His letters were a success, and President Roosevelt established the Manhattan Project. .
Physicists from 1939 onward conducted much research to find answers to such questions as how many neutrons were emitted in each fission, which elements would not capture the neutrons but would moderate or reduce their velocity , and whether only the lighter and scarcer isotope of uranium (U-235) fissioned or the common isotope (U-238) could be used. They learned that each fission releases a few neutrons. A chain reaction, therefore, was theoretically possible, if not too many neutrons escaped from the mass or were captured by impurities. To create this chain reaction and turn it into a usable weapon was the ultimate goal of the Manhattan Project.
In 1942 General Leslie Groves was chosen to lead the project, and he immediately purchased a site at Oak Ridge, Tenn., for facilities to separate the necessary uranium-235 from the much more common uranium-238. Uranium 235 was an optimal choice for the bomb because of its unusually unstable composition. Thus, the race to separate the two began. During that time, the work to perfect the firing mechanism and structure of the bomb was also swiftly underway. .
General Groves' initial task had been to select a scientific director for the bomb project. His first two choices, Ernest O. Lawrence, director of the electromagnetic separation project, and Arthur H. Compton, director of Chicago Metallurgical Laboratory, were not available. Groves had some doubts regarding the next best candidate, J. Robert Oppenheimer (Wood 2). Finally, Groves gambled on Oppenheimer, a theoretical mathematician, as director of the weapons laboratory, built on an isolated mesa (flat land area) at Los Alamos, New Mexico. .
After much difficulty, an absorbent barrier suitable for separating isotopes of uranium was developed and installed in the Oak Ridge gaseous diffusion plant. Finally, in 1945, uranium-235 of bomb purity was shipped to Los Alamos, where it was fashioned into a gun-type weapon.