492 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 22 



larger instruments, is greater than 10 km. This is readily accomplished with 

 wedge-shaped pole faces giving a magnetic field that is bowed outwardly over 

 the region of the electron orbits, as shown in Fig. 134. 



The amount of focusing that can be provided is limited by the accompany- 

 ing radial decrease in magnetic field since the concurrent stability to radial 



C H 



Fig. 134. Schematic diagram of betatron construction. .4, electron injector; B, "dough- 

 nut" vacuum chamber; C, auxiliary coil for extracting beam; D, equilibrium orbit position; 

 E, magnet yoke; F, flux path; G, pole faces, wedge-shaped for focusing; 77, magnet ener- 

 gizing coils; I, bowed magnetic field in region of orbit showing horizontal component 

 necessary for focusing. 



oscillations is possible only when the restraining force of the magnetic field 

 falls off with radius less rapidly than the centrifugal force on the electron. 

 The magnetic field, therefore, must decrease with radius at a rate less than 

 1/V, or in general, stable, focused orbits are obtainable when 



H 



1 



< n < 1 



The defocusing effects of space charge and scattering are strongest at low 

 electron velocities when, fortunately, magnetic focusing is most effective. 



REFERENCES FOR CHAP. 22 



1. Kerst, D. W.-.Phys. Rev., 58, 841 (1940). 



2. Kerst, D. VJ.-.Phys. Rev., 60, 47 (1941). 



3. Kerst, D. W.: Rev. Sci. Instruments, 13, 387 (1942). 



4. Kerst, D. W., and R. Serber: Phys. Rev., 60, 53 (1941). 



