﻿Ionization by Alpha Rays. 4G3 



Part I. 



Qualitative Measurements. 



2. It was thought that additional information might be 

 obtained as to the nature of the delta rajs by experiments 

 on ionization by collision. It' the power of an ion to form 

 new ions when it collides with neutral atoms depends on 

 its velocity when it collides, two ions will have different 

 powers in the same electric field if they start with different 

 velocities. 



Consider a layer of gas contained between parallel elec- 

 trodes A and B, composed of different materials, a rays pass 

 through A and the layer of the gas, being finally absorbed 

 in B ; they excite delta rays at both A and B, and produce 

 ionization in the gas. An electric field is established between 

 A and B of an intensity sufficient to cause ionization by 

 collision ; the former experiments in a vacuum have shown 

 that it will also be sufficient to prevent any of the delta rays 

 leaving the positive electrode, and to ensure that all the delta 

 rays leave the negative. The current arriving at the elec- 

 trodes due to the ionization in the gas must be the same for 

 both directions of the electric field if the ionization in the 

 gas is uniform ; but if the delta rays emitted from A are 

 different, either in quantity or in quality, from those emitted 

 by B, the current due to the delta rays from the electrodes 

 will differ according to the direction of the field. When A 

 is positive this part of the ionization will be caused by the 

 delta ra} r s from B ; when A is negative it will be caused by 

 the delta rays from A. 



If the delta rays differ in initial velocity the difference 

 between the currents in the two directions should be almost 

 independent of the intensity of the field, so long as it is 

 sufficient to cause ionization by collision at all ; for after the 

 electrons, which are liberated as delta rays, have made a few 

 collisions all trace of their original velocity will be lost ; 

 any difference in initial velocity will appear only in a dif- 

 ference in the fraction which ionize at their first collision. 

 But if the rays differ in quantity, there will always be more 

 ions in the gas in one case than in the other, and the difference 

 between the currents in the two directions should increase 

 steadily with the intensity of the field. 



3. The electrodes of the apparatus in which the measure- 

 ments were made are drawn to scale in fiff. 1. A and B are 



