PRINCIPLES OF RADIOLOGICAL PHYSICS 



37 



2-2a. Compton Scattering. Electromagnetic radiation exerts a rapidly 

 variable electric force on any charged particle and drives it to oscillate 

 according to the variation of the electric force. The forced motion of 

 the particle constitutes a variable current and therefore it acts as an 

 antenna which sends out radiation in all directions. Consequently, 

 energy from the incident radiation becomes diffused or "scattered" all 

 around in new directions. 



Nuclei, being much heavier than electrons, respond far less than elec- 

 trons to the driving force of the radiation. The scattering effect of 



^ 6 X to-" 

 in 



en 

 (n 

 o c 



tr p 



u 



UJ 



< 

 O 

 (J1 



< 



I- 



o 



0.01 



100 



1.0 10 



PHOTON ENERGY, Mev 



Fig. 1-24. Plot of the Compton scattering cross section for photons of different 

 energies. 



nuclei should be roughly a million times less intense than the scattering 

 due to electrons and has in fact not yet been observed. All the observed 

 scattering of electromagnetic radiation by free particles (until 1950) has 

 been attributable to electrons. 



The response of an electron to the driving force of radiation is limited 

 by the inertia of the particle, and, accordingly, it decreases as the radia- 

 tion frequency increases. On the other hand, a particle radiates energy 

 away the more effectively the more rapid the variation of its motion. 

 These two circumstances have compensating effects and tend to make the 

 net scattering effectiveness of an electron independent of the frequency of 

 the radiation (Fig. 1-24). ^ 



5 The probability of processes, such as Compton scattering, due to the interaction 

 of a beam of radiation with individual atoms or atomic particles within a material, is 

 represented conveniently as a "cross section" according to the following argument. 

 The frequency of processes which are observed experimentally or expected theoreti- 

 cally is proportional to the flux of particles or photons in the incident radiation and 

 to the density of atomic particles in the irradiated material. Therefore this frequency 

 equals the frequency with which a particle, or a photon, as the case may be, would 

 traverse a target area of suitable size attached to each of the pertinent atomic particles 

 (e.g., electrons) within the material. The area of such imaginary targets measures the 

 probability of the process under consideration and is called the "effective cross 

 section" or simply the "cross section" of the process. 



