40 McCriettanp anp Hackert—The Absorption of B Radium Rays by Matter. 
We must carefully distinguish between these two coefficients ; and it is obvious 
that in studying the stopping power of different substances or different forms of 
atoms we should deal with the coefficient ~ and not with the different coefficient 
b/l—k, 
the value of which depends on the efficiency of the substance as a source of 
secondary radiation. 
The value of « for a large number of elements was determined in a 
previous paper* dealing with secondary radiation, so that all we require to 
do is to find the coefficient p,/1—« accurately. Experiments of the usual 
type dealing with absorption do not, however, give an accurate value of this 
coefficient. In the usual method of determining the coefficient of absorption, 
the intensity of a pencil of rays is measured, and then a plate of the substance 
under investigation is interposed, and the intensity again measured. The 
quantity ’ which satisfies the equation 
—Nd 
li — Roe : 
is then calculated, where &, is the intensity after traversing the plate of 
thickness d, R, is the incident radiation, and \ is called the coefficient of 
absorption. The quantity )’ refers to the total radiation, primary and 
secondary, but if is not the same as our coefficient 
p/l — kK, 
and in fact the value of \’ depends on the thickness of plate used even when 
the radiation 1s perfectly homogeneous. 
This is easily seen as follows :— 
We have as above 
— iW Poe Var = b2 
R= Ae UVa 4b A'e” a 5 
or writing \ for /@ — 0 
1 = Agrew? Le Avex. 
We can show that )’ differs from A, and that the difference depends on 
the thickness d of the plate used to absorb the rays. 
When z=0, wehave #,=A-+ A’; 
18s lgo 7 a A'(e” = mie) 
* Trans. Roy. Dublin Soc., vol. ix., part ii., 1906. 
