Section III., 1908. [ S3 ] Trains. R. S. C. 



YI. — The Absorption of the Different Types of Beta Rays together with, 

 a Study of the Secondary Bays Excited by them. 



By V. E. Pound, M.A., University of Toronto. 



(Communicated by Prof. J. C. McLennan and read May 26, 1908.) 



I. Introduction. In a paper in the Phil. Mag., of July, 1907, 

 Prof. MacKenzie gives an account of some observations which he made 

 on the secondary radiation issuing from each side of plates of lead 

 upon which a pencil of /? rays was allowed to fall. Using plates of 

 increasing thickness, he found that the secondary radiation issuing 

 from the side of the plate upon which the /3 rays fell, gradually in- 

 creased in intensity and reached a maximum value when a plate .2 mms. 

 in thickness was used. With plates of still greater thickness, this 

 secondary radiation remained constant in intensity. He obtained, 

 however, an entirely different result on investigating the secondary 

 radiation from the back of the plate upon which the /? radiation was 

 allowed to fall. Under these conditions the secondary radiation fell 

 off very slowly as the thickness of the plates increased, and was still 

 quite measurable with plates of lead 15 mms. in thickness. 



In arriving at these results, MacKenzie' investigated the secondary 

 radiation issuing from each side of the plates, first, when both /3 and 



Y rays were allowed to fall on them, and second, when y rays alone 

 were allowed to fall on the plates, and the results quoted by him, and 

 ascribed by him to the action of the /? rays were obtained by sub- 

 tracting the effects due to the y rays alone from those due to the 

 combined /5 and y radiations. 



With the arrangement he adopted it was possible that in cutting 

 off the /i rays in order to study the effect of the y radiation alone he 

 also cut off a greater proportion of the latter than he estimated. If 

 this were so it would result in ascribing to the /? radiation a part of the 

 secondary radiation, which properly should have been ascribed to the 



Y rays. 



In view of the importance of his results in their relation to theories 

 of secondary radiation now being put forward by Bragg^ and others, 

 it was thought well to make a more extended examination of the 

 secondary radiations excited by both ^ and by y rays, and in the follow- 

 ing paper an account is given of some experiments in which the 

 secondary radiation both from the back and the front of metal plates 

 was studied, when these were traversed by y rays alone, and also when 

 pencils of /5 rays of different types were allowed to fall on them. 



1 Phil. Mag., July 1907. 



2 Phil. Mag., May 1908. 



