[ 12<3 ] 



VI. The Absorption of the Different Types of Beta Rays, 

 together with a Study of the Secondary Rays excited by them. 

 By V. E. Pound, 31. A., University of Toronto*. 



[Plate II. j 



I. Introduction. 



IN a paper in the Phil. Mag. of July 1907, Prof. Mac- 

 kenzie 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 j3 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 ft rays fell, gradually increased in intensity 

 and reached a maximum value when a plate *2 mm. in 

 thickness was used. Wilh plates of still greater thickness, 

 this secondary radiation remained constant in intensity. He 

 obtained, however, an entirely different result on investi- 

 gating the secondary radiation from the back of the plate 

 upon which the /3 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 f investigated the 

 secondary radiation issuing from each side of the plates, first, 

 when both ft and 7 rays were allowed to fall on them, and, 

 second, when 7 rays alone were allowed to fall on the plates ; 

 and the results quoted by him, and ascribed by him to the 

 action of the ft rays, were obtained by subtracting the effects 

 due to the 7 rays alone from those due to the combined /3 

 and 7 radiations. 



With the arrangement he adopted, it was possible that in 

 cutting off the /3 rays in order to study the effect of the 

 7 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 /3 radiation a part of the secondary radiation 

 which properly should have been ascribed to the 7 rays. 



In view of the importance of his results in their relation 

 to theories of secondary radiation now being put forward by 

 Bragg J and others, it was thought well to make a more 

 extended examination of the secondary radiations excited by 

 both /3 and by 7 rays; and in the following paper an account 



* Communicated by Prof. J. C. McLennan. Read before the Royal 

 Society of Canada, May 16, 1908. 

 t Phil. Mag. July 1907. 

 t Phil. Mag-. May 1908. 



