Two associates of Soviet physicist Igor Kurchatov reported to the Physical Review in June 1940 that they had observed rare spontaneous fissioning in uranium. “The complete lack of any American response to the publication of the discovery,” writes the American physicist Herbert F. York, “was one of the factors which convinced the Russians that there must be a big secret project under way in the United States.”
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[An experiment] left the German project with two possible moderator materials: graphite and heavy water. In January a misleading measurement reduced that number to one. At Heidelberg Walther Bothe, an exceptional experimentalist who would eventually share a Nobel Prize with Max Born, measured the absorption cross section of carbon using a 3.6-foot sphere of high-quality graphite submerged in a tank of water. He found a cross section of 6.4 × 10−27 cm^2, more than twice Fermi’s value, and concluded that graphite, like ordinary water, would absorb too many neutrons to sustain a chain reaction in natural uranium. Von Halban and Kowarski, now at Cambridge and in contact with the MAUD Committee, similarly overestimated the carbon cross section — the graphite in both experiments was probably contaminated with neutron-absorbing impurities such as boron — but their work was eventually checked against Fermi’s. Bothe could make no such check. The previous fall Szilard had assaulted Fermi with another secrecy appeal:
“When [Fermi] finished his [carbon absorption] measurement the question of secrecy again came up. I went to his office and said that now that we had this value perhaps the value ought not to be made public. And this time Fermi really lost his temper; he really thought this was absurd. There was nothing much more I could say, but next time when I dropped in his office he told me that Pegram had come to see him, and Pegram thought that this value should not be published. From that point the secrecy was on.”
It was on just in time to prevent German researchers from pursuing a cheap, effective moderator. Bothe’s measurement ended German experiments on graphite.
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Leo Szilard was known by now throughout the American physics community as the leading apostle of secrecy in fission matters. To his mailbox, late in May 1940, came a puzzled note from a Princeton physicist, Louis A. Turner. Turner had written a Letter to the Editor of the Physical Review, a copy of which he enclosed.1365 It was entitled “Atomic energy from U238” and he wondered if it should be withheld from publication. “It seems as if it was wild enough speculation so that it could do no possible harm,” Turner told Szilard, “but that is for someone else to say.”
Turner had published a masterly twenty-nine-page review article on nuclear fission in the January Reviews of Modern Physics, citing nearly one hundred papers that had appeared since Hahn and Strassmann reported their discovery twelve months earlier; the number of papers indicates the impact of the discovery on physics and the rush of physicists to explore it. Turner had also noted the recent Nier/Columbia report confirming the attribution of slow-neutron fission to U235. (He could hardly have missed it; the New York Times and other newspapers publicized the story widely. He wrote Szilard irritably or ingenuously that he found it “a little difficult to figure out the guiding principle [of keeping fission research secret] in view of the recent ample publicity given to the separation of isotopes.”1368) His reading for the review article and the new Columbia measurements had stimulated him to further thought; the result was his Physical Review letter.
...Szilard… answered Turner’s letter on May 30… [and] told him “it might eventually turn out to be a very important contribution” — and proposed he keep it secret. Szilard saw beyond what Turner had seen. He saw that a fissile element bred in uranium could be chemically separated away: that the relatively easy and relatively inexpensive process of chemical separation could replace the horrendously difficult and expensive process of physical separation of isotopes as a way to a bomb.
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“Oppenheimer wanted me to be the associate director,” [I.I. Rabi] told an interviewer many years later. “I thought it over and turned him down. I said, ‘I’m very serious about this war. We could lose it with insufficient radar.’” The Columbia physicist thought radar more immediately important to the defense of his country than the distant prospect of an atomic bomb. Nor did he choose to work full time, he told Oppenheimer, to make “the culmination of three centuries of physics” a weapon of mass destruction. Oppenheimer responded that he would take “a different stand” if he thought the atomic bomb would serve as such a culmination. “To me it is primarily the development in time of war of a military weapon of some consequence.” Either Oppenheimer had not yet thought his way through to a more millenarian view of the new weapon’s implications or he chose to avoid discussing those implications with Rabi. He asked Rabi only to participate in an inaugural physics conference at Los Alamos in April 1943 and to help convince others, particularly Hans Bethe, to sign on. Eventually Rabi would come and go as a visiting consultant, one of the very few exceptions to Groves’ compartmentalization and isolation rules.
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Work toward an atomic bomb had begun in the USSR in 1939. A thirtysix-year-old nuclear physicist, Igor Kurchatov, the head of a major laboratory since his late twenties, alerted his government then to the possible military significance of nuclear fission. Kurchatov suspected that fission research might be under way already in Nazi Germany. Soviet physicists realized in 1940 that the United States must also be pursuing a program when the names of prominent physicists, chemists, metallurgists and mathematicians disappeared from international journals: secrecy itself gave the secret away.
More (#4) from The Making of the Atomic Bomb:
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