EARLY BOMB DESIGN
Los Alamos: Laboratory (1943-1944)
Events: Bringing It All Together, 1942-1945

Early work on the design of the atomic bomb began even as scientists continued to arrive at Los Alamos throughout 1943.  The properties of uranium were reasonably well understood, those of plutonium less so, and knowledge of fission explosions entirely theoretical.  That 2.2 secondary neutrons were produced when uranium-235 fissioned was accepted, but while Glenn Seaborg's team had proven in March 1941 that plutonium underwent neutron-induced fission, it was not known yet if plutonium released secondary neutrons during bombardment.  Further, the exact sizes of the "cross sections" of various fissionable substances had yet to be determined in experiments using the various particle accelerators then being shipped to Los Alamos.  The theoretical consensus was that fission chain reactions (below) did take place with sufficient speed to produce powerful releases of energy (and not simply result in the explosion of the critical mass itself), but only experiments could test this theory.  The optimum size of the critical mass remained to be established, as did the optimum shape.  When enough data were gathered to establish optimum critical mass, optimum effective mass still had to be determined.  That is, it was not enough simply to start a chain reaction in a critical mass; it was necessary to start one in a mass that would release the greatest possible amount of energy before it was destroyed in the explosion.  

In addition to calculations on uranium and plutonium fission, chain reactions, and critical and effective masses, work needed to be done on the ordnance aspects of the bomb, or the "Gadget" as it came to be known.  Two subcritical masses of fissionable material would have to come together to form a supercritical mass for an explosion to occur.  Furthermore, they had to come together in a precise manner and at high speed.  Measures also had to be taken to ensure that the highly unstable subcritical masses did not predetonate because of spontaneously emitted neutrons or neutrons produced by alpha particles reacting with lightweight impurities.  The chances of predetonation could be reduced by purification of the fissionable material and by using a high-speed firing system capable of achieving velocities of 3,000 feet per second.  A conventional artillery method of firing one subcritical mass into the other (above) was under consideration for uranium-235, but this method would work for plutonium only if absolute purification of plutonium could be achieved.  

A "gun-type" design of this sort was thus designed for uranium.  Unable to solve the purification problem, however, bomb designers feared that they would have to turn instead to the relatively unknown implosion method (right) for plutonium.  With implosion, symmetrical shockwaves directed inward would compress a subcritical mass of plutonium into a smaller, now-critical sphere.  This sphere would be surrounded by a heavy "tamper" that would reflect neutrons back into the active volume and restrain the explosion for a few crucial moments, thereby increasing the efficiency of the blast.  An initiator placed at the center of the sphere would ensure that the chain reaction began at precisely the right moment (rather than relying on "background neutrons" caused by spontaneous fission or background radiation).  

Always in the background loomed the hydrogen bomb, a thermonuclear device considerably more powerful than either a uranium or plutonium device.  Any hydrogen bomb would likely require an atomic fission bomb as a detonator, however.  For this reason research on the hydrogen bomb, or "Superbomb," was always a distant second in priority at Los Alamos, but Robert Oppenheimer concluded that it was too important to ignore.  After considerable thought, he gave Edward Teller permission to devote himself to the nuclear weapon that ultimately would dominate the Cold War.