Sec. 12.2] IONIZATION CHAMBERS 349 



gas. If the gas is air, the average energy absorbed per ion pair is known for 

 various radiations and the total energy absorbed in the sensitive gas volume 

 is readily calculated. 



The second special type of ionization chamber provides a means for 

 measuring the amount of energy absorbed from the primary radiation in 

 traversing solid substances and therefore is of the most profound importance 

 to dosimetry and to the absolute measurement of energy flux. For this 

 reason it is described here in somewhat greater detail. 



It has been shown by Gray [8,9,13] that for a small air-cavity surrounded 

 by a medium M the energy absorbed in M per unit volume per second from 

 the primary radiation is related to the ionization observed in the cavity by 

 the expression 



E — SJW ergs or mev/cc/sec 



where E = energy absorbed per unit volume per second in medium M (wall 

 material) 

 S = ratio of stopping power for secondary particles in medium M to 



stopping power of the gas 

 W = average energy absorbed from secondary particles to form one 



ion pair in the gas (see Sec. 3.9) 

 / = number of ion pairs formed in the cavity per unit volume per 

 second 

 The quantity S is the ratio of energy loss per unit length of path in the 

 medium M to that in the gas. This may be expressed most conveniently in 

 terms of the atomic stopping powers B and densities p of the two media. 

 Thus, 5 = p m B m /p g B g . In many instances the values of B are not known 

 with certainty, but if the atomic composition of the gas can be made similar 

 to that of M, then B m = B g and 5 is given directly as the ratio of only the 

 densities. 



The simple but important relation above is valid only when the cavity 

 ionization chamber meets the following physical conditions: 



1. The dimensions of the cavity must be smaller than the average range 

 of the secondary corpuscular radiation in the gas contained in the cavity. 



2. The wall thickness of material M surrounding the cavity must be equal 

 to or greater than the maximum range of the secondary particles in M. 



3. The primary radiation should not be appreciably attenuated in trav- 

 ersing the chamber. 



4. The relative stopping power must be independent of the velocity of the 

 secondary particles. 



It is apparent from these conditions that for the equation to be valid the 

 intensity of secondary corpuscular radiation passing through the cavity 

 must exactly equal its intensity within the wall material when it is in radiative 

 equilibrium with the penetrating primary component. 



