92 APPLIED RADIOACTIVITY 



and if H and R are equal to 10,000 oersteds* and 10 cm, respectively, 

 and the value of e/m for protons is used, it will be observed that the 

 protons found traveling in a semicircle of 10-cm radius are those which 

 possess energy equal to that produced by an electric field between two 

 points in a vacuum of nearly half a million volts. 



These very high-speed ions emerge at G, where the edge of the dee 

 is cut away. Here they are deflected by the oil-cooled copper deflection 

 plate G, which is maintained at a steady potential of 50,000 volts nega- 

 tive relative to the vacuum chamber, at earth potential. 



The ions are deflected into the target chamber T, where they can 

 either be used for the bombardment of a substance mounted as a target 

 or be passed through a thin metal window out of the vacuum chamber 

 into the air. 



Artificial Radioactivity 



The cyclotron is therefore a source which can supply controlled 

 charged-particle projectiles of enormous energy content. With these 

 projectiles it becomes possible to penetrate the nuclear barrier of many 

 atoms so that in their union they may enter into a reaction with each 

 other, and convert them into artificially radioactive bodies. 



Among the artificially radioactive processes, beta-ray radioactivity 

 is most common. This consists of the emission either of negative elec- 

 trons or of positrons. A second type of spontaneous transmutation 

 is electron capture, in which the atomic nucleus combines with and 

 destroys one of its atomic electrons. Finally, there is the so-called 

 isomeric transition, which involves only a shift in the configuration of 

 the neutrons and protons in the nucleus without any change in their 

 numbers. 



The experimental results obtained with the aid of high-speed deuteron 

 projectiles show that radioactive nuclei were formed in which neutrons 

 or alpha particles were emitted. In many cases it was found that the 

 neutron of the deutron was captured and the proton emitted with high 

 speed. This general type, the neutron-capture reaction, is illustrated 

 by the radiosodium and radiophosphorus reactions. 



Radiosodium may be obtained by bombarding sodium chloride by 

 a beam of high-speed deuterous from a cyclotron. The sodium atom 

 captures the high-speed deuteron, as illustrated in Fig. 11-16, with the 

 emission of a proton, and forms an unstable isotope nucleus, Na 24 . This 

 radiosodium disintegration product has a half-life of 14.8 hours (Van 



* Unit field intensity. The oersted (formerly called the gauss) is that field which 

 exerts a force of 1 dyne on unit magnetic pole. 



