Atomic Bomb
An atomic bomb is a weapon of mass destruction which uses nuclear fission to produce vast amounts of energy. Its basic principle is assembling a critical mass of a radioactive element with properties to spark a swift chain reaction. Its devastating effect is twofold: first, from the heat of the thermonuclear reaction, and second, from the radioactive fallout which continues after the bomb has been deployed.
The difference between an atomic bomb and a nuclear reactor is in the speed and control of the release of energy. It is precisely those features which are undesirable in a nuclear reactor that are vital in an atomic weapon. An atomic weapon must chain react swiftly enough that the entire reaction happens in a matter of minutes. Uranium-235 and plutonium are generally used for atomic bombs because of their ability to fission when hit by "slow" neutrons. That is, these substances can fission at all energy levels, not just at very high ones.
In order for an atomic bomb to only harm its targets and not its creators, the critical mass must remain separated until the bomb is supposed to go off. There were two original design solutions for this problem: the gun method and the implosion method. The gun method involves shooting a plug of radioactive matter at lower than critical mass into a cavity in a block of the same substance. The plug and block together then form critical mass. The implosion method packs explosive around the outside of two hemispheres. When the explosives go off, the hemispheres are pushed together into a sphere of critical mass.
In a typical uranium bomb, the critical mass of uranium is assembled inside a moderator. The moderator provides neutrons, which are captured by the uranium atoms. The atoms then undergo nuclear fission, producing daughter nuclei and more neutrons. The neutrons produced by the fission have to be going at the right speed to be captured by other uranium atoms, which continues the process.
The first atomic bomb was exploded on July 16, 1945, in Alamogordo, New Mexico. It was called the Trinity Test. The bomb had been designed by members of the Manhattan Project, the American wartime effort in atomic physics. In August of the same year, American Air Force troops dropped two bombs over the Japanese cities of Hiroshima and Nagasaki. Since the Second World War, atomic weapons have been developed by many countries but not used in warfare. The doctrine of Mutually Assured Destruction originally held that the nuclear weapons hold the world's superpowers in check because each side was aware of the devastating power of the atomic bomb. The absence of a second superpower has done nothing to decrease fear and awe of this work of science and technology. The creation of the atomic bomb did, however, prompt the formation of several professional groups of physicists opposed to nuclear warfare.
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