The Manhattan Project: Making the Atomic Bomb
Part IV: The Manhattan Engineer District in Operation
Early experiments on both uranium and plutonium provided welcome results. Uranium emitted neutrons in less than a billionth of a second- just enough time, in the world of nuclear physics, for an efficient explosion. Emilio Segré later provided an additional cushion with his discovery in December 1943 that, if cosmic rays were eliminated, the subcritical uranium masses would not have to be brought together as quickly as previously thought; nor would the uranium have to be as pure. Muzzle velocity for the scaled down artillery piece could be lower, and the gun could be shorter and lighter.43 Segré's tests on the first samples of plutonium demonstrated that plutonium emitted even more neutrons than uranium due to the spontaneous fission of plutoniurn-240. Both theory and experimental data now agreed that a bomb using either element would detonate if it could be designed and fabricated into the correct size and shape. But many details remained to be worked out, including calculations to determine how much uranium-235 or plutonium would be needed for an explosive device.
Bacher's engineering division patiently generated the essential cross-sectional measurements needed to calculate critical and efficient mass. (The cross section is a measurement that indicates the probability of a nuclear reaction taking place). The same group utilized particle accelerators to produce the large numbers of neutrons needed for its cross-sectional experiments. Bather's group also compiled data that helped identify tamper materials that would most effectively push neutrons back to the core and enhance the efficiency of the explosion. Despite Los Alamos's postwar reputation as a mysterious retreat where brilliant scientists performed miracles of nuclear physics, much of the work that led to the atomic bombs was extremely tedious.
The chemists' job was to purify the uranium-235 and plutonium, reduce them to metals, and process the tamper material. Only highly purified uranium and plutonium would be safe from predetonation. Fortunately purification standards for uranium were relatively modest, and the chemical division was able to focus its effort on the lesser known plutonium and make substantial progress on a multi-step precipitation process by summer 1944. The metallurgy division had to turn the purified uranium-235 and plutonium into metal. Here, too, significant progress was made by summer as the metallurgists adapted a stationary-bomb technique initially developed at Iowa State University.
Parsons, in charge of ordnance engineering, directed his staff to design two artillery pieces of relatively standard specifications except for their extremely light barrels-one for a uranium weapon and one for a plutonium bomb. The weapons needed to achieve high velocities, but they would not have to be durable since they would only be fired once. Here again early efforts centered on the more problematic plutonium weapon, which required a higher velocity due to its higher risk of predetonation. Two plutonium guns arrived in March and were field-tested successfully. In the same month, two uranium guns were ordered.