﻿Energy Decrease of ot Particles passing through Matter. G81 



travelled were obtained which showed the same general form 

 as the experimental curves, and from these an estimate of the 

 number of electrons in the atom was made. 



On the other hand, Bohr considered the time of passage of 

 the a particles past an electron to be the determining factor, 

 and assumed that as long as this time of passage was small, 

 compared with the period characteristic of the electron in 

 dispersion phenomena, the electron could be considered as 

 free. When the time of passage was comparable with this, 

 however, the electron could no longer be considered as free, 

 and in this way an upper limit to p was introduced. The 

 calculated velocity curves showed good agreement with 

 experiment over most of the range of the ot. particle. 



§ 2. Method adopted. 



In this paper the law of decrease of energy of an a 

 particle is developed along lines somewhat different from 

 those of the writers mentioned. 



According to the modern ideas of atomic structure, due 

 to Bohr, the electrons are thought to be arranged in various 

 stationary states or energy levels. An electron leaving one 

 of these stationary states can only move to another such 

 state or completely out of the atom (to infinity). Thus 

 the energy which an electron can take up is limited to a 

 number of finite amounts characteristic of the atom. 



These views furnish a simple method of fixing the upper 

 limit to the radius of action of an a particle upon the 

 electrons of matter. If the electron is to be moved from 

 one stationary state to another by the passage ot an a 

 particle near it, then with the finite amount of energy 

 which the electron must take up in order to effect the 

 change there may be associated an upper limit to the 

 radius of an a particle upon the electron. 



The mechanism which is involved in this transfer of energy 

 from a particle to electron may be difficult to conceive. 

 However, when an electron is moved from its stationary 

 state to infinity by the action of light, the frequency v of 

 the light must be such that the quantum hv is greater 

 than the finite difference of energy between the initial and 

 the final states of the electron. Here the rule governing 

 the transfer of energy is known, although the mechanism 

 involved is not. A similar statement holds for the case 

 when the transfer of electrons from one state to another 

 is caused by electron impacts (elastic and inelastic impacts). 

 Similarly, in the case of « particle impacts the transfer of 



