Sec. 2.5] 



GAMMA RAYS 



41 



an electron transition to positive energy states where it is then observed 

 together with the "hole," or positron, as a pair of particles of identical mass 

 but opposite charge. The energy in excess of 2m c 2 appears as kinetic energy, 

 but it need not be shared equally by the two particles. When the positron 

 is brought to rest by the normal processes of energy loss from ionization and 

 radiation, it recombines with an electron, or more precisely, a positive 

 energy electron fills the unoccupied level, and two gamma photons are 

 emitted (annihilation radiation) each with a characteristic energy hv = m c 2 . 



The existence of such "holes" or positrons were first observed by Anderson 

 [8] in cosmic radiation. 



2.5. Secondary Particle Production. The total radiation observed at any 

 depth in an absorber consists of primary gamma rays together with their 



z 



UJ 



z 



DISTANCE 

 Fig. 4. Diagram indicating the change in secondary radiation intensity when gamma rays 

 pass through media of different electronic densities — in this case, from air to lead to air. 

 Intensity curves are not to scale. 



secondary radiation of electrons (photo-, Compton, and pairs) and x-rays 

 produced as a result of these electrons. Since the range of secondary elec- 

 trons in all absorbers is very small compared with the half -value thickness for 

 gamma rays, the number of recoil particles formed per unit time equals the 

 number absorbed when radiative equilibrium is reached between the primary 

 and secondary radiation intensity. The observed secondary intensity there- 

 fore decreases exponentially at the same rate as the gamma-ray intensity 

 although its actual absorption coefficient is very much greater. The relative 

 magnitudes of the intensity of the gamma-ray beam and its secondary elec- 

 trons when they are in equilibrium depends on the atomic number and density 



