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XLIIL On the Absorption of the ft and y Rays of Actinium. 

 By T. Godlewski, Ph.D. {Cracow)*. 



THE numerous investigations o£ the absorption of j3 rays 

 emitted by thorium and radium have established the 

 fact that the j3 rays of these elements are complex in their 

 character and consist of electrons projected with a wide range 

 of velocities. It has been shown that their penetrating 

 power does not remain constant but increases with the thick- 

 ness of matter traversed. Consequently, the absorption does 

 not follow an exponential law, such as is shown by the more 

 homogeneous rays of uranium. In the case of radium and 

 thorium, the value of the absorption constant is dependent on 

 the thickness of the matter traversed. For instance, the 

 following values of the absorption constant X for aluminium 

 were found by Eve f for the /3 rays from radium : — 



Thickness in mms. 



X cm.- 



to -85 



6-5 



•85 „ 2-5 



5-7 



2-5 „ 4-1 



1-6 



As we see, the value of \ varies in a marked manner with 

 the thickness of matter previously traversed. 



In my previous paper on actinium, I pointed out that the 

 activity of actinium, measured by the (3 rays, decreases 

 accurately according to an exponential law with the thickness 

 of traversed matter. Thus, in this respect, the j3 rays of 

 actinium are distinguished from the (3 rays of other elements. 



The measurements were made in the following way : — 

 About 2 grams of emanating substance of Giesel (of activity 

 about 300 times that of uranium) were spread uniformly on 

 the bottom of a dish (7 cms. in diameter). The dish was 

 covered with a very thin plate of mica, which was sealed 

 with wax, in order to prevent the escape of the emanation. 



The measurements of activity were made by means of 

 sensitive electroscope of the type of G. T. R. Wilson. The 

 bottom of the electroscope was removed and replaced by an 

 aluminium plate, 0*08 mm. thick, which absorbed all the 

 a rays. The dish, containing the actinium, was placed under 

 the electroscope and covered with the plates of metal whose 

 absorption power was to be examined. 



* Communicated by Prof. E. Rutherford, F.R.S. 

 t Phil. Mag-. Dec. 1904. 



