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Manhattan Project - Part 3

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Physicist and Nobel laureate Arthur Holly Compton organized the Metallurgical Laboratory at the University of Chicago in early 1942 to study plutonium and fission piles. Compton asked theoretical physicist J. Robert Oppenheimer of the University of California to study the feasibility of a nuclear weapon.

In the spring of 1942, Oppenheimer and Robert Serber of the University of Illinois, worked on the problems of neutron diffusion (how neutrons moved in the chain reaction) and hydrodynamics (how the explosion produced by the chain reaction might behave).

To review this work and the general theory of fission reactions, Oppenheimer convened a summer study at the University of California, Berkeley in June 1942. Theorists Hans Bethe, John Van Vleck, Edward Teller, Felix Bloch, Richard Tolman and Emil Konopinski concluded that a fission bomb was feasible. The scientists suggested that such a reaction could be initiated by assembling a critical mass - an amount of nuclear explosive adequate to sustain it - either by firing two subcritical masses of plutonium or uranium 235 together or by imploding (crushing) a hollow sphere made of these materials with a blanket of high explosives. Until the numbers were better known, this was all that could be done.

Teller saw another possibility: By surrounding a fission bomb with deuterium and tritium, a much more powerful "superbomb" might be constructed. This concept was based on studies of energy production in stars made by Bethe before the war . When the detonation wave from the fission bomb moved through the mixture of deuterium and tritium nuclei, they would fuse together to produce much more energy than fission, in the process of nuclear fusion, just as elements fused in the sun produce light and heat.

Bethe was skeptical, and as Teller pushed hard for his "superbomb" and proposed scheme after scheme, Bethe refuted each one. When Teller raised the possibility that an atomic bomb might ignite the atmosphere, however, he kindled a worry that was not entirely extinguished until the Trinity test, even though Bethe showed, theoretically, that it couldn't happen.

The summer conferences, the results of which were later summarized by Serber in "The Los Alamos Primer" (LA-1), provided the theoretical basis for the design of the atomic bomb, which was to become the principal task at Los Alamos during the war, and the idea of the H-bomb, which was to haunt the Laboratory in the postwar era. Seldom has a physics summer school been as portentous for the future of mankind.

With the prospect of a long war, a group of theorists under the direction of J. Robert Oppenheimer met at Berkeley during the summer of 1942 to develop preliminary plans for designing and building a nuclear weapon. Crucial questions remained, however, about the properties of fast neutrons. John Manley, a physicist at the University of Chicago Metallurgical Laboratory, was assigned to help Oppenheimer find answers to these questions by coordinating several experimental physics groups scattered across the country.

The measurements of the interactions of fast neutrons with the materials in a bomb are essential because the number of neutrons produced in the fission of uranium and plutonium must be known, and because the substance surrounding the nuclear material must have the ability to reflect, or scatter, neutrons back into the chain reaction before it is blown apart in order to increase the energy produced. Therefore, the neutron scattering properties of materials had to be measured to find the best reflectors.

Estimating the explosive power required knowledge of many other nuclear properties, including the cross-section (a measure of the probability of an encounter between particles that result in a specified effect) for nuclear processes of neutrons in uranium and other elements. Fast neutrons could only be produced in particle accelerators, which were still relatively uncommon instruments in physics departments in 1942.

The need for better coordination was clear. By September 1942, the difficulties involved with conducting preliminary studies on nuclear weapons at universities scattered throughout the country indicated the need for a laboratory dedicated solely to that purpose. The need for it, however, was overshadowed by the demand for plants to produce uranium-235 and plutonium - the fissionable materials that would provide the nuclear explosives.



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