1912] Secondary Rays Excited by the Alpha Rays. 181 



ON THE SECONDARY RAYS EXCITED BY THE ALPHA RAYS 

 FROM POLONIUM— n. 



{Read 13th January, 1Q12). 



By V. E. Pound, M.A.* 



I . — Introduction. 



In a previous paper by the writer*^ it has been shewn that when 

 the a rays of polonium strike a carbon or a brass plate, secondary rays 

 are produced which are similar to the S rays of polonium. It has been 

 shewn, too, that a very considerable part of this secondary radiation is 

 due to the presence of gas occluded at or deposited on the surface of the 

 brass or carbon plate. The present paper describes some further ex- 

 periments on the influence of this gaseous layer on the secondary radi- 

 ation excited in different substances when these are subjected to bom- 

 bardment by the « rays of polonium. 



11. — Description of Apparatus. 

 The apparatus used for the investigation was composed of a brass 

 tube A about 3 cm. in diameter and 18 cm. long, one end of which was 

 closed air tight by an ebonite plug sealed in with wax. Centrally through 

 this plug was led a brass rod, B, carrying a brass plate C upon which 

 a thin circular piece of polonium coated brass about 1.5 cm. in diameter 

 was fixed. A brass ring D passed through the ebonite and connected 

 to earth served as a guard ring. At the other end of the tube A, a 

 smaller tube E, 1.5 cm. in diameter was inserted and soldered in position 

 at the top so as to be co-axial with the former. In this way the space 

 between the tubes A and E was made air tight. The lower end of the 

 tube E was closed by means of a very thin brass sheet and a flat circular 

 plate K of carbon or metal was held against this thin brass sheet by 

 means of a flanged collar F provided with a screw. The distance be- 

 tween the polonium and the lower face of the carbon or brass plate was 

 .6 cm. The whole apparatus was connected by a tube G to a Gaede 

 pump and a McLeod gauge. The apparatus was insulated by mica 

 and supported in such a way that the polonium and the circular plate 



*Presented by Prof. J. C.^McLennan, 

 *'Trans. Canadian Institute, 1912. 

 Phil. Mag-. 1912. 



