752 



Prof. A. H. Compton on the 



While it is possible to account in this manner for the 

 difference in penetrating power o£ the primary and secondary 

 radiation if a sufficiently heterogeneous primary beam is 

 postulated, it is clear that, as a result o£ scattering, there can 

 be no transformation of radiation of one frequency into 

 radiation of another frequency. That is, the scattered rays 

 can be no softer than the softest components of the primary 

 rays, and removal by filtering of the softer components of 

 the primary radiation must harden also the secondary 

 beam. 



An experimental method of determining the relative 

 amount of scattering and fluorescence has been applied to a 

 study of the secondary radiation excited by the hard gamma 

 ra} T s from radium C. In figure 1 is shown diagrammatically 

 the arrangement of the experiment. A source of hard 



Fig. 1. 



I 



• 



s 

 W 



1 



mmmt- 



l)l-]l|H 



gamma rays S excites secondary radiation in a block R, and 

 the intensity of this radiation is measured by an ionization 

 chamber I, which is screened by heavy lead blocks from the 

 direct beam of gamma rays, and which in the final experi- 

 ments is surrounded on four sides by about 4 cm. of lead to 

 keep out secondary radiation from the walls of the room. 

 The ionization current when the radiator R is removed, is 

 approximately balanced by the ionization current produced 

 by an adjustable source of gamma rays S in a second 

 chamber I'. The intensity of the secondary radiation is then 

 measured by the difference in the readings of the electro- 

 meter E when the radiator R is in place and when removed. 

 The test for the presence of fluorescent radiation was 

 made by comparing the intensity of the secondary radiation 

 when an absorption screen was placed alternately in position 

 A, in front of the source of gamma rays, and position B, in 



