GENERATION OF X-RAYS 5 



metal is brought continuously before the electron stream. This form 

 of target allows an exceedingly small effective focus, 1 mm or 2 mm 

 square, to be used in a tube operating at high electrical energy. 



Generation of X-Rays 



X-rays are produced by the sudden stoppage or deceleration of high- 

 speed electrons. In this process the very high kinetic energy of the 

 moving electrons is converted into radiant energy of very short wave- 

 length, while some of the energy due to the deceleration is converted 

 into heat. 



In modern practice, the generation of x-rays takes place in an evacu- 

 ated tube having two electrodes, one emitting electrons and the other 

 acting as a target upon which the electrons are projected by a high 

 difference of potential placed between the electron emitter and the 

 target. 



The electron emitter, in a commercial form of x-ray tube, is a spirally 

 or helically wound filament of tungsten wire, which is heated to any 

 desired temperature by means of a variable current from a 6-volt step- 

 down transformer usually connected to a commercial 110-volt alter- 

 nating-current source. Its operating temperature may be regulated 

 by a current control, which in turn controls the number of electrons 

 emitted. 



If the positive accelerating potential of the target is comparatively 

 small, the current flowing from filament to target will be less than that 

 calculable from Richardson's equation. This reduction is due to the 

 repulsion of these negative electrons in the region between filament and 

 target on the electrons coming out of the hot-wire filament. An electron 

 cloud is formed in the space between target and filament with its greatest 

 electron density just in front of the filament. This electron cloud is the 

 space charge. 



If the voltage of the target is very great, the electron current passing 

 between filament and target will be appreciably greater than that given 

 by Richardson's equation. This increase is due to the large potential 

 gradient at the surface of the filament which pulls electrons out of the 

 filament and adds them to the normal emission. This so-called field 

 emission may become greater than the thermionic emission calculable 

 from the Richardson equation. 



The highly accelerated electron stream is guided by a properly de- 

 signed electrical field to fall on a small area of the target. The interposed 

 target suddenly decelerates these electrons. Some lose their kinetic 

 energy by a head-on collision with an atom of the target so that at a 

 single encounter they give up all their kinetic energy. Under these 



