Wave-Length of Hard Gamma Rays. 



715 



to that from another is practically independent of the angle *, 

 An examination was therefore made of the relative intensity 

 of the secondary radiation from copper and lead at angles 

 less than 30°. The experimental arrangement is shown in 

 fig. 2. A strong source of hard gamma rays S (usually about 



Fig. 2. 



— rV 



100 millicuries) is placed at a point on the axis of the 

 ionization chamber I, and a lead cylinder C is placed between 

 to cut off the primary gamma rays. The sample of copper 

 or lead under examination is in the form of a ring R supported 

 coaxially on the lead cylinder by a piece of cardboard. It is 

 clear .that all parts of the ring will scatter gamma rays into 

 the ionization chamber at approximately the same angle. 

 In order to secure the greatest possible intensity, the dimen- 

 sions of the apparatus were so adjusted that the maximum 

 differences in the angle 6 made with the primary beam by 

 the secondary rays entering the ionization chamber were 

 between 0'2 and 30/2. By this arrangement ionization due 

 to the secondary radiation from the ring R can be obtained 

 which is very considerably greater than that due to stray 

 rays from the walls of the room, etc. It was found con- 

 venient, however, to balance the stray radiation against 

 ionization produced by a small source S' of gamma rays in a 

 second chamber V . The secondary radiation was then 

 measured by the difference in the readings of the electro- 

 meter when the radiator R was in place and when removed. 

 The results of the experiments are shown in the following 

 table. The values of the observed ratios of the intensity of 



* Cf., e. </., M. Ishino, Phil. Mag. xxxiii. p. 140(1914); K. W. F. 

 Kohlrausch, Phys. Zeitschr. xxi. p. 193 (1920); A. 11. Compton, supra, 

 p. 749. 



