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The Manhattan Project was an effort during World War II in the United States to develop the first nuclear weapon. It was directed by American physicist Dr. Julius Robert Oppenheimer.
The industrial problem was centered around the production of sufficient fissile material, of sufficient purity. This effort was two-fold, and is represented in the two bombs that were dropped.
The Hiroshima bomb, Little Boy, was uranium-235, a minor isotope of uranium that has to be physically separated from more prevalent uranium-238, which is not suitable for use in an explosive device.
The separation was effected mostly by gaseous diffusion of uranium hexafluoride (UF6), but also by other techniques. The bulk of this separation work was done at Oak Ridge.
The Nagasaki bomb, Fat Man, in contrast, consisted primarily of plutonium-239, a synthetic element which could be induced to supercriticality only by implosion. The design of an implosion device was at the center of the efforts by physicists at Los Alamos during the Project.
The property of uranium-238 which makes it less suitable directly for use in an atomic bomb is used in the production of plutonium -- with sufficiently slow neutrons, uranium-238 will absorb neutrons and transmute into plutonium-239. The production and purification of plutonium was at the center of wartime, and post-war, efforts at the Hanford Site, using techniques developed in part by Glenn Seaborg.
The choice of civilian instead of military targets has often been criticized. However, the U.S. already had a policy of massive incendiary attacks against civilian targets in Japan. These dropped 20% explosives, to break up wooden structures and provide fuel, and then dropped 80% (by weight) small incendiary bombs to set the cities on fire.
The resulting raids completely destroyed many Japanese cities, including Tokyo, even before atomic weapons were deployed. The allies performed such attacks because Japanese industry was extremely dispersed among civilian targets, with many tiny family-owned factories operating in the midst of civilian housing.
In the years between World War I and World War II, the United States had risen to pre-eminence in nuclear physics, driven by the work of recent immigrants and local physicists. These scientists had developed the basic tools of nuclear physics -- cyclotrons and other particle accelerators - and many new substances using these tools, including radioisotopes like carbon-14.
Enrico Fermi recalled the beginning of the project in a speech given in 1954 when he retired as President of the APS.
I remember very vividly the first month, January, 1939, that I started working at the Pupin Laboratories because things began happening very fast. In that period, Niels Bohr was on a lecture engagement in Princeton and I remember one afternoon Willis Lamb came back very excited and said that Bohr had leaked out great news.
The great news that had leaked out was the discovery of fission and at least the outline of its interpretation. Then, somewhat later that same month, there was a meeting in Washington where the possible importance of the newly discovered phenomenon of fission was first discussed in semi-jocular earnest as a possible source of nuclear power.
US President Franklin D. Roosevelt was presented with a letter signed by Albert Einstein (transcribed by Leo Szilard) on October 11, 1939, which urged the United States to rapidly develop an atomic bomb program. The president agreed. The Navy awarded Columbia University the first Atomic Energy funding of $6,000, which grew into the Manhattan Project under Oppenheimer and Enrico Fermi's work.
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