﻿92 



Properties of the a Hays of Radiotliorium. 



air of the a particles from the emanation, i£ determined by 

 the electrical method, would, consequently, be about 5'2 + 0'3 

 or 5'5 centims. 



In the previous paper I determined the ranges of the a rays 

 from thorium B and thorium C to be 4' 7 and 8' 6 centims. 

 respectively. The range 4*7 centims. was deduced by direct 

 inspection of the ionization-distance curve o£ the active 

 deposit of thorium. Since, however, the curve due to the 

 rays from thorium C slopes towards the origin at the point 

 distant 4*7 centims., the range 4*7 centims. for thorium B is 

 too small and is more probably 5*0 centims. 



The ranges of the a particles from the five a ray products 

 of thorium are collected in the following table. For com- 

 parison, the ranges of the a particles from the radium products 

 determined by Bragg and Kleeman are also added : — 





Eange of the a particles 







Product. 



in centims. in air at 



atmospheric pressure 



and temperature. 



Product. 



Range of the 



a particles. 



Thorium 



ray less. 

 3"9 centims. 



Radium 



3'50 centims. 



Radiotliorium ... 



Emanation .. 



4-23 „ 



Thorium X 



5"7 ,, 



Radium A... 



4-83 „ 



Emanation 



5-5 



B... 



only (3 ra vs. 



Thorium A 



rayless. 



„ c... 



7-06 



Thorium B 



5"0 centims. 



D... 



rayless. 



Thorium 



8-6 



„ E... 



only (3 rays. 







„ F... 



3-86 * 



* The range of the « particles from radium F has recently been determined 

 by Dr. Levin in this laboratory. 



The results obtained in this investigation of the a rays from 

 thorium are in complete harmony with the theory of absorp- 

 tion of the a rays advanced by Bragg and Kleeman. The 

 a rays emitted by any single product all travel at the same 

 speed and cease ionizing the gas fairly suddenly after passing 

 through a definite distance of air. The a particles from the 

 different products differ only in the speed of projection, which 

 determines the range of the ionization of the a, particles 

 in air. 



If we assume that the « particles emitted by the thorium 

 and radium are identical in all respects, the results show that 

 the a particles from the thorium products are, on the average, 

 projected with greater velocity than the a. particles from the 

 radium products. In each case, the velocity^ of the a particles 

 from the corresponding products radium C and thorium C 



