PRINCIPLES OF RADIOLOGICAL PHYSICS 



41 



atomic number than in the proximity of nuclei of low atomic number or 

 of electrons (Figs. 1-29 and 1-30). 



The production of pairs is confined to the immediate proximity of the 

 charged particles if the frequency of the incident radiation barely exceeds 

 the minimum required value. The fraction of space within each atom 

 where pairs may arise increases progressively as the X-ray frequency and 

 the photon energy increase, until it includes the whole atom. 



Pair production is reversible; a positive and a negative electron may 

 neutralize, or "annihilate," each other in the space surrounding another 



10 X 10"25 



UJ 



•Z. c 

 OUJ " 

 d -J - 



G< < 



aoco 



cncrt c^ 



o 



<r)\- If) 



W^ CD 



cc -* 



FRACTION OF TOTAL KINETIC ENERGY 



GIVEN TO NEGATIVE ELECTRON 



0.8 0.6 0.4 0.2 



Pb 

 4 X I0'2' 



0.2 0.4 0.6 0.8 



FRACTION OF TOTAL KINETIC ENERGY 

 GIVEN TO POSITIVE ELECTRON 



1.0 







Fig. 1-30. Cross section for the production of pairs with positron and electron energies 

 in a specified range. Example: The shaded area, which is equal to the product of a 

 0.05 range of energy fraction and a mean ordinate of 2.3 X 10"^* cm^, measures the 

 cross section 1.15 X lO'^^ cm^ for the production of a pair by a 5-Mev photon in the 

 space surrounding an aluminum nucleus with 30 to 35 per cent of the available kinetic 

 energy (5 - 2mc^ = 4 Mev) imparted to the positron and the remaining 65 to 70 

 per cent (namely 2.6 to 2.8 Mev) imparted to the electron. For data on lead use 

 the right-hand scale. For data on other materials expand or contract the scale in 

 proportion to Z^. (Courtesy G. R. White.) 



charge and thereby irradiate all their energy in the form of a high-energy 

 photon. More frequently, however, a positive electron ends its life by 

 combining with a negative electron through a somewhat different process 

 which does not require the proximity of another charge. This process 

 releases the energy of the two particles by the simultaneous emission of 

 two photons. 



Pair production may also result, with low probability, from the collision 

 of two charged particles. For further details on pair production see, for 

 example, Heitler (1944), Sect. 20. 



2-2c. Elastic Collision of Charged Particles ("Rutherford Scattering''). 

 When an electrically charged particle of a corpuscular radiation 

 approaches another charged particle within a material, the electrical 

 attraction or repulsion tends to deflect the incident particle off its path 



