112 BIOLOGICAL EFFECTS OF RADIATION 



of the radiation with the matter in the enclosure. Remembering that a 

 secondary electron spends its energy in a relatively short distance (the 

 ionization locus) by ionizing the atoms in its path, it is evident that those 

 secondary electrons which are produced in the walls of the chamber and 

 then reach the enclosed air will contribute materially to the ionization 

 of the air. The same thing applies to the secondary electrons produced 

 by the interaction of photons and the air itself. On the other hand, 

 most of the secondary photons produced in the walls — or in the air — 

 contribute relatively little to the ionization which is being measured. 

 It is clear, therefore, that such a chamber does not measure, in general, 

 the ionization which would be produced by the radiation in the same 

 volume of air if the material walls of the chamber were not present. 

 From what has been said so far, it might be inferred that (on account 

 of the contribution of secondary electrons by the walls) the ionization in 

 the chamber is greater than it would be in free air. This, however, is not 

 necessarily so for reasons which will be apparent presently. 



The relative proportions of secondary electrons ionizing the air which 

 originate in the walls, and those which originate in the air itself, depend 

 on the geometrical factors of the chamber and the quality of the radiation. 

 (That the atomic composition of the materials of which the chamber is 

 made must play an important part is obvious. In our discussion we are 

 considering only ionization chambers which are made of organic materials 

 or such combinations of organic materials as will approximate the atomic 

 composition of air or living tissues.) When the radiation is soft, the 

 secondary electrons carry little energy and the ionization loci are very 

 short. Hence only those electrons which are liberated at or very near 

 the inner surface of the chamber walls reach the air within it. In 

 traversing this thickness of matter, the (soft) radiation loses considerable 

 energy and its intensity within the chamber is therefore much less than 

 it would be at the same point in the beam if the chamber walls were not 

 present. The secondary electrons liberated in the air by this weaker 

 radiation will be fewer than otherwise and hence the total ionization 

 produced in the air of the chamber may be too small in spite of the con- 

 tribution made by the wall electrons. When the radiation is hard, the 

 situation may be reversed and the measured ionization may be too high. 

 However, this is not so likely to happen, for a reason which deserves 

 special attention. 



X-rays of the hardness commonly employed in practice {i.e., X-rays 

 produced at 200 kv. peak and filtered by 0.5 mm. Cu) in their passage 

 through atmospheric air, liberate secondary electrons which may travel 

 several or even many centimeters before their energy is all utilized in 

 producing ions. Whether liberated by photoelectric or Compton encoun- 

 ters, the electrons which travel farthest are emitted initially in the same 



