754 Prof. A. H. Oompton on the 



increased. For wholly scattered radiation, as we have just 

 seen, the value of this ratio should remain approximately 

 unity for all thicknesses of the absorption screen. If the 

 secondary radiation is a mixture of the two types, it will be 

 seen that the ratio I'/I should approach, for large thicknesses 

 of the absorption screen, the constant value 



r/i=cj(c s +c t ), (i) 



where c s is the fraction of the primary beam scattered into 

 the ionization chamber with no absorption screen at B, and 

 fit is the corresponding fraction for the fluorescent radiation. 

 Thus by measuring the ratio of the intensity of the secondary 

 radiation when suitable absorption screens 'are placed altern- 

 ately in front of the ionization chamber and the source, it is 

 possible to determine the relative magnitude of the scattered 

 and the fluorescent radiation. 



The results of measurements of this ratio at three different 

 angles with the primary beam are shown in Table I. 



Table I. 



Thickness of 



Ratios I' 



/I for iron 



radiator. 



lead screen. 



45°. 



90°. 



t35°. 







1 



1 



1 



0-15 cm. 







0-45 



0-5 





0-30 



-0-02 



1-0 



052 



0-13 



-0-02 



2-0 



0-39 



0-02 





30 



0-26 







41 



0-20 





,, , 



At each angle the measurements were continued until the 

 intensity was too low for accurate determinations of I'/I* 

 The probable error of the final measurements of the ratio at 

 135° was about O02, at 90° about 03, and at 45° about 

 0*04. On the basis of the above discussion, \\ e may therefore 

 conclude that for gamma rays which have traversed several 

 centimetres of lead the secondary radiation at angles 

 greater than 90° is, except for the small probable error, all 

 of the fluorescent type. 



At 45° it appears that the value of the ratio F/I is 

 approaching a constant value for large thicknesses of the 

 absorption screen. The limiting value of this ratio would 

 seem to be of the order of 5 or 10 per cent., which, according 

 to expression (1), would represent approximately the fraction 

 of the secondary radiation at this angle which is due to true 

 scattering. It is clear, in view of the magnitude of the 



