the (3 Rays from a Thick Layer of Radium E. 873 



the rays were homogeneous there would be no such change, 

 since for any strength o£ field the percentage change iu 

 ionization would be the same for the same absorbing screen. 



The change in the position of the maximum is quite 

 marked, being at Hp = 2,400 in curve a, and at Hp = 3,650 

 in curve e. 



It will be noticed that the rays which produced the 

 maximum ionization when no aluminium was placed under 

 the electroscope are practically all absorbed by a thickness 

 of '73 mm. Al, while for rays corresponding to the higher 

 fields appreciable quantities are still transmitted. 



In these experiments the bottom of the electroscope was 

 closed by a sheet of tinfoil so that the maximum point even 

 with no aluminium under the electroscope occurs at a rather 

 higher field than it would if no matter whatever were placed 

 in the path of the rays. An experiment was therefore made 

 with a dutch-metal leaf closing the bottom of the electroscope, 

 and the maximum point, which was carefully determined, was 

 found to occur for a value of Hp 2,200 gauss cm. 



This value is in very good agreement with those obtained 

 by Schmidt* 2,200 and Gray 2,300. Experiments made 

 with tin as absorbing medium gave the same type of 

 result. 



Absorption curves were taken for rays corresponding to 

 various field strengths and are shown in fig. 3, curves a, b 

 and d, in which the log of the ionization in arbitrary units is 

 plotted against the thickness of absorbing material. The 

 curves are for rays corresponding to Hp = 3,200, 2,400, and 

 1620 gauss cm. respectively. It will be seen that the 

 different pencils of rays have very different penetrating 

 powers, the initial absorption coefficients being 13*0, 22'2 f 

 and 62*5 cm. -1 for curves a, b, and d respectively. 



It is especially significant that bundles of rays forming 

 a large percentage of the whole can be separated out which 

 have initial absorption coefficients much less than that of the 

 ichole beam. If homogeneous rays were absorbed according 

 to an exponential law the absorption coefficient of the whole 

 beam would continually decrease, which is not found to be 

 the case. 



The source of radium E was a thick layer of radium D, 

 so that any soft rays are practically all absorbed in the 

 material. As a further precaution the active material was 

 covered with # 1 mm. of aluminium. 



* Schmidt, Phys. Zeit. viii. 1907, p. 361. 

 Phil. Mag. S. 6. Vol. 20. No. 119. Nov. 1910. 3 M 



