PRINCIPLES OF RADIOLOGICAL PHYSICS 95 



the radiative collisions become paramount before scattering becomes 

 unimportant. 



4-2c. Extremely High Energy: Predominance of Radiation Losses, Cas- 

 cade Shower Effects. The energy loss by X-ray production begins to 

 predominate over the dissipation by inelastic collision at an energy which 

 depends on the atomic number Z of the material and is of the order of 



lOyZ ev (34) 



according to Eq. (12), Sect. l-4b. Whenever most of the electron energy 

 goes into X rays, most of the X-ray energy goes into pair production, A 

 "cascade shower" of high-energy electrons, positrons, and X-ray photons 

 is then formed as described in Sect. 3-2. There remains to be seen how 

 the energy of a shower becomes distributed in successive layers of a 

 material. 



Scattering of the shower radiation in different directions is rather 

 unimportant, except at the lower energy end of showers in heavy mate- 

 rial. In the first place, electron deflection progresses slowly, as com- 

 pared to X-ray production, at all energies above approximately 20 Mev. 

 (The relative importance of electron scattering and X-ray production is 

 independent of the atomic number since both effects vary mainly in pro- 

 portion to Z".) Similarly, Compton scattering of the X rays is unimpor- 

 tant at high energies as compared to pair production; furthermore, the 

 photon energy drops to a low level in the unlikely event of Compton 

 scattering by a large angle (see Sects. 2-2a and 3-2). In the second place 

 the new particles and photons which are generated in a shower travel 

 approximately in the same direction as the generating radiation. 



As a result, particles or photons of energy E within a cascade shower 

 normally travel within a narrow cone whose aperture is of the order of 



2/E radians = 115/^ degrees (35) 



where the energy E is expressed in million electron volts. 



A shower dissipates its energy to the material traversed primarily 

 through the inelastic collision losses of the electrons and positrons. The 

 positrons behave essentially like the electrons, as will be discussed in 

 Sect. 4-2d. Since the rate of energy dissipation by high-energy electrons 

 or positrons is roughly independent of their energies and roughly equal to 

 1.5p Mev/cm [expression (33)], the rate of energy dissipation by the whole 

 shower equals approximately 1.5p times the number of particles which 

 traverse each particular layer of material. 



The number of particles in showers which develop under identical conditions is 

 subject to large statistical variations. The energy lost by electrons in radiative 

 collisions is evenly distributed, on the average, among photons of all energies up 

 to the total. However, any one electron may transfer most, or even all, of its 

 energy to a single photon, or it may distribute its energy among many smaller 



