Energy of a particles from Radioactive Substances. Ill 



determined by Bragg and Kleeman*. Levin f has deter- 

 mined the range of the particles from polonium (radium F), 

 while Hahn % has investigated in detail the ranges of the 

 a particles expelled by the various products of thorium and 

 actinium. 



The velocities of the u particles in a homogeneous source of 

 rays are all reduced by the same definite amount in passing 

 through a material screen. As a result of the variation of 

 velocity of the a particles from radium C in traversing matter, 

 I have shown § that the velocity V of the a particle having 

 a range r cms. in air after passing through a screen is 



given by 



V/V = -348 v/V+ 1-25, 



where Y is the initial velocity of the « particles emitted 

 from radium G which have a range in air of 7*06 cms. 



This relation has been confirmed by direct measurement 

 of the velocities of the a particles from the various a ray 

 products which have different ranges of ionization. The 

 value of Y has been determined and shown to be 2*06 X 10 s * 

 cms. per second. Consequently the velocity V of an oc 

 particle which is able to ionize air for a distance of r cms. is 

 <>iven by 



V='717y'V + l"25 X 10 9 cms. per second. 



This is based on the assumption, which has been verified 

 in a number of instances, that the ex. particles from the a ray 

 products of the various radioelements have an identical mass. 



The initial velocity of expulsion of the « particle from each 

 a ray product is thus a definite constant which, like its- 

 period of transformation, serves to distinguish it from all 

 other products. 



The velocities of the « particles emitted from the various 

 products are collected in the table given below (p. 112). The 

 period of transformation of the product and the range of its 

 a particles in air are added for comparison. The fourth column 

 gives the kinetic energy of the a particle at the moment 

 of its expulsion. Since the kinetic energy \mv 2 is equal to 



. e. and the value of - — ■ is determined directly from 



2 e e 



the known values of u and e/m. the kinetic energy is expressed 



in terms of e. the charge (in electromagnetic units) carried 



* Phil. Mag. Sept. 1905. 

 f Amer. Journ. Sci. July 1906. 

 X Phil. Mag. July and Sept. 1906. 

 $ Phil. Mau-. An jr. 190G. 



