156 BELL SYSTEM TECHNICAL JOURNAL 



To guess at the total number of neutrons emitted (say) from beryl- 

 lium it is necessary to know the excitation-curve and to make an esti- 

 mate of the factor aforesaid. I confine myself to quoting from Chad- 

 wick: "The greatest effect is given by beryllium, where the yield is 

 probably about 30 neutrons for every million alpha-particles of polonium 

 which fall on a thick layer," 



Transmutation with production of positive electrons 



This mode of transmutation, as I mentioned earlier, has been ob- 

 served by the Joliots with Be, B and Al, the primary corpuscles being 

 polonium alpha-particles. Nothing has yet been published about 

 distribution-in-range or disintegration-function. Positive electrons 

 of energy as high as 3.1 MEV have been observed proceeding from 

 aluminium. 



Aluminium thus affords a case of an atom which under alpha-particle 

 bombardment may emit from its nucleus a particle of any of three 

 kinds: a proton, a neutron, a positive electron. It has been suggested 

 by Joliot that there is actually only one process, in which a proton 

 emerges either intact, or else split into a neutron and a positive electron 

 which are its hypothetical components. If this can be verified it will 

 have important bearings on various fundamental questions, including 

 that of the mass of the neutron.^*' Boron also emits particles of all 

 three kinds, but here the situation is complicated by the possibility 

 that not all of the three proceed from the same isotope. 



Transmutation by Neutrons 



Transmutation by neutrons has been observed only with the Wilson 

 chamber, and therefore rarely: there are a few scores of recorded cases, 

 the fruit of twenty or thirty thousand separate photographs taken 

 some by Feather at the Cavendish, some by Harkins and his colleagues 

 at Chicago. What is observed is a pair of tracks diverging from a 

 point in the midst of the gas contained in the chamber; it is inferred 

 that the (invisible) path of a neutron extends from the neutron-source 

 to the point of the divergence, and that the observed tracks are those 

 of two fragments of a nucleus which that particle has struck. "Frag- 

 ment ' ' must be taken in the generalized sense of page 117: the substance 

 of the neutron may be comprised in either or both of the two. Each 

 case must be separately analyzed, taking into account the directions 

 and the ranges of the fragments (it is here that the question of the 

 range-z^5-energy relations of massive nuclei, footnote 20, becomes 

 crucial). It is possible to infer that in many cases the neutron is ab- 



^^ See the reference in Footnote 27. 



