LOS ALAMOS NATIONAL LABORATORY / SCIENCE PHOTO LIBRARY LOS ALAMOS NATIONAL LABORATORY / SCIENCE PHOTO LIBRARY
Boy", an atomic fission bomb of the type dropped over Hiroshima, Japan on August 6th 1945. It was this act, along with a similar attack on Nagasaki, that forced the capitulation of Japan on September 2nd of that year. The bomb was triggered using a 'gun' method whereby a subcritical mass of uranium is fired, using an explosive charge, at a target consisting of another similar subcritical mass. The assembly of this supercritical mass of fissionable material initiated a nuclear chain reaction which produced vast quantities of energy. It weighed only 4 tonnes, but had a yield equivalent to nearly 20,000 tonnes of high explosive.
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