PRINCIPLES OF RADIOLOGICAL PHYSICS 



71 



energy electron (see Sect. 3-1) as well as by an X-ray photon (Fig. 

 1-44). 



When a high-energy electron or positron transfers a large share of its 

 energy to a newly emitted X-ray photon and then the photon produces a 



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 Fig. 1-42. (a) Schematic representation of the degradation of a 7-ray photon by 

 successive Compton scatterings and the eventual photoelectric effect. (6) Number 

 of electrons of various energies released in water, on the average, by every 7-ray 

 photon emitted by Co'^" (initial photon energy 1.17 or 1.33 Mev). The dotted line 

 gives the spectrum of Compton electrons, (c) Distribution of the energy of Co^" 7 

 rays in water among electrons of different energies produced by the Compton or the 

 photoelectric effect. {Courtesy L. V. Spencer and F. Stinson.) 



pair, this pair added to the initial particle gives a total of three particles. 

 By the time the pair is generated by the photon, the initial particle has 

 an even chance, on the average, of emitting still another photon. Thus 



