RADIOACTIVITY— RUTHERFORD IQQ 



ARTIFICIAL TRANSMUTATION 



We now come to a discovery, the proof of the artificial transmu- 

 tation of the elements, which has led to a wide extension of the field 

 of radioactivity. A few words should first be said of the theo- 

 retical aspects of this problem as they appeared to be in 1918. On 

 the nuclear theory, an atom could only be changed by altering in 

 some way the charge or mass of the nucleus, or both together. It 

 was known that the nucleus was of exceedingly minute dimensions 

 with a radius of the order 10"^^ cm and must be held together by 

 exceedingly powerful forces in order to prevent its spontaneous dis- 

 ruption. In order to transform an atom, it thus seemed clear that 

 a very concentrated source of energy must be brought to bear on the 

 individual nucleus. Actuated by these ideas, I began experiments 

 in 1917 to test whether the bombardment of light elements by ener- 

 getic a-particles might lead to the occasional transformation of a 

 nucleus as the consequence of a direct collision between the nuclei 

 concerned. It could be calculated that in such an encounter the 

 a-particle must come very close to the nucleus, even if it did not 

 penetrate its structure. Such a close approach must in any case give 

 rise to enormous forces between the a-particle and the nucleus which 

 might be expected to produce such a distortion of its structure as to 

 result in the disintegration of the nucleus. The scintillation method 

 was employed to detect whether any fast particles appeared under 

 such an intense a-particle bombardment. No effect was noticed for 

 carbon or oxygen, but a number of fast particles were observed in 

 the case of nitrogen, which were identified as swift hydrogen nuclei, 

 now known as protons. It seemed clear that these protons could 

 only arise as the result of the transformation of the nitrogen nucleus. 

 In the light of later results, the essential processes involved in this 

 transformation are now clear. Occasionally an a-particle actually 

 enters the nitrogen nucleus and forms a new atom like fluorine of 

 mass 18 and nuclear charge 9. This new nucleus is unstable and 

 instantly breaks up with explosive violence, hurling out a fast proton 

 and leaving behind a stable nucleus corresponding to a stable isotope 

 of oxygen of mass 17. On an average only one a-particle in 100,000 

 is effective in producing such a transformation. 



With the help of Chadwick, it was soon found that 12 of the light 

 elements could be transformed in a similar way with the emission 

 in each case of protons, but with different speeds and numbers. 

 Time does not allow me to discuss later important developments 

 which have shown that groups of protons of different velocities 

 are ejected from each element and which have proved that reso- 

 nance levels exist within the struck nucleus favoring the capture of 

 a-particles of definite speed. Further investigation led to a dis- 



