Building the Bomb
Berkeley Summer Conference
In the summer of 1942, J. Robert Oppenheimer convened a study conference in his University of California offices to explore the possibility of developing an atomic bomb. Notable physicists such as Hans Bethe and Edward Teller attended, and the conferees concluded that an atomic bomb was possible and could be assembled by firing one piece of uranium or plutonium at a second piece of the same material. When the two pieces of material collided, a nuclear explosion would occur.
A gun-type assembly was both well understood and relatively simple, thus early Los Alamos efforts centered almost exclusively on developing such a weapon. Since plutonium was a much more difficult material to work with than uranium, Oppenheimer concentrated further work on a gun using plutonium. This early gun type was known by its codename, Thin Man.
Spontaneous Fission
In the spring of 1944 Emilio Segre discovered that light element impurities in plutonium, which could not be eradicated, would cause a premature, low-order detonation of a plutonium-gun bomb — a fizzle. This discovery, known as spontaneous fission, was devastating. Already facing a crippling shortage of uranium, a combat weapon might be significantly delayed or not available for use during the war.
Implosion
Recognizing that a supersonic shock wave created by high explosives could be used to implode, or crush, a ball of plutonium (to initiate a fission chain reaction), Oppenheimer reorganized Los Alamos in August 1944. He refocused most of the laboratory's efforts on developing a successful method to implode plutonium. By early spring 1945, the design for the implosion gadget was set. This new plutonium bomb, called Fat Man, was such a radical departure from established technology that doubts about its success made necessary a test, codenamed Trinity, which was conducted in July 1945.
Although most of the effort at Los Alamos after August 1944 centered on implosion development, work continued on a uranium gun device, which was renamed Little Boy. Uranium presented few technical problems and success seemed certain enough that no proof test of Little Boy was required. Since only enough uranium was available in 1945 for one bomb, a test would have kept Little Boy out of combat.