316 BELL SYSTEM TECHNICAL JOURNAL 



these numbers somewhat, but it seems fairly assured that the higher 

 values first appear later than the lower. Yet the lower and even the 

 lowest appear repeatedly all through the list; for the five most massive 

 atoms for which / is known, its values are 1/2 for TP*'^ TF''^ and Pb'"^ 

 9/2 for Bi-"^ and 3/2 for Pa-''^ There is no sign of a periodic variation ; 

 the random fashion in which the display or the lack of detectable 

 hyperfine structure are sprinkled among the elements would be 

 sufficient proof that it is of nuclear origin, even were there no theory. 

 We recognize thus that 7 is a quality of nuclei which depends upon 

 nuclear mass (not charge!) Moreover, everything connected with 

 the concept of nuclear angular momentum — the behavior of the 

 extranuclear momenta L and 5 and / of which it is an analogue, the 

 nature of the phenomena which it is contrived to explain, the values of 

 / deduced from these phenomena — all these things imply that / is a 

 quantum-vector compounded out of the individual angular momenta 

 of individual particles composing the nucleus, the quantum-numbers 

 of these individual momenta being, like that of their resultant, integer 

 multiples of 1/2. The very simplest model would consist, of course, 

 of particles all having identical angular momenta of quantum-number 



1/2. 



Now there are two leading schemes for imagining a nucleus — let 

 us say, of atomic number Z and mass-number A — as systems of 

 minuter particles. According to the one, it consists of A protons and 

 {A — Z) negative electrons; according to the other, of Z protons and 

 [A — Z) neutrons. 1' It would be correct to say that the former was 

 the leading scheme until about two years ago, the latter now; but as the 

 facts about nuclear momenta and nuclear magnetic moments have had 

 much to do with bringing on this change of favor, I will not take it for 

 granted in advance. 



If we take the nucleus to be a congeries of protons and negative 

 electrons, then we are postulating a system of which all the particles 

 are known to have spins of quantum-number 1/2 — the simplest 

 conceivable system, as I said; and we should still have in reserve the 

 possibility of assigning orbital motions with orbital angular momenta 

 to some or all of the particles, should it ever seem desirable. But if 

 we take the nucleus to be a congeries of protons and neutrons, we are 

 introducing particles of a kind for which the spin is unknown, and 

 must be fixed by assumption. Since we can put 1/2 for the spin of the 

 neutron, this affords little basis for choice. 



" The fact that alpha-particles are often mentioned as constituents of nuclei is not 

 in contradiction with this statement, since an alpha-particle is interpreted as 4 

 protons and 2 electrons by the one scheme, 2 protons and 2 neutrons by the other. 



