium is fired at a larger, target piece. Upon impact, the two pieces fuse together briefly, forming what is called a supercritical mass (a mass slightly greater than what is necessary to sustain a chain reaction). The rapid release of massive amounts of energy in a limited volume creates the explosion. In the Little Boy bomb, a mass of uranium about the size of a baseball produced an explosion as powerful as 20 kilotons of TNT.