100 BIOLOGICAL EFFECTS OF RADIATION 



the same.^ But (a) the total photonic energy is less than it was before 

 the beam entered the thin layer of material, and (6) most of the resultant 

 low-energy photons do not travel in the direction of the primary photons. 

 In fact, some travel in the opposite direction. Furthermore, these 

 photons transfer their energy to matter within a very short distance and 

 most of them cannot leave the thin layer of material in which they were 

 formed. 



In the first thin layer of the material there occur also transformations 

 of the Compton type. Here again the total number of photons at the 

 end is the same, but their energy is less since some was transmitted to 

 the secondary electrons ejected in the process. As in the case of the 

 secondary photoelectric photons, those produced in Compton encounters 

 may be emitted in any direction but with one very important limitation. 

 The direction of emission and the proportion of the primary-photon 

 energy carried by the secondary photon are governed by the laws of 

 elastic collisions and must therefore bear certain relations to the direction 

 of the primary photon and its energy content. The aggregate distribu- 

 tion of the secondary photons in this case is such that those of highest 

 energy travel in substantially the same direction as the primary photons, 

 those of lowest energy travel in the opposite direction, while those of 

 intermediate energy travel in appropriate directions between these two 

 extremes. In this case, most of the secondary photons are able to leave 

 the thin layer of material.^ However, only those emitted substantially 

 in the direction of the primary photons reach the next layer. Thus, 

 while the total number of photons produced in the first layer by photo- 

 electric or Compton encounters is practically the same as the number of 

 photons which have been transformed, the number^ which enters the 

 next layer is appreciably lower than the number of primary photons. 

 Since in addition some of the photons incident on the second layer have 

 less energy than the original ones, the intensity of radiation at this level 

 is still less. 



Similar transformations take place in the next thin layer. More of 

 the primary photons lose their identity by transferring their energy 

 partly to electrons and creating new photons of lower energy. Con- 

 tinuing in this way it is evident that at each step more and more of the 

 primary photons disappear and a larger proportion of the transmitted 

 radiation consists of lower-energy photons. At the same time the num- 

 ber of photons traveling in directions differing from that of the primary 

 beam increases. These photons consist of those which have been pro- 

 duced through photoelectric and Compton energy interchanges (largely 



^ This is not strictly true on account of the "Auger effect" (cf. K. K. Darrow). 

 " The actual proportion depends, of course, on the wave-length of the original 

 beam and the thickness of the layer under consideration. 



