490 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 22 



resonant or a controlled-tank circuit. A generator is coupled to the circuit 

 through a secondary winding to compensate for losses in the iron and losses 

 due to ohmic resistance. 



A structure of laminated transformer iron is required for the yoke and 

 pole pieces in order to reduce excessive heating and losses by induced eddy 

 currents. Similarly, large metallic parts cannot be used in or near the region 

 of the gap since eddy currents induced in conductors in this region cause 

 sufficient alteration in the magnetic field to disturb the circulating electrons. 

 The vacuum chamber or doughnut, therefore, is constructed wholly of non- 

 conductive materials such as glass, quartz, or porcelain. 



22.2. Electron Injection and Beam Extraction. Electrons are injected into 

 the accelerating region from a source that is pulsed for a few microseconds 



Fig. 133. Electron paths in the betatron. .4, equilibrium orbit; B, tungsten target; C, 

 injector containing filament and electrostatic accelerating slit; D, electrons leaving injector 

 on oscillating orbits. 



at the proper instant in the magnetic-field cycle. Normally the electrons 

 are given an energy of 20 to 70 kev by electrostatic acceleration in the injector 

 in order to provide electrons with a velocity near that of light. A compara- 

 tively simple source is used consisting of a hot cathode for emitting electrons 

 and a slit anode through which a narrow beam of electrons is accelerated, as 

 shown in Fig. 133. 



Several possible mechanisms have been proposed for extraction of the 

 beam when the maximum energy is attained. In all cases, the beam is 

 deflected from the equilibrium radius either to smaller radii where it ulti- 

 mately impinges on a tungsten target to be converted into x-radiation, or to 

 larger radii to strike a target mounted on the rear of the injector. The 

 simplest scheme for extraction requires saturation of the pole pieces just 

 before the maximum flux through the orbit is reached. The flux conditions 

 are then no longer maintained; the electrons gain momentum faster than the 



