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Authors: Jim Baggott
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uranium had intensified in Germany since the outbreak of war, that nuclear research had been taken over by the German government and was being conducted in great secrecy. The implications were reasonably clear: whether they liked it or not, they were now locked in a race with the Nazis to build an atomic bomb. The letter also contained a threat: unless there was a change of policy, Szilard would publish his latest research on nuclear chain reactions in the open literature.
The letter was sent to Sachs on 7 March 1940. A week later, Sachs wrote of these new developments to Roosevelt, who called for a further meeting of the Advisory Committee. Progress was still painfully slow: the meeting was not scheduled until 27 April. Einstein was again invited, but again declined. At least the further letter to Roosevelt prompted the release of the $6,000 that had been promised.
By the time the meeting was held, Alfred Nier at the University of Minnesota and John Dunning at Columbia had gathered experimental evidence confirming that U-235 is indeed responsible for slow-neutron fission in uranium, vindicating Bohr and Wheeler’s original hypothesis. They had used tiny quantities of U-235 and U-238 obtained from uranium compounds of chlorine and bromine. They went on to conclude that a fission chain-reaction would not be possible without separation of U-235.
The opinion of the Advisory Committee was split. Briggs expressed doubts that a chain reaction would be possible in natural uranium. Sachs urged that they should nevertheless move ahead with experiments on the uranium–graphite reactor that Szilard had proposed. All agreed that they should wait for the results of measurements on neutron absorption by graphite.
The funds were transferred to Columbia University and used to purchase a quantity of purified graphite. Szilard had been careful to specify high levels of purity. At lunch with representatives of the National Carbon Company, Szilard had probed for details about likely impurities in commercially-available graphite. He specifically mentioned potential contaminants that would absorb neutrons and render meaningless anyattempts to measure neutron absorption by graphite itself. Half-jokingly, he said: ‘You wouldn’t put boron into your graphite, or would you?’
His visitors looked at each other in embarrassed silence. One of the principal uses of graphite is in the manufacture of electrodes for electric arcs, and boron is typically a component in the manufacturing process. Any graphite they supplied would therefore likely be contaminated. They agreed to supply a quantity of graphite manufactured using different methods, without the use of boron.
Four tons of graphite duly arrived at the Columbia laboratory in the form of carefully-wrapped bricks. The simple process of unwrapping and stacking the bricks in a neat pile was enough to give the researchers the appearance of coal miners. The results of the neutron absorption measurements were, however, strongly positive: graphite could indeed be used satisfactorily as a moderator. The idea of a nuclear reactor in the form of a uranium–graphite ‘pile’ took a critically important step towards becoming a reality. 2
Szilard urged Fermi not to publish the results of these experiments. The relationship between the two had to this point been quite tense, but now it reached breaking point. They were two quite different personalities. Szilard was a loner, always ready to challenge conventional wisdom and norms of behaviour, sometimes outrageously. Fermi was an out-and-out scientist, much more collaborative and polite, caring little for the world outside the domain of science. Szilard’s experiences had led him to be extremely wary of the world outside science, and he fervently believed that scientists had a duty to behave responsibly in matters likely to have a significant impact on this world. ‘Fermi and I had disagreed from the very start of our collaboration about every issue
The letter was sent to Sachs on 7 March 1940. A week later, Sachs wrote of these new developments to Roosevelt, who called for a further meeting of the Advisory Committee. Progress was still painfully slow: the meeting was not scheduled until 27 April. Einstein was again invited, but again declined. At least the further letter to Roosevelt prompted the release of the $6,000 that had been promised.
By the time the meeting was held, Alfred Nier at the University of Minnesota and John Dunning at Columbia had gathered experimental evidence confirming that U-235 is indeed responsible for slow-neutron fission in uranium, vindicating Bohr and Wheeler’s original hypothesis. They had used tiny quantities of U-235 and U-238 obtained from uranium compounds of chlorine and bromine. They went on to conclude that a fission chain-reaction would not be possible without separation of U-235.
The opinion of the Advisory Committee was split. Briggs expressed doubts that a chain reaction would be possible in natural uranium. Sachs urged that they should nevertheless move ahead with experiments on the uranium–graphite reactor that Szilard had proposed. All agreed that they should wait for the results of measurements on neutron absorption by graphite.
The funds were transferred to Columbia University and used to purchase a quantity of purified graphite. Szilard had been careful to specify high levels of purity. At lunch with representatives of the National Carbon Company, Szilard had probed for details about likely impurities in commercially-available graphite. He specifically mentioned potential contaminants that would absorb neutrons and render meaningless anyattempts to measure neutron absorption by graphite itself. Half-jokingly, he said: ‘You wouldn’t put boron into your graphite, or would you?’
His visitors looked at each other in embarrassed silence. One of the principal uses of graphite is in the manufacture of electrodes for electric arcs, and boron is typically a component in the manufacturing process. Any graphite they supplied would therefore likely be contaminated. They agreed to supply a quantity of graphite manufactured using different methods, without the use of boron.
Four tons of graphite duly arrived at the Columbia laboratory in the form of carefully-wrapped bricks. The simple process of unwrapping and stacking the bricks in a neat pile was enough to give the researchers the appearance of coal miners. The results of the neutron absorption measurements were, however, strongly positive: graphite could indeed be used satisfactorily as a moderator. The idea of a nuclear reactor in the form of a uranium–graphite ‘pile’ took a critically important step towards becoming a reality. 2
Szilard urged Fermi not to publish the results of these experiments. The relationship between the two had to this point been quite tense, but now it reached breaking point. They were two quite different personalities. Szilard was a loner, always ready to challenge conventional wisdom and norms of behaviour, sometimes outrageously. Fermi was an out-and-out scientist, much more collaborative and polite, caring little for the world outside the domain of science. Szilard’s experiences had led him to be extremely wary of the world outside science, and he fervently believed that scientists had a duty to behave responsibly in matters likely to have a significant impact on this world. ‘Fermi and I had disagreed from the very start of our collaboration about every issue
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