NUCLEAR FISSION 287 



the test of experiment. About all that the several answers have in 

 common is, that more neutrons emerge than are spent. Zinn and 

 Szilard say, two or three times as many; Anderson, Fermi and Szilard 

 say, between one and two; the Paris school, three or four. A yet 

 higher value (eight) published from Paris seems to comprise some 

 " tertiary " neutrons produced by the secondaries. 



But if every fission produces a fresh neutron to replace the one 

 which caused it, and then some extras in addition, must we not an- 

 ticipate a self-sustaining, nay even a self-amplifying effect? Must we 

 not fear, in fact, a cataclysmic explosion? 



Were anything of the sort to happen, we may take it for granted 

 that the world would know of it, though in all probability the experi- 

 menter would not himself survive to report it. Evidently then it has 

 not happened, and there must be a brake or brakes in Nature which 

 impede the slide toward the catastrophe, and have thus far averted it. 

 In other words, there must be ways in which neutrons are made harm- 

 less by some innocuous type of capture, before they ever produce a 

 fission. 



Some of these other ways are known already. If the uranium is 

 mixed with other elements — as, in Nature, it invariably is — the nuclei 

 of these can take up some of the neutrons. Whether the composite 

 nuclei so formed are stable or radioactive is in this connection not 

 important ; they give no neutrons out in exchange for the ones absorbed, 

 and so the chain is broken. But if all other elements are carefully 

 extracted, do any brakes remain? 



Two surely do, and one is the fact that the newborn neutrons are 

 rapid, and cannot be efficacious as agents of fission until they are 

 slowed down to thermal energies. In pure uranium the slowing-down 

 can only be extremely gradual, so unfavorable is the huge mass-ratio — 

 238 to 1 — for the energy-transfer in the elastic impacts. Yet if the 

 volume of purified metal were great enough, this brake would relax. 

 Thus the durability of small-size pieces of uranium made chemically 

 pure, well attested as it is, is not by itself a proof that much larger pieces 

 would be safe. Those who are trying to approach the catastrophe, 

 while hoping not to provoke it, are engaged in piling up uranium in 

 greater and greater masses. 



The other brake is supplied by the "reaction of pure neutron- 

 capture," which I mentioned on an early page (p. 270). Every now 

 and then, when a neutron enters a nucleus of uranium, the composite 

 nucleus finds itself able to live on without fissure. It survives for a 

 time, then emits a negative electron of energy trivial compared with 



