314 BELL SYSTEM TECHNICAL JOURNAL 



radium A, radium B and so on. The "so on" covers more than it 

 did in the other two cases, this chain continuing to the terminus here 

 marked as radium G, though usually known by a different name. 



Surveying the scene of these massive radioactive nuclei, one is 

 struck by the fact that not all of the arrows are vertical. Many are 

 slanting, and by their slant and their length they show that they 

 represent the emission of alpha-particles. It is a feature of some (not 

 of all) of the unstable nuclei of mass-numbers greater than 200, that 

 they strive toward stability by emitting these. For this feature we 

 should be very grateful, since it was by the use of alpha-particles from 

 natural radioactive bodies that Rutherford achieved the first of 

 transmutations; though physicists now can transmute without their 

 aid, no one can guess how long they would have waited without trying 

 had they not had that encouragement. Vertical arrows also are seen, 

 but again there is a contrast to the lighter isotopes; all of the electrons 

 emitted by radioactive nuclei of mass-numbers beyond 200, or by 

 natural radioactive isotopes of whatever mass, are negative. But for 

 the fact that positive electrons had been observed among the cosmic 

 rays in 1932, they would have been discovered along with the first 

 examples of artificial radioactive isotopes in 1934, and what a sensation 

 that would have been ! 



More than by anything else, probably, one is impressed by the 

 concatenation of these radioactive nuclei. A long journey to stability 

 lies ahead of thorium A and actinium A, a longer one still ahead of 

 radium A; but the total lengths of the journeys are greater yet, for 

 they begin farther back. In Fig. 15 we behold the three series of 

 radioactive isotopes in their entirety, and it is seen that the three 

 "A-products," as they are called, are midway in the evolution and 

 not at its beginning. The manner of drawing of this figure is changed 

 from the preceding, atomic number being laid off along the horizontal 

 axis and mass-number along the vertical; also, crosses and circles and 

 dots are used to mark the members of the actinium, radium and 

 thorium series respectively, and have no bearing on stability or 

 instability. The actinium series should lie lower than it is drawn, 

 with its terminus AcD lying midway between ThD and RaG; the 

 mistake is incurred so as to diminish the overlappings which would 

 otherwise confuse the picture. 



Except for a few created in the last three years by transmutation, 

 every known nucleus-type of mass-number greater than 209 and 

 atomic number greater than 83 (as well as a few of slightly lower 

 values) is found in Fig. 15. It appears that 83 and 209 are critical 

 values of nuclear charge and mass, beyond which the constituents of 



