Resonance in uranium and thorium disintegration and the phenomenon of nuclear fission. Offprint from: Physical Review, Vol. 55, No. 4, 15 February 1939 [letter dated 7 February].

[Lancaster, PA & New York, NY: American Institute of Physics for the American Physical Society, 1939.

First edition, extremely rare offprint issue, of arguably Bohr’s most important paper on nuclear fission, the first recognition of the crucial importance played by uranium-235, the only isotope found in nature capable of sustaining a chain reaction and the fissile material used in the first atomic bombs. “In the first week of [February 1939], in response to early experimental data from [the Carnegie Institute of Washington] showing uranium fission by ‘slow’ neutrons, Bohr suggest[ed that] the very rare uranium-235 (U235) isotope, which would fission under all circumstances, is solely responsible for slow-neutron fission, while the abundant uranium-238 (U238) isotope requires ‘fast’ neutrons of over 1 MeV in energy in order to fission. Because U238 generally scatters incident neutrons, most neutrons will lose energy until they have an energy matching the ‘absorption resonance’ energy of U238, at which point they will be absorbed and removed from the reaction. However, slow neutrons already below the absorption resonance will continue to scatter until they fission a U235 nucleus” ( OCLC lists Chicago only. No other copies at auction.

Working at the Kaiser Wilhelm Institute for Chemistry in Berlin, the nuclear chemists Otto Hahn and Fritz Strassmann found in December 1938 that when uranium nuclei are bombarded with neutrons, they seem to transmute into much lighter elements rather than nearby elements, as would be the case with most nuclei. They sent the results to their physicist colleague Lise Meitner, who was in Sweden having escaped Nazi persecution. She and her visiting nephew, Copenhagen physicist Otto Frisch, quickly developed a physical model of the nucleus-splitting, or ‘fission’, process and published it in Nature in February.

On January 16, 1939, Niels Bohr and Léon Rosenfeld arrived in New York from Copenhagen, having been given the news of the fission discovery just prior to their departure. Bohr had discussed the fission process with Rosenfeld on their journey, and he continued to think intensely about it after his arrival in the United States. This resulted in a further major step forward in the theoretical understanding of the process, reported in the offered paper.

“When, one day in early February [1939], Placzek came to Princeton, Bohr remarked to him that now all the confusion about transuranic elements belonged to the past. No, said Placzek, there is still a mystery: as a function of energy, neutron capture by U[ranium] shows a sizeable resonance peak at neutron energies of about 25 eV – the formation of the compound nucleus U239 found earlier by Hahn, Meitner and Strassmann. One must therefore expect – said Placzek – that fission, a decay mode of U239, should also show a peak at 25 eV – of which, however, there was no sign. That discussion took place over breakfast at the Princeton Club. When Bohr heard that he became restless. ‘Let us go to Fine Hall,’ he said.’ During the five minutes it took to get there, Bohr was silent, deep in thought. When he came to his office, joined by Placzek, Rosenfeld, and Wheeler, he said: ‘Now hear this, I have understood everything.’

“Bohr explained: the dominant U isotope with weight 238 has the resonance, but at that energy fission does not take place; the rare isotope of weight 235, making up only 0.7 per cent of natural U, becomes fissile at that energy but does not resonate.

“A short note to this effect was published on 15 February [the offered paper]. It is written in typical Bohr style: qualitative, no formulas whatever. It should be stressed that there existed as yet no experimental foundation for Bohr’s idea of the role of 235. In fact, at that time ‘very few physicists accepted Bohr’s explanation. Fermi in particular disagreed strongly.’ A direct test would necessitate the enrichment of 235 in a natural U sample which then should exhibit an enhanced rate of fission by slow neutrons. That confirmation came only in March 1940” (Pais, Niels Bohr’s Times, pp. 456-7). 

Bohr explained further in the present paper why it was that only uranium 235 underwent fission by slow neutrons, while uranium 238 and thorium 232 underwent fission only by fast neutrons. “When U238 absorbed a neutron it became a nucleus of odd mass number, U239. When Th232 absorbed a neutron it also became a nucleus of odd mass number, Th233. But when U235 absorbed a neutron it became a nucleus of even mass number, U236. And the vicissitudes of nuclear arrangements are such, as Fermi would explain one day in a lecture, that “changing from an odd number of neutrons to an even number of neutrons released one or two MeV.” Which meant that U238 had an inherent energetic advantage over its two competitors: it accrued energy toward fission simply by virtue of its change of mass; they did not” (Rhodes, The Making of the Atomic Bomb, p. 286).

“Bohr’s conclusion that the slow-neutron effect was due to the rare isotope, U235, with which Fermi and others disagreed, was, of course, of enormous practical importance, because it implied, on the one hand, that there was no danger of an explosive chain reaction from natural uranium, but that, on the other hand, separated U235 would easily sustain such a reaction. The argument between Bohr and Fermi could be definitely settled by experimental evidence. For a final decision between the effects of the two uranium isotopes one had to wait for samples enriched in one or other isotope and that was possible only later. Meanwhile any new experimental details about the fission process might give some further clue, and Bohr followed the progress of experiments with great interest” (Collected Papers 9, p. 67).

“The levels of argument that Bohr wove into a two-page paper are dizzying. In his own words, it all reduced to “allowing us to account both for the observed yield of the process concerned for thermal neutrons and for the absence of any appreciable effect for neutrons of somewhat higher velocities. For fast neutrons … because of the scarcity of the isotope concerned [U235] the fission yields will be much smaller than those obtained from neutron impacts on the abundant isotope [U238].” The details of Bohr’s analysis would be revised as further experimental data accumulated, but by the spring of 1939, general outlines of understanding of the response of different uranium isotopes to neutron bombardment and the prospects for a chain reaction were beginning to become clear, at least in principle” (Reed, The History and Science of the Manhattan Project, p. 87).

4to (267 x 200 mm), pp. 418-419. Single sheet, without binding, as issued (horizontal fold, part above fold browned, minor split, annotated #21 in blue crayon).

Item #5037

Price: $3,000.00

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