IONIZATION AND BIOLOGICAL EFFECTS 113 



general direction followed by the primary photons. In their subsequent 

 motion through the air some of these fast electrons are deviated and may 

 travel eventually in the opposite direction. There are also Compton 

 electrons (of lower speed) which are emitted more or less sideways with 

 respect to the line of motion of the primary photons. Accordingly, one 

 may picture a beam of hard X-rays passing through air as being accom- 

 panied by a swarm of electrons darting in all directions, in a region which 

 extends beyond the confines of the beam itself. It should be remem- 

 bered, of course, that at any one time there are more electrons traveling 

 forward than backward and that the former carry most of the energy which 

 subsequently appears as ionization. By analogy with the case of second- 

 ary photons previously discussed, it is evident that after the radiation 

 has traversed a certain thickness of air, a sort of equilibrium has been 

 established between the primary photons and the secondary electrons. 

 This thickness depends on the quality of the radiation and is greater the 

 harder the radiation. The actual value is rather difficult to determine 

 in the case of very hard X-rays and gamma rays. Obviously, it must be 

 closely related to the path of the secondary electrons, but this is so long 

 for some of them that in the same thickness of air the secondary photons 

 will also liberate an appreciable proportion of electrons. One must then 

 take into account also the equilibrium between primary and secondary 

 photons and between the latter and their secondary electrons. To make 

 the picture more concrete let us suppose that we can take an instantaneous 

 photograph of the photons and high-speed electrons in a beam of radia- 

 tion passing through air. In 1 cm.^ of air we would then find a certain 

 number of: (a) primary photons, (6) secondary photons, and (c) beta 

 particles. ^^ The number of primary photons per cubic centimeter (in 

 the picture) decreases with the distance from the source, due to the 

 inverse square law, as already explained. Therefore it is necessary to 

 consider the relative numbers of the three energy carriers in 1 cm.' at 

 different distances from the source. Let us assume that (by means of a 

 strong magnetic field) all electrons have been eliminated at a certain 

 cross section of the beam. At this point, then, our photograph will show 

 only photons in the cubic centimeter of air, and for simplicity we shall 

 assume that they are primary photons. A short distance beyond this 

 cross section, there will be in the cubic centimeter of air in addition to 

 photons, some beta particles, but their number is small in comparison 

 with the number of photons. From this point on the number of beta 

 particles per primary photon increases with the distance, rapidly at first 

 and then slowly. The number of secondary photons per primary photon 



1* This term is used here instead of "secondary electrons" because some of the 

 high-speed electrons are produced by secondary photons and might therefore be 

 considered to be "tertiary electrons." 



