350 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 12 



The significant properties of chambers for which the formula and condi- 

 tions above are valid are [9] (1) for sufficiently small chambers the ionization 

 per unit volume is independent of the size of the cavity; (2) the ionization 

 is directly proportional to the gas pressure; (3) for the same primary radiation 

 the ionization in chambers with different wall materials is proportional to 

 the energy absorbed per unit volume of wall material and inversely propor- 

 tional to its stopping power. 



It is apparent that size is a distinguishing and crucial feature of the cavity 

 chamber, but it is not always clear just how small the dimensions must be 

 since the minimum range of recoil electrons in the case of gamma rays, 

 and recoil protons from neutrons, is essentially zero. In principle the size 

 should be "infinitesimal," but in practice cavity dimensions of the order of 

 millimeters are usually permissible without introducing appreciable error. 

 In any actual application this can be tested with cavities of progressively 

 larger dimensions. The largest cavity that still indicates a constant ioniza- 

 tion per unit volume can be regarded as sufficiently small for that particular 

 wall material and radiation. 



The exactness of the cavity formula above is also influenced by two factors 

 in addition to finite cavity size. These are (1) the ionization produced in 

 the gas by the primary radiation directly is, in general, not equal to that 

 which would be produced in an equivalent volume of wall material, and 

 (2) the presence of the cavity alters slightly the energy and directional dis- 

 tribution of the secondary corpuscular radiation. These factors are only 

 significant when the constituents of the wall material and gas differ markedly 

 in atomic number, such as in the combination of air and lead. In practice 

 their influence can be ascertained by observing the ionization as a function 

 of pressure. If pressure and ionization are proportional, the cavity formula 

 can be assumed to be valid. The most accurate results are to be expected 

 with wall material and gas of the same atomic composition — a combination 

 that is sometimes difficult to achieve. 



In applications of the cavity chamber to problems of dosimetry the wall 

 materials usually consists of compositions containing several kinds of atoms 

 in proportions similar to the composition of tissue, and in addition, the 

 primary radiation frequently is heteroenergetic. If these conditions are 

 to be taken into account explicitly, the cavity formula, though still valid, is 

 modified [10] into the form 



n 



E = $iV \ fiji<Ji ev/cc/sec 



i = \ 



where $ = flux of primary radiation, ev per cm 2 per sec 

 N = number of atoms per cc of wall material 



