348 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 12 



filled chambers are among the most sensitive instruments for measuring 

 C 14 and H 3 , respectively. 



The second category of applications involves gamma radiation and fast 

 neutrons but may also include charged particles with very high energies such 

 as those encountered in cosmic rays and from high-energy accelerators. The 

 interpretation of the measured ionization in this case is not always obvious 

 since the chamber walls perform an important function in affecting the 

 observed intensity of ionization. 



When the range, or more correctly the half-value thickness, of the primary 

 radiation in the gas contained in the chamber is very much greater than the 

 chamber dimensions, the ionization produced and measured is, in the most 

 general case, the sum of the ionization produced in the gas volume by the 

 primary radiation plus that produced by recoil and scattered particles from 

 the walls. With chambers of wholly arbitrary dimensions and materials 

 it is difficult to assign values for the energy and flux of primary radiation from 

 the measured values of the ionization because it is nearly impossible to ascer- 

 tain what fraction of the ionization is contributed by secondary corpuscular 

 radiation produced in the walls. Only when the chamber has been calibrated 

 for a specific type of primary radiation and for a particular energy can the 

 indicated ionization be interpreted in terms of dose or as energy flux. 



The contribution of the walls to the observed ionization depends prin- 

 cipally on the following factors: 



1. Atomic number of wall materials. 



2. Thickness of the wall relative to the range of secondary corpuscular 

 radiation. 



3. Dimensions of the chamber relative to range of secondary corpuscular 

 radiation. 



4. Attenuation of primary radiation effected in the walls. 



5. In the case of neutrons, possible nuclear reactions. 



The only simple conclusion that can be drawn concerning these factors is 

 that the maximum wall effect is attained when the wall thickness equals the 

 range of secondary particles produced in it and the material is chosen to give 

 the highest yield of particles; for gamma rays these are materials possessing 

 the highest electron densities such as copper and lead, and for fast neutrons 

 these are hydrogenous substances. 



In two special cases the foregoing difficulties in evaluating the physical 

 significance of the measured ionization reduces to results that permit simple 

 interpretation. In the first case, consider a collimated beam of primary 

 radiation passing between but not striking the electrodes of a parallel plate 

 chamber. If the walls are placed at large distances compared to the range 

 of the secondary particles in the chamber gas, then the ionization produced 

 in the sensitive volume corresponds exactly to the energy absorbed in the 



