158 RADIATION BIOLOGY 



From the foregoing, it becomes clear that, under appropriate experi- 

 mental conditions, it is possible to recognize in the general expression (3) 

 the following relation:^ 



p™ = C<J<-< (10c) 



If the wall of the chamber is adequately thick and the attenuation of the 

 photon flux in it is neghgible, 



e™ = no'(o"« + o-m) in the medium 

 and (lOd) 



e^ = nl^<^a + T„) in the gas 



where aa = Compton absorption coefficient (per electron) 



Tm, Ty = photoclectric absorption coefficient (per electron in the 

 medium and in the gas, respectively) 

 rio" = number of electrons per unit mass in the medium 

 n% = number of electrons per unit mass in the gas 



It becomes possible, therefore, to measure both the energy flux F or 

 photon flux l^hv whenever the energy of the primary quanta is known 

 since Oa and r are well established by theory and experiment. The 



tion, more generally the following average should be considered, 



rhv 



j^m I <r{hv,Eo)Ea dEo 



if measurement of monochromatic radiation is involved; <r(hv,Eo) is the differential 

 electronic cross section for the production of a secondary electron of energy Eo by a 

 quantum of energy hv, namely: 



aihv,Eo) = 4.93 X io-25a-2(i - f/ag + .P/'2otY + P/^g) 



where a = hv/moC^;! = E^/hv; g = \ - /; S^{E) and «"(£') refer to stopping power 

 per electron of energy E in the gas and in the medium, respectively; and A'" and Nl 

 are the number of electrons per unit mass in the two media. The extent of the 

 variation of Pm with energy of the incident photon can be estimated from the variation 

 of B in Fig. 2-5 since essentially 



p„ = N^/NIB (10b) 



In the most general case, when both p„ and W are functions of energy and are for 

 heterogeneous radiation, the following expression holds, 



iVr y T" <^ihv,E)Eo dEo 



JF= '"^ 



I \ / cr{hv,E) dEo j 



dE 



where the summation extends to all photons. 



6 Comparison of Eqs. (10a), (lOcl, and (lOd) will show that for the case of a vanishing 

 cavity composed of wall and filled with gas, for both of which t « (r„, the ratio 6™/«2 

 equals the ratio of the integrals of expression (10a). 



