PRINCIPLES OF RADIOLOGICAL, PHYSICS 103 



Sect. 3-2, the photoelectric effect predominates for lower photon energies, 

 the Compton effect for intermediate, and pair production for the higher 

 energies. 



The scattering of X rays and the penetration of secondary electrons 

 complicate the study of the distribution of X-ray energy in different 

 portions of a material. The difficulties encountered resemble those met 

 in the study of electron penetration, but our present knowledge is bet- 

 ter regarding the penetration of X rays than that of electrons. 



4-3a. Law of Penetration for Monochrotnatic X Rays {^'Narrow Beam" 

 Penetration). Whereas charged particles have a good chance of experi- 

 encing minute deflections or minute energy losses as they traverse even a 

 very thin layer of matter. X-ray photons may traverse even a thick layer 

 of matter without being changed at all. The study of penetration 

 becomes much simpler if only that fraction is considered of the intensity 

 of an X-ray beam which traverses a layer of matter without experiencing 

 any change at all. Interaction with matter by photoelectric effect or pair 

 production leads to outright absorption of X-ray photons ; interaction by 

 Compton effect leads to deflection and to a change of photon energy. 

 We consider here the ideally simple process of attenuation of a mono- 

 chromatic X-ray beam, in which any change is regarded as a total loss. 



Each X-ray photon has a constant probability of being absorbed or 

 scattered in successive layers of equal thickness, no matter how much 

 material it has traversed previously. This condition is the same which 

 governs the occurrence of collisions of a particle along its track (except 

 that particle collisions are much more frequent). Therefore the con- 

 siderations developed in Sect. 3-6a and illustrated in Fig. 1-43 apply to 

 the penetration of X rays. The total intensity of a beam of mono- 

 chromatic X rays decreases in the course of penetration in proportion to 

 the probability of attaining a greater and greater depth without being 

 absorbed or scattered. 



Thus the intensity varies as a function of the depth of penetration x 

 according to a law having the same form as Eq. (27). The probability 

 for a photon to be absorbed or scattered per unit distance of penetration 

 in a material is usually indicated by the symbol /x and is called the 

 "absorption coefficient." This probability n corresponds to the prob- 

 ability of collision per unit length, in the discussion in Sect. 3-6a, which 

 is equal to the expected number of collisions v over a section of track of 

 unit length. Therefore the variation of intensity / as a function of x is 

 described by the formula 



I(x) = 7(0)6-"- (40) 



where 7(0) represents the intensity at the depth x = 0, i.e., the incident 

 intensity. 



The intensity may be expressed in any appropriate unit. From the 



