ARTIFICIAL TRANSMUTATION 89 



emitted when boron was bombarded with alpha particles. The charged 

 particles were electronic in nature but carried a positive charge. These 

 positive charged electrons had just previously been discovered by Ander- 

 son at the California Institute of Technology and named positrons by 

 him. The uncharged particles were shown to have protonic mass. 

 These neutral particles of protonic mass had been identified only a few 

 months before by Chadwick in England, who gave them the name 

 neutrons. 



The unusual part about the emissions discovered by the Curie-Joliots 

 was the continued positron emission even after the bombardment by 

 alpha particles had ceased. Thus the first artificial radioactivity had 

 been produced as a property of a disintegration product. 



The reaction equation for this experiment is shown in Fig. 11-14. 

 The alpha particle joins the boron nucleus, with the emission of a neu- 

 tron. An unstable nitrogen nucleus results, which in turn explodes 



a lle+ b B X0 > 7 N 13 + Neutron 



then 7 N' 3 > 6 C' 3 + +1 e°+ Energy 



Neutron 



Alpha particle ^ Resu|t 



Boron 

 stable nucleus ^^ y 



+i e Positron 

 (positive electron) 



[ N 13 r 

 \ 



Unstable (radioactive) Stable 



nucleus nucleus 



Fig. 11-14. The Curie-Joliot experiment. 



with the emission of a positron (+ie°) and the formation of a new stable 

 nucleus carbon, C 13 . 



The success of the Curie-Joliot experiment raised a question of funda- 

 mental importance. If the boron atom can be converted by alpha- 

 particle bombardment into a new radioactive element, what would 

 prevent other atoms from becoming radioactive if bombarded with 

 any form of high-speed atomic projectile? 



The alpha particles, emitted by RaC', used in the above experiments 

 possess kinetic energy of 1.23 X 10 -5 erg, corresponding with the energy 

 acquired by an electron in passing between two points in a vacuum 

 differing in potential by 7.66 million volts. It is obviously impracticable 

 to impart to a charged particle, by a single high-voltage operation, 

 energy comparable to that possessed by an alpha particle emitted by 

 natural radioactive elements. It is practicable, however, for a voltage 



