﻿the Rays producing Aurora Borealis. 213 



Bragg and Kleeman *, a-rays from a thin layer of radium 

 bromide in radioactive equilibrium with its disintegration 

 products, consist of a mixture of homogeneous groups corre- 

 sponding to the various steps of transformation which are 

 accompanied by the expulsion of a-rays. 



The way in which a-rays are absorbed by matter is very 

 characteristic for this type of radiation, and is very different 

 from the law of absorption of /3-rays. 



Suppose a bundle of a-particles strikes a homogeneous 

 layer of matter. The a-particle for the greatest length of 

 its path will penetrate the matter in nearly straight-lined 

 orbits, and only a comparatively small scattering takes 

 place. 



During the passage the velocity gradually diminishes, and 

 after having traversed a certain thickness of matter called 

 the range the a-particle loses its power of ionizing and of 

 producing photochemical effects, and stops. 



Br igg and Kleeman f have determined the range for 

 various groups of a-radiation, and from the knowledge of 

 the initial velocity we can find the relation between range 

 and velocity. H. Geiger % recently measured the velocity (r) 

 of the a- particle at different points of its path through 

 matter, and found the following simple relation 



v* = k(r-x), (1) 



where x is the thickness traversed and r is very nearly equal 

 to the range, and k is independent of x and v. If the 

 various a-rays merely differ with respect to velocity, the 

 relation between initial velocity Y^ and the range li in a 

 certain substance would be 



V 3 = £R (2) 



Bragg and Kleeman measured the relative amount of 

 ionization produced by a homogeneous pencil of a-rays per 

 unit length at various points along its path, and found the 

 ionization to increase as the velocity diminished, first slowly, 

 then more rapidly, and it assumed a very marked maximum 

 close up to the point where the rays are stepped. On the 

 assumption that the ionization (i) per unit length of path is 

 proportional to the loss of kinetic energy, Geiger finds from 



equation (1) that ?' 3 = — - 

 1 r — x 



* Phil. Mag. [6] x. p. 318 (1905). 



t Loc. cit. 



\ H. Geiger, Proc. Roy. Soc. A. lxxxiii. p. 505 (1010). 



