Nuclear Fission 



By KARL K. DARROW 



This article pertains to the most newly-discovered and most 

 sensational mode of transmutation, in which the entry of a neutron 

 into a massive atom-nucleus brings about an internal explosion in 

 which the nucleus is " fissured " or divided into two fragments which 

 share the total mass and charge between them in nearly equal pro- 

 portions. (In all other modes of transmutation except those 

 affecting the very lightest elements, the division is into fragments 

 of very unequal mass and charge.) The conversion of rest-mass 

 into kinetic energy, or (as is more commonly said) the release of 

 energy, is unprecedented in scale. A multitude of radioactive 

 bodies, many hitherto unknown, is formed; and there is spontaneous 

 emission of fresh neutrons in great quantities, possibly sufficient to 

 convert the process once initiated into a self-perpetuating one under 

 realizable conditions. 



EVERY now and then a physicist is liable to receive a letter from 

 some yearbook or other, in which he is invited to write x thousand 

 words on the "most important developments in physics during the 

 year just ending." The only safe reply is of course that for ten years 

 at least and perhaps for fifty it will be impossible to tell which is the 

 most important development in physics during the year just ending. 

 This year, however, it looks as though one need not be so cautious; for 

 ever since the first few weeks of the year many have felt pretty sure 

 that one particular discovery would long be recognized as the most 

 important to be made, or at any rate to be revealed, in 1939. It came 

 early — the first publication was on the sixth of January, and there was 

 a rain or perhaps I should say a deluge of others before the end of 

 February. Inasmuch as these others proceeded from laboratories 

 sprinkled all of the way from Copenhagen to Berkeley, it is literally 

 true for once that a discovery commanded immediate attention. Nor 

 is attention even yet diverted, though the pace of publication has 

 grown less. 



The phenomena of fission are as yet confined to the last three ele- 

 ments of the periodic table: thorium, protactinium, uranium. I show 

 their chemical symbols, their atomic numbers or nuclear charges, and 

 the mass-numbers — to wit, the nearest integers to the actual values 

 of the masses — of their several isotopes (charges expressed of course 

 as multiples of e, masses as multiples of one-sixteenth the mass of the 



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