70 



RADIATION BIOLOGY 



production depends greatly upon the energy of the X-ray photons and 

 upon the atomic number of the material traversed. The photoelectric 

 effect predominates greatly at lower energies, the Compton effect at 

 intermediate ones, and pair production at higher energies. Data on this 

 subject are shown in Fig. 1-41. Figure l-42a through c shows a diagram 



g 



I- 

 a. 



CE 



o 



(/> 



CD 



< 



O 



u. 

 o 



LlJ 



< 



o 

 q: 



a. 



100 



80 



60 



40 



20 



0.01 



100 

 80 

 60 

 40 

 20 



0.1 1.0 10 



PHOTON ENERGY. Mev 



100 



0.01 



100 



0.1 1.0 10 



PHOTON ENERGY. Mev 



Fig. 1-41. Relative probability of different effects for X rays of all energies in carbon 

 and lead. (Courtesy G. R. White.) 



of a chain of degradation processes and gives results on the over-all effects 

 of degradation of Co^" y rays in water. 



When the photoelectric effect is most important, the energy of each 

 X-ray photon is primarily distributed by a single photoelectron. When 

 the Compton effect is most important, each X-ray photon produces in 

 succession a series of secondary electrons. 



At high photon energies, where pair production is most important, the 

 electron and the positron which arise by pair production from one photon 

 are likely to lose most of their energy in the production of more X rays. 

 If these X rays have a still sufficiently high photon energy, they produce 

 more pairs. The process may go on and on: X rays make pairs, the pairs 

 make more X rays, the X rays make more pairs, . . . (see Fig. 1-43). 

 The group of particles and photons which results from this process is 

 called a "shower," or, more specifically, a "multiplicative shower" or 

 "cascade shower." A cascade shower can be started by a single high- 



