CONTEMPORARY ADVANCES IN PHYSICS 117 



Detection and Measurement of Transmutation 



While thus the scope of transmutation has been so vastly extended 

 in the past eighteen months, there is one limit which has not yet been 

 passed. No product of transmutation has yet been detected by any 

 chemical means. Many a plate of metal has been bombarded with 

 protons or with alpha-particles, but no man has seen it change into a 

 plate of another metal, nor alter in any of its chemical properties; 

 many a tubeful of gas has been bombarded, but no man has observed 

 the qualities or the spectrum-lines of another gas appearing in the 

 content of the tube. All that is ever observed is an outpouring of 

 material particles from the piece of bombarded matter; particles of 

 such a nature, that they must come from the nuclei of the atoms. 

 One expects this statement to go out of date from one morning to 

 the next; but at the moment of this writing it is still as true as it 

 was in 1919 when Rutherford first disintegrated nuclei, and broader 

 in one respect only. From 1919 until 1932, one would have said 

 "charged particles"; but since the winter of 1932, it is known that 

 either charged particles or uncharged may be driven out of nuclei, 

 by the appropriate impacts. 



Thus there are two great experimental problems, and not one only; 

 beside the problem of producing the streams of bombarding corpuscles, 

 there is that of detecting and of recognizing the particles which fly 

 forth from the bombarded nuclei — the "fragments," I will say. There 

 is a grave objection to this term, and to the common name "disintegra- 

 tion" for the process. Both suggest a picture of the nucleus as a 

 structure of pre-existing pieces which the impact breaks apart and 

 scatters. This picture is surely incorrect, for there are cases in which 

 the fragments contain the susbtance of the impinging corpuscles. In 

 fact, if we define "fragment" — as we should — to include the part which 

 in most of the experiments does not escape from the target bulk, we 

 may say that this kind of case is frequent, and perhaps indeed that 

 there is no other kind ! Nevertheless we seem to be unable to get along 

 without the words "disintegration" and "fragment." 



For detecting protons and more massive fragments which are 

 charged, there are three methods. 



Tho: first method (A) is that of observing the scintillations, which fast 

 charged particles produce when they impinge on fluorescent screens. 

 This is the classic and historic method, by which were made the 

 earliest proofs of transmutation by impact of alpha-particles (which 

 I described at length in the earlier article) and also the earliest proof 

 of transmutation by protons. Of late years this method has been 

 largely displaced by the others. Few people outside of the Cavendish 



