Contemporary Advances in Physics, XXVIII 

 The Nucleus, Third Part * 



By KARL K. DARROW 



This article deals first with the newer knowledge of alpha-particle emis- 

 sion: that common and striking form of radioactivity, in which massive 

 atom-nuclei disintegrate of themselves, emitting helium nuclei (alpha- 

 particles) and also corpuscles of energy in the form of gamma-rays or high- 

 frequency light. There follows a description of the contemporary picture 

 of the atom-nucleus, in which this appears as a very small region of space 

 containing various charged particles, surrounded by a potential-barrier; and 

 the charged particles within, or those approaching from without, are by the 

 doctrine of quantum mechanics sometimes capable of traversing the barrier 

 even when they do not have sufficient energy to surmount it. The expo- 

 nential law of radioactivity — to wit, the fact that the choice between dis- 

 integration and survival, for any nucleus at any moment, seems to be alto- 

 gether a matter of pure chance — then appears not as a singularity of nuclei, 

 but as a manifestation of the general principle of quantum mechanics: the 

 principle that the underlying laws of nature are laws of probability. More- 

 over it is evident that transmutation of nuclei by impinging charged par- 

 ticles, instead of beginning suddenly at a high critical value of the energy of 

 these particles, should increase very gradually and smoothly with increasing 

 energy, and might be observed with energy- values so low as to be incompre- 

 hensible otherwise; and this agrees with experience. 



Diversity of Energies in Alpha- Particle Emission 



ON EVERYONE who studied radioactivity some twenty years 

 ago, there was impressed a certain theorem, an attractively 

 simple statement about the energy of alpha-particles: it was asserted 

 that all of these which are emitted by a single radioactive substance 

 come forth from their parent atoms with a single kinetic energy and a 

 single speed. When beams of these corpuscles were defined by slits 

 and deflected by fields for the purpose of measuring charge-to-mass 

 ratio, nothing clearly contradicting this assertion was observed: the 

 velocity-spectrum of the deflected corpuscles appeared to consist of a 

 single line. In studying the progression of alpha-particles across 

 dense matter, it was indeed observed that not all of those proceeding 

 from a single substance had sensibly the same range. It is, however, 

 to be expected that if two particles should start with equal energy 

 into a sheet of (let us say) air or mica, they would usually traverse 

 unequal distances before being stopped ; for the stopping of either would 



* This is the second and concluding section of "The Nucleus, Third Part," begun 

 in the July, 1934 Technical Journal. 



"The Nucleus, First Part" was published in the July, 1933 issue of the Bell Sys. 

 Tech. Jour. (12, pp. 288-330), and "The Nucleus, Second Part" in the January, 1934 

 issue (13, pp. 102-158). 



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