1044 Mr. Shenstone and Prof. Schlundt on the Number 



the coefficients of absorption given in the literature on the 

 subject apply only to thicknesses of lead greater than 2 cm., 

 and are far from the truth in the region below a centimetre. 

 Curves I. and II. of fig. 2 are the result of 7-ray measure- 

 ments on thorium and radium C sources when increasing 

 thicknesses of lead were placed between the source and the 

 electroscope. It has been seen that the ratio of ^-activities 



for equal 7-activities was computed as =- thorium C -=- s- 

 radium C. 



•0 











I 



'ig.2 













•9 

 •8 

 •7 

 -6 

 •5 

 •4 





































































\ \ Th.C 

 II Pa. C 





















III Pa. Standard. 



IV fiat/o Pa.C'Tf 



>.C. 













^ 



V " /Pa. St: P< 



VI » /?a.St:T> 



9.C. 















•? 



Mms. Lead. 



1 













3-3 4-3 63 8-3 10-3 



-y-Ray Absorption Curves. 



12-3 



i4-a 



T ■ ,, . -, ,. Xy radium G mi 



In this expression the ratio ~ .-, — ; Ti appears. There- 



r 2,y thorium C ri 



lore if, from curves I. and II., we plot curve IV., the ratio of 



7 radium C .., . 



-—r — ; ^ , that curve will oive us the factors bv which the 



7 thorium L>' ° J 



ratio of a-emissions for equal 7-activities will be changed 

 when comparisons are made of 7-activities between radium 

 and thorium C through greater thicknesses of lead. The 

 curve obtained in this way, by multiplying the ordinates of 

 curve IV. by *75, is given in fig. 3. 



The activity of a radioactive source is usually measured 

 by comparing its 7-activity with that of a radium standard. 

 The radiation of a radium standard consists in the main of 

 two 7-radiations — a soft radiation from radium B and the 

 hard radiation from radium C. The soft radiation is reduced 



