288 BELL SYSTEM TECHNICAL JOURNAL 



fission-energies, and relapses into permanent stability. This is espe- 

 cially likely to happen when the neutron-energy is about 10 ev. Sup- 

 pose then a volume of pure uranium so great, that the rapid neutrons 

 released within it can make collisions numerous enough to bring their 

 energies down to the thermal range where they are dangerous. Before 

 they reach this range they must pass successfully through that other 

 where they are liable to be disarmed — or put away in prison, rather. 

 This second brake does not diminish in its strength as the volume of 

 uranium is raised. 



Perhaps the second by itself is powerful enough to avert the ex- 

 plosion. In this case there is no danger of incurring the cataclysm by 

 piling up uranium, however pure. There remains however the chance 

 of separating the two isotopes 235 and 238, verifying that the fission 

 by thermal neutrons occurs only in the one and the reaction of pure 

 neutron -capture only in the other, and then accumulating the danger- 

 ous one by itself. Enough has just now been separated, as I said in an 

 earlier footnote, for the verification to begin. To separate enough for 

 the dangerous trial will take a good deal longer in the doing. After 

 it is done, there is yet another brake which may avert catastrophe. 

 When the cumulative processes begin, the heating of the metal may 

 and probably will so affect the energies of the neutrons, that their 

 efficiency for fissuring the nuclei will be greatly abated and so the 

 processes find a natural limit. Otherwise it is to be hoped that those 

 who build up great masses of sensitive uranium will recognize prelim- 

 inary signs that the danger-point is close, before they actually attain it. 



Literature 

 (Year always 1939 unless otherwise stated) 



Early Chemical Identifications of Fission-Products: Hahn and Strassmann, Nalur- 

 wiss. 27, passim; also Curie and Savitch, Jour, de Phys. 9, 355 (1938). 



Isolation of Fission-Products by their Spontaneous Egress from Bombarded 

 Uranium or Thorium: Joliot, C.R. 208, 341; Meitner and Frisch, Nature 143, 471; 

 McMillan, Phys. Rev. 55, 610; Segre, Phys. Rev. 55, 1104. 



Detection of Fission-Products in lonization-Chamber: Meitner and Frisch, Nature 

 143, 239, 276; Columbia school, Phys. Rev. 55, 511; Roberts, Meyer and Hafstad, 

 Phys. Rev. 55, 416; Green and Alvarez, Phys. Rev. 55, 417; Fowler and Dodson, ibid. 



Fission-Products Detected in Expansion-Chamber: Joliot, C.R. 208, 647; Corson 

 and Thornton, Phys. Rev. 55, 509. 



Distribution-in-Energy of Fission-Products: Booth, Dunning and Slack, Phys. Rev. 

 55, 981, 982, 1273; Kanner and Barschall, Phys. Rev. 57, 372 (1940). 



Further Chemical Studies of Fission-Products: Hahn and Strassmann, Naturwiss. 

 27, 451; Preuss. Akad. 1939, no. 12; Savitch, C.R. 208, 646; Thibaud and Moussa, 

 C.R. 208, 652; Heyn, Aten and Bakker, Nature 143, 517; Bretscher and Cook, 

 Nature 143, 559; Dodson and Fowler, Phys. Rev. 55, 880; Glasoe and Steigman, 

 Phys. Rev. 55, 982; Abelson, Phys. Rev. 56, 1. 



Decay-Curves for Totality of Fission-Products Unseparated : Bjerge, Brostrom 

 and Koch, Nature 143, 794. 



Identification of Iodine among Fission-Products by X-rays: Abelson, Phys. Rev. 

 55, 418; Feather and Bretscher, Nature 143, 516. 



