Berkeley Summer Study Group
|"My wife [Rose], who was then twenty-four, knew vaguely what we were talking about. On a walk in the mountains at Yosemite National Park, she asked me to consider carefully whether I really wanted to continue to work on this. Finally, I decided to do it. The 'Super' [hydrogen bomb] was a terrible thing, but the fission bomb had to come first in any case and 'the Germans were presumably doing it'." - Hans Bethe
As discussed previously, American scientists' concerns about the uses Adolf Hitler might make of the German discovery of fission in December 1939 led to investigations of nuclear energy production in the United States. Fission, in which a slow neutron splits a heavy atom into two atoms of approximately half the weight of the original, released tremendous amounts of energy that might be used in a power-producing pile (a reactor) or a bomb.
By the time the United States entered World War II in December 1941, several projects were under way to investigate the separation of fissionable uranium 235 from uranium 238, the manufacture of plutonium, and the feasibility of nuclear piles and explosions.
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 his former postdoctoral student, Robert Serber of the University of Illinois, worked with Oppenheimer's students Eldred Nelson and Stan Frankel 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 chief uncertainties lay in the experimental values for neutron cross-sections -the probabilities that neutrons would strike a fissionable atom and either cause it to fission, be absorbed or be scattered -and neutron multiplication -the numbers of neutrons that would be produced in fission and cause other atoms to fission in a rapid chain reaction.
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. "Everyone seemed to be saying, well, that's all settled, let's talk about something interesting," Serber recalled.
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 made by Bethe before the war of energy production in stars. 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, just as elements fused in the sun produce light and heat.
Bethe was skeptical, and as Teller proposed scheme after scheme for a "superbomb," 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.