Editing Trinity (nuclear test) (section)

==Background==
{{Main|Manhattan Project}}
The creation of [[nuclear weapon]]s arose from the scientific and political developments of the 1930s. The decade saw many new discoveries about the nature of atoms, including the existence of [[nuclear fission]]. The concurrent rise of [[fascist]] governments in Europe led to a fear of a [[German nuclear weapon project]], especially among scientists who were refugees from [[Nazi Germany]] and other fascist countries. When their calculations showed that nuclear weapons were theoretically feasible, the British and United States governments supported an all-out effort to build them.{{sfn|Szasz|1992|pp=3–8}}

These efforts were transferred to the authority of the [[U.S. Army]] in June 1942 and became the [[Manhattan Project]].{{sfn|Jones|1985|pp=30–31}} [[Brigadier General (United States)|Brigadier General]] [[Leslie R. Groves, Jr.]] was appointed its director in September.{{sfn|Jones|1985|p=76}} The weapons development portion of this project was located at the [[Los Alamos Laboratory]] in northern [[New Mexico]], under the directorship of physicist [[J. Robert Oppenheimer]]. The [[University of Chicago]], [[Columbia University]] and the [[Lawrence Berkeley National Laboratory|Radiation Laboratory]] at the [[University of California, Berkeley]] conducted other development work.{{sfn|Jones|1985|p=63}}

Manhattan Project scientists had identified two [[fissile isotope]]s for potential use in bombs: [[uranium-235]] and [[plutonium-239]].{{sfn|Jones|1985|pp=8–10, 28–29}} Uranium-235 became the basis of the [[Little Boy]] bomb design, first used (without prior testing) in the [[bombing of Hiroshima]]; the design used in the Trinity test, and eventually used in the [[bombing of Nagasaki]] ([[Fat Man]]), was based on plutonium.{{sfn|Jones|1985|pp=522–523, 535–537}} The original design considered for a weapon based on plutonium-239 was [[Thin Man (nuclear bomb)|Thin Man]], in which (as in the Little Boy uranium bomb) two subcritical masses of fissile material would be brought rapidly together to form a single [[critical mass]].{{sfn|Jones|1985|pp=508–509}}

Plutonium is a [[synthetic element]] with complicated properties about which little was known at first, as until 1944 it had been produced only in [[cyclotron]]s in very pure microgram amounts, whereas a weapon would require kilogram quantities bred in a reactor.{{sfn|Baker|Hecker|Harbur|1983|p=142}} In April 1944, Los Alamos physicist [[Emilio Segrè]]{{sfn|Hawkins|Truslow|Smith|1961|p=101}} discovered that plutonium produced by the [[X-10 Graphite Reactor]] at [[Clinton Engineer Works]] contained [[plutonium-240]] as an impurity.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=235–239}} Plutonium-240 undergoes [[spontaneous fission]] at thousands of times the rate of plutonium-239, and the extra [[neutron]]s thereby released made it likely that plutonium in a [[gun-type fission weapon]] would detonate too soon after a critical mass was formed, producing a "[[fizzle (nuclear test)|fizzle]]"—a [[nuclear explosion]] many times smaller than a full explosion.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=235–239}} The Thin Man design would therefore not work.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=240–242}}

Project scientists then turned to a more technically difficult [[implosion-type nuclear weapon|implosion design]]. In September 1943, mathematician [[John von Neumann]] had proposed surrounding a fissile [[pit (nuclear weapon)|"core"]] by two different [[high explosives]] which produced [[shock wave]]s of different speeds. Alternating the faster- and slower-burning explosives in a carefully calculated configuration would produce a compressive wave upon their simultaneous detonation. This so-called "[[explosive lens]]" focused the shock waves inward with sufficient force to rapidly compress the solid plutonium core to several times its original density. The increase in density caused the core{{snd}} previously subcritical{{snd}}to become supercritical. At the same time, the shock wave activated a small [[modulated neutron initiator|neutron source]] at the center of the core, thereby assuring that the [[Nuclear chain reaction|chain reaction]] would begin in earnest immediately at the moment of compression. Such a complicated design required substantial research and experimentation in engineering and [[hydrodynamics]],{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=130–138}} and in August 1944 the entire Los Alamos Laboratory was reorganized to focus on this work.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=245–247}}