758 Prof. A. H. Oompton on the 



magnitude *. It seems premature to attempt any detailed 

 ■explanation of the failure of the usual electron theory until 

 more definite information is available with regard to the 

 wave-length of the hard gamma rays. 



The Characteristics of the Fluorescent Radiation. 



Let us now consider the properties or the fluorescent 

 radiation excited by the hard gamma rays. The observed 

 relative intensities of the secondary radiation from aluminium, 

 iron, and lead at different angles' with the primary beam, 

 when gamma rays from radium C filtered through a centi- 

 metre of lead are employed, are shown in Table IV. 



Table IV. 



Secondary Angle 



radiator. 30°. 45°. 60°. 75°. 90°. 120°. 135°. 150°. 



Al , ... 6-2 4-0 1-7 (1-0) 7 0-4 0-3 



Fe 10 7-6 4-6 2-0 (1-0) 0-6 0-5 04 



Pb Jl 68 4-3 2-2 (1-0) 08 ... 07 



In comparing the intensity of the scattered beam at any 

 two angles, i and 6 2 , the effect of absorption was eliminated 

 as completely as possible by the well-known method of 

 placing the radiating plate with its normal at an angle 

 {6i + 0^)l'2 with the primary beam. In this case the 

 absorption is the same at the two angles at which the 

 secondary radiation is compared. Since the window of 

 the ionization chamber consisted of 0*15 cm. of lead, any 

 soft fluorescent radiation was strongly absorbed. 



Data similar to those given in this table have been 

 published by Florance f and Kohlrausch J, except that in 

 the present case the effect of absorption by the radiator has 

 been largely eliminated, and the primary beam was rendered 

 homogeneous by filtering through a suitable lead screen §. 



* C. f. e. g. Hull & Rice, Phys. Rev. viii. p. 326 (1916). Barkla & 

 White, Phil. Mag. xxxiv. p. 277 (1917). 



+ D. C. H. Florance, Phil. Mag. xx. p. 921 (1910). 



t K. W. F. Kohlrausch, loc. cii. 



§ It should be noted that on account of the differing hardness of the 

 secondary radiation at different angles, the relative intensity observed at 

 a given angle depends upon the fraction of the radiation absorbed by 

 the ionization chamber. The ionization chamber used in the present 

 experiments was so designed that it absorbed a large part of even the 

 hard primary rays. This probably accounts for the fact that the writer's 

 experiments show relatively more intense radiation at the smaller angles 

 where the secondary rays are hard, than do the experiments of Florance 

 and Kohlrausch, whose ionization chambers presumably absorbed only a 

 small fraction of the incident radiation. 



