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With the prospect of a long war, a group of theorists under the direction of J. Robert Oppenheimer met at Berkeley during the summer of 1942 to develop preliminary plans for designing and building a nuclear weapon.The United States was fully engaged in war in the Pacific and Atlantic oceans in the summer of 1942.Crucial questions remained, however, about the properties of fast neutrons. John Manley was assigned to help Oppenheimer find answers to these questions by coordinating several experimental physics groups scattered across the country.Keep in mind that an explosive nuclear chain reaction occurs when a sufficient quantity of nuclear fuel, such as uranium or plutonium, is brought together to form a critical mass. This is the minimum amount of fissionable material needed to start a chain reaction. The chain reaction starts when neutrons strike the heavy uranium or plutonium nucleus which splits releasing a tremendous amount of energy along with two or more neutrons which, in turn split more nuclei, and so on. In this gun-type device, the critical mass is achieved when a uranium projectile which is sub-critical is fired through a gun barrel at a uranium target which is also sub-critical. The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins.