158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



I have saved the most sensational item for the last. Not only is 

 fission caused by slow neutrons, but it produces fresh neutrons among 

 its many products. Could these fresh neutrons produce fission in 

 their turn? Presumably they could; we know of nothing to differ- 

 entiate them from other neutrons. Could they produce new fissions 

 and these in turn new fissions and so onward in geometrical progres- 

 sion, so that a whole massive piece of uranium might blow up in a 

 sudden explosion of unparalleled fury touched off by so seemingly in- 

 nocent an event as the entry of a single neutron ? 



This is perhaps the most important of the unsolved questions of 

 physics. Let us begin by asking after a certain necessary though not 

 sufficient condition. The fissions cannot proceed in geometrical pro- 

 gression, the explosion of the whole mass cannot occur, unless each 

 fission results (on the average) in more than one free neutron to 

 replace the one neutron which is consumed in producing it. Is it so ? 

 Well, the few people whose opinions are worth taking agree that it. 

 is. They do not agree well as to how many fresh neutrons there are 

 over and above one, but they do agree that there is an excess. 



With this as a basis, let us turn the question around. Why has not 

 the great explosion happened as yet, since there are neutrons enough 

 to achieve it? 



One reason apparently is, that the fresh neutrons are moving with 

 the wrong speeds when they are released. Fission is performed 

 mainly by very slow neutrons, while the new-born ones are very rapid. 

 But if the piece of uranium were very large, even the fresh neutrons 

 would be slowed down by their repeated collisions with nuclei; and 

 therefore those who are trying to make the explosion, or trying to 

 approach it without quite making it, are heaping up great masses of 

 uranium. If, however, the uranium is mixed with other elements — as 

 in nature it always has been — the neutrons are liable to be captured 

 and rendered harmless by the nuclei of these others. Therefore, the 

 next step is to purify the uranium. This would be easy enough were 

 it not that "purity" in this connection means something more stringent 

 than even chemical purity. Within the last few weeks it has been 

 proved that only one isotope of uranium is sensitive to slow neutrons, 

 and this is a rare one — fortunately, I feel like saying. One must 

 perform a process of isotope-separation in which the two isotopes 

 differ in mass by less than 2 percent, and one is more than a hundred 

 times as abundant as the other. Probably this will take a long time 

 in the doing. If and when it is done, shall we find that human artifice 

 has succeeded in removing or relaxing the last brake provided by 

 nature to impede the slide toward catastrophe? Perhaps not even 

 then, for the rare isotope of uranium may have ways of its own for 

 capturing neutrons and rendering them harmless before the most of 



