Section III, 1922 



[113] 



Trans. R.S.C. 



The Absorption and Effective Range of the ^-Rays from Radium E 



By Miss A. V. Douglas, M.Sc. 



Presented by J. A. Gray, F.R.S.C. 



(Read May Meeting, 1922) 



Introduction 



It was originally supposed that the /3-rays emitted from some 

 radioactive source, such as Ra.E., were homogeneous, that is, of a 

 definite velocity. When absorption curves were first taken it was 

 natural to try a law of the exponential type, / = 7oe~*"' where /o 

 is the initial intensity of the radiation, I the intensity of the rays 

 transmitted through an absorbing plate of thickness x, and ^u the 

 coefficient of absorption. The intensity is not directly measurable, 

 but the assumption is made that it is proportional to the ionization 

 which is produced in an electroscope. The experimental procedure 

 is as follows: 



The active material is placed at A 

 and the ionization is measured by the. 

 rate of fall of the gold-leaf in the electro- 

 scope E (Fig. 1). The absorbing plate B, 

 of thickness x, is placed in the position as 

 shown, and the ionization as before. The 

 ionization, and hence the intensity, is 

 found to decrease as x is increased. To 

 understand the nature of the absorption 

 it is necessary to determine the relation 

 between / and x. 



From the equation given above, it follows that: 

 dl= — jxlQe~^^ dx = —fj. I dx 



Fig. 1 



or 



J' 



■ jj, dx. 



dl 



Hence for equal increments of thickness dx the ratio ~j is constant. 



A further relation is obtained thus : log /+m x = log Jo. Hence if 

 16^ / be plotted against x a straight line curve should result. 



Also if /r, /s, /t, etc., represent the relative intensities of rays 

 transmitted through thicknesses Xo, Xq+x^, Xc + 2 x^, etc., then the 



8— c 



