Nuclear Fission

T? VERY 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 elements 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 267 268 BELL SYSTEM TECHNICAL JOURNAL commonest kind of oxygen atom); charge appears as a subscript before the symbol, mass as a superscript after it: 90Th 232 ; 9iPa231; 234 92U ; 92U 235 ; 238 92U From this list I omit several very unstable isotopes of which we shall probably never be able to assemble enough to observe their fission.