PRINCIPLES OF RADIOLOGICAL PHYSICS 



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l-4b. Production of X Rays. The internal electrons of atoms radiate 

 X rays of definite frequencies when they are set in oscillating motion. 

 These frequencies are characteristic for each kind of atom and are much 

 higher for heavy than for light atoms because higher nuclear charges 

 hold the internal electrons more rigidly (see Sect. 2-la). The correspond- 

 ing photon energies range up to 120 kev. Oscillating motions of internal 

 electrons arise when an internal atomic electron is knocked off by the 

 impact of a fast electron or other particle. 



Atomic nuclei also emit monochromatic X rays, or y rays, of character- 

 istic frequencies determined by their internal oscillating currents. This 

 happens only when the nuclei Ijave 

 been excited by a collision or as the 

 consequence of a spontaneous disinte- 

 gration. The monochromatic y rays 

 obtained in this way usually have very 

 low intensity. The highest photon 

 energies are released in the formation of 

 He^ and Be^ nuclei and amount to ap- 

 proximately 20 and 17.6 Mev, respec- 

 tively. They are obtained by bom- 

 barding with protons the unstable 

 isotope of hydrogen, H.^, or the stable 

 isotope of lithium, Li'. 



Whether these monochromatic X rays 

 originate from internal atomic electrons 

 or from nuclei, the primary agent is 

 always a high-energy corpuscular radia- 

 tion. On the other hand, whenever 

 high-energy electrons strike a target material, X rays are also produced 

 directly by the electrons through an entirely different mechanism and in 

 much larger amounts, at least if the material has a high atomic weight. 



In the course of their flight through the atoms the electrons experience 

 sudden accelerations as they find themselves momentarily attracted by 

 the atomic nuclei. Such a jerky motion of charged particles constitutes 

 the source of pulsed, irregular, rapidly variable electromagnetic disturb- 

 ances, namely, the X rays. This is actually the main practical mecha- 

 nism of X-ray production. X rays produced in this manner are often 

 called Bremsstrahlung. 



A typical X-ray tube is shown in Fig. 1-15. Electrons emitted by the 

 filament are attracted by the positive pole which is usually called the 

 anode or "target." Potential differences up to several milHon volts may 

 be applied to tubes of this general type. 



Most of the kinetic energy acquired by the electrons in flight is turned into 

 heat. Water or oil cooling of the anode is generally employed to allow for 



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 2000 6000 10,000 14,000 



WAVE LENGTH, A 



Fig. 1-14. Spectrum of light from 



a mercury lamp. Notice the lines 



and the continuous spectrum. 



(Glasser, 1944; Forsylhe el al., 1930.) 



