Contemporary Advances in Physics, XXVIII 

 The Nucleus, Third Part * 



By KARL K. DARROW 



Transmutation, the major subject of the Second Part of this sequence on 

 the nucleus, assumes again a leading role in the present article. Remark- 

 able cases have been discovered since the first of the year, including a great 

 number in which the impact of one nucleus upon another (or of a neutron 

 "on a nucleus) provokes an instantaneous transmutation which is followed 

 after seconds, minutes or hours by the spontaneous breaking-apart of one 

 of the resultant nuclei. One may say that these last are the nuclei of new 

 kinds of radioactive elements, and the phenomena are often called " induced 

 radioactivity"; but many of these new unstable elements differ from all 

 radioactive .bodies hitherto known in that they emit positive electrons. 

 Some additional examples of transmutation are described at the end of 

 this article. 



Induced Radioactivity 



UP to the end of last year (1933) it was taken for granted that 

 transmutation is practically instantaneous: that when two nuclei 

 collide, the ensuing fusion and disruption (if any there be) are ended 

 within a time inappreciably short. Nowadays, however, many cases 

 are being discovered, in which a disruption occurs a long time — 

 several minutes or even hours, possibly not for days — after the collision. 

 We must suppose that at the moment of the collision something 

 happens, which entails the eventual disruption. In a very few cases 

 we may be reasonably sure that this initial "something" is itself a 

 transmutation, resembling those previously known in that it is 

 instantaneous, but differing from them in that one of the resulting 

 fragments is an unstable nucleus, of which the eventual spontaneous 

 disruption is that which is observed. This may be the course of 

 events in all cases, but it is also conceivable that in the collision one 

 of the original nuclei may be put into an unstable state without the 

 occurrence of an initial transmutation. 



The first-to-be-known of these phenomena was discovered by M. 

 and Mme. Joliot at the very start of 1934, when they exposed samples 

 of aluminium (and boron and magnesium) to the bombardment of the 

 5.3-MEV alpha-particles from polonium, and after a few minutes of 

 exposure removed them from the bombarding beam and placed them 



* In this issue is published the first section of "The Nucleus, Third Part." The 

 paper will be concluded in the October, 1934 issue. 



"The Nucleus, F"irst 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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