

Secondary y Radiation. 427 



sufficient to interfere with the deduction just made. Conse- 

 quently we may say that Zn and Al both give out more 

 secondary emergence y radiation than does Pb, provided 

 of course that the quality of the radiation in the two cases 

 is not so different as to balance the effect. 



In order to find the amount and nature of the radiation 

 which proceeds from the sides of the conical hole, Pb domes 

 were placed as at //, //, or Pb cylinders as at i, i, fig. 1. 

 When the Jatter were used, their equivalent thickness was 

 found in terms of corresponding domes. A cylinder was 

 found to be approximately equivalent to a dome 1*5 times 

 its thickness. 



The results of experiments upon the radiation from the 

 sides of the cone are shown in fig. 4, curve A, in which 

 thicknesses of dome are shown horizontally and the corre- 

 sponding leaks per 30 sees, vertically. 



The curve A can be represented approximately by the 

 expression 500 -f- ol()e~ l ' 2hd . 



That the radiation comes from the sides of the cone can 

 be shown by placing at e (fig. 1) Pb plates in which circular 

 holes are cut so as not to intercept the main stream of y 

 radiation coming from the Ra. 



The results now obtained for different dome thicknesses 

 are shown by curve B, fig. 4, which it will be seen can 

 again be represented approximately by the expression 500 + 

 430<?- 1,25 ". 



It seems reasonable to suppose that besides the constant 

 normal leak in the ionization-chamber, secondary y radiation 

 from the sides of the conical hole can enter the chamber, 

 and that this radiation may for the purposes of correction 

 to which it is to be applied later on, be taken as equivalent 

 to a homogeneous radiation for which X=l*25. 



If now a Pb plug, 4 cm. deep, be placed in the conical 

 hole in the position shown by a, fig. 1 , so as to intercept 

 the main stream of y radiation, but still leave the sides 

 of the cone exposed to the ionization-chamber, and a set of 

 experiments similar to the above be performed, the results 

 shown in curve (J, fig. 4, are obtained. These again are 

 approximately represented by 500 + 1 70<? _1 ' 25r '. 



It is not surprising that the constant normal leak has not 

 been reduced to any appreciable extent, for it will be seen 

 from fig. I, that the plug would hardly intercept any y 

 radiation passing directly from the Ra to the ionization- 

 chamber. On the other hand, we obtain a rather important 

 result, for assuming, of course, that the interpretation put 

 upon these experiments is correct, even when the Ra is 



