PRINCIPLES or RADIOLOGICAL PHYSICS 



39 



momentum (Fig. 1-26). The recoil is appreciable only when the photon 

 has sufficiently high energy, i.e., when it is an X-ray photon, and is 

 deflected by a sufficiently large angle. Otherwise, when the recoil is 

 weak, the scattering electron may not be regarded as free from its bonds 

 within an atom and no Compton effect is observed. 



The probability of scattering in different directions is indicated in 

 Fig. 1-27. More complete data and formulas are given, for example, by 

 Nelms (1953). 



8x10 



-26 



O 



UJ 

 CD 



CO 

 <Sl 



o 

 cc 

 o 



o 



CE 



< 



o 



IT) 



LU 



cn 



UJ 



u. 



Ll 



30 



150 



180 



60 90 120 



ANGLE, degrees 

 Fig. 1-27. Angular distribution of the Compton-scattered photons. Example: The 

 cross section for the scattering of a 1-Mev photon by an electron toward a l-cm^ 

 detector located at a distance of 10 cm and at an angle of 60° with respect to the 

 initial direction equals the product of the ordinate, 1.8 X 10~^* cm^ per unit solid 

 angle, and the solid angle 1/10^ = 0.01 covered by the detector, namely, 1.8 X 



10^28 cm2. 



2-2b. Pair Production. According to general experience, electromag- 

 netic radiation is not affected by the electric or magnetic forces prevailing 

 in the space in which the radiation propagates. However, a radically 

 new experience occurs when very-high-freciuency radiation, with photons 

 above 1 Mev, traverses the space within atoms which is the seat of strong 

 forces varying rapidly from point to point. 



The propagation of high-frequency X rays in the space surrounding a 

 charged particle leads to the simultaneous outright generation of an elec- 

 tron and a positron, i.e., of two particles, one bearing a negative and one 

 bearing a positive charge (Fig. 1-28). An X-ray photon disappears for 

 every electron-positron "pair" produced. The energy of the absorbed 

 photon ecjuals the combined energy of the end products of the process, 

 namely : (a) the kinetic energy of the electron and of the positron ; (b) the 



