The Manhattan Project: Making the Atomic Bomb
Part IV: The Manhattan Engineer District in Operation
Recruiting the Staff
Oppenheimer spent the first three months of 1943 tirelessly crisscrossing the country in an attempt to put together a first-rate staff, an effort that proved highly successful.41 Even Bacher signed on, though he promised to resign the moment militarization occurred; Rabi, though he did not move to Los Alamos, became a valuable consultant. As soon as Oppenheimer arrived at Los Alamos in mid-March, recruits began arriving from universities across the United States, including California, Minnesota, Chicago, Princeton, Stanford, Purdue, Columbia, Iowa State, and the Massachusetts Institute of Technology, while still others came from the Met Lab and the National Bureau of Standards. Virtually overnight Los Alamos became an ivory tower frontier boomtown, as scientists and their families, along with nuclear physics equipment, including two Van de Graaffs, a Cockroft-Walton accelerator, and a cyclotron, arrived caravan fashion at the Santa Fe railroad station and then made their way up to the mesa along the single primitive road. It was a most remarkable collection of talent and machinery that settled this remote outpost of the Manhattan Project.
Theory and the "Gadget" The initial spartan environment of "the Hill" (which included box lunches and temporary housing) was without doubt quite a contrast to the comfortable campus settings so familiar to many on the staff. But the laboratory's work began even as the Corps of Engineers struggled to provide the amenities of civilized life. 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 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. The theoretical consensus was that chain reactions took place with sufficient speed to produce powerful releases of energy and not simply explosions of the critical mass itself, but only experiments could test the 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 "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 in sure 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 was under consideration for uranium-235, but this method would work for plutonium only if absolute purification of plutonium could be achieved.
A variation of the explosion method was designed for uranium. Bomb designers, unable to solve the purification problem, turned to the relatively unknown implosion method for plutonium. With implosion, symmetrical shockwaves directed inward would compress a subcritical mass of plutonium packed in a nickel casing (tamper), releasing neutrons and causing a chain reaction.
Always in the background loomed the hydrogen bomb, a thermonuclear device considerably more powerful than either a uranium or plutonium device but one that needed a nuclear fission bomb as a detonator. Research on the hydrogen bomb, or Super, was always a distant second in priority at Los Alamos, but Oppenheimer concluded that it was too important to ignore. After considerable thought, he gave Teller permission to devote himself to the Super. To make up for Teller's absence, Rudolf Peierls, one of a group of British scientists who reinforced the Los Alamos staff at the beginning of 1944, was added to Bethe's theory group in mid-1944. Another member of the British contingent was the Soviet agent Klaus Fuchs, who had been passing nuclear information to the Russians since 1942 and continued doing so until 1949 when he was caught and convicted of espionage (and subsequently exchanged). 42