270 BELL SYSTEM TECHNICAL JOURNAL 



electron or a negative electron, or an alpha-particle; the corresponding 

 changes in Z are — 1, + 1, — 2, those in A are 0, 0, 4. 



Now it is high time that we get on to uranium. This, like thorium, 

 was one of the elements that Fermi exposed to slow neutrons, and to 

 which he observed that something was happening. As he continued 

 these researches, and as the great institutes of nuclear physics of Hahn 

 in Berlin and the Joliots in Paris followed suit, it became evident that a 

 great deal was happening. With nearly all of the other elements, there 

 occurred just one of the reactions which I symbolized above, or maybe 

 one reaction with some of the nuclei and another with others. Some- 

 times the reaction would lead to a stable nucleus-type; in such a case, 

 when the neutron-bombardment ceased all the excitement was in- 

 stantly over. Sometimes it would lead to a radioactive nucleus-type; 

 in such a case, the radioactivity would continue after the bombardment 

 ceased, but it would steadily die away to nothing, in accordance with 

 the well-known law. But with uranium and thorium there was a 

 swarm of radioactive products, so many that to this day they have not 

 all been separated and identified. Moreover, some of these were 

 descendants of others, for after the bombardment ceased they grew in 

 strength for a while before declining. All of them were emitters of 

 electrons, and the electrons (in every case in which their sign is known) 

 were negative. 



Owing to the theorem which has just been stated, it was taken for 

 granted that the immediate effect of the neutron entering the nucleus 

 of uranium was to provoke one of the three reactions which I lately 

 listed. Of these the one most commonly assumed was the reaction, 



the so-called "reaction of pure neutron -capture" — called pure, be- 

 cause no massive particle goes forth. I mention it here because it is 

 still believed in, and we shall meet it later. Uranium 239 is radioactive, 

 and some of the other radioactive products were believed to be direct 

 or indirect descendants of it. Well, every one of the radioactive sub- 

 stances resulting from the reaction or reactions U{n) — for so I will 

 symbolize them in general — is an emitter of negative electrons. There- 

 fore each has a greater positive charge on its nucleus than does its 

 predecessor; therefore by this theory, each descendant from U^^^ must 

 have a greater positive charge than the 92g of the uranium nucleus. 

 But uranium is the final element of the periodic table; therefore by this 

 theory the radioactive bodies in question had to be isotopes of new 

 elements beyond uranium. 



