PRINCIPLES OF RADIOLOGICAL PHYSICS 



11 



l-2b. Acceleration of Charged Particles. 

 accelerating charged particles is to subject 

 them to the electric force between two con- 

 ductors with a sufficiently large potential 

 difference. For example, if a source of singly 

 ionized atoms lies at an electric potential 1 

 million volts above that of a target and if the 

 ions can move freely, their kinetic energy 

 increases to 1 million electron volts (Mev) 

 by the time they reach the target. This 

 method serves, for example, to provide beams 

 of high-energy cathode rays in the ordinary 

 cathode ray or X-ray tubes (Fig. 1-6). The 

 main limitation to this method lies in the 

 technical difficulty of generating and main- 

 taining potential differences as large as 

 might be desired. 



Large potential differences are produced by a 



suitable arrangement of a-c transformers followed by rectify- 

 ing stages, or by electrostatic generators (Van de GraafE 

 machines) . The main component of an electrostatic gener- 

 ator is a mechanically driven insulator belt which picks up 

 electric charges at one point and deposits them onto a high- 

 voltage conductor (see Fig. 1-7) . Multimillion-volt accelera- 

 tors are normally enclosed in high-pressure tanks which 

 make it easier to maintain a high voltage without discharge 

 through the surrounding space. The highest performance 

 yet attained by this method is a steady potential difference of 

 approximately 12 million volts. 



In the acceleration of charged particles the voltage differ- 

 ence should be distributed in such a way that the particles 

 follow paths concentrated in a narrow beam. Figure 1-8 

 shows the usual method for achieving this in the construc- 

 tion of very-high-voltage accelerator tubes. The voltage 

 differences between successive metallic sections are about 

 equal. 



In order to achieve very high kinetic energies without 

 having to produce correspondingly high voltage differences, 

 the acceleration of the particles can be subdivided in a large 

 number of steps, each step involving only a moderately 

 high potential difference. For example, a group of particles 

 traveUng along the axis of a series of cylindrical electrodes 

 (see Fig. l-9b) can receive one step of acceleration at each 

 gap between successive electrodes. For this purpose a 



^-N 



+ 



+ 



III 



+ 



Fig. 1-7. Diagram 

 of a Van de Graaff 

 generator. The 

 belt accumulated 

 positive charges on 

 the upper conduc- 

 tor and carries 

 negative charges 

 down to the 

 ground. (Richt- 

 ■myer and Kennard, 

 1947.) 



set heed not be maintained. The potential difference at each gap may oscillate 



steady, very high potential difference distributed along the 



