﻿688 Energy Decrease of a Particles passing through Matter. 



For the same reason calculations have not at present been 

 carried out for substances other than air. In this connexion 

 the remarkable agreement obtained by Bohr in the case of 

 hydrogen, making use only of known data, should not be 

 lost sight of. Unfortunately, data regarding resonance and 

 ionization potentials for gases in the molecular state are still 

 incomplete. 



It is regretted that results of a more decisive character 

 have not as yet developed from this application of the con- 

 cepts of resonance and ionization potentials. However, it is 

 felt that the possibility of explaining, along the lines followed 

 here, much of the experimental data on the motion of a 

 particles should be pointed out. Tlie equation of motion is 

 obtained comparatively simply, as will be seen from § 3. 



The above remarks concerning agreement with experiment 

 referred to velocities not lower than # 5Vo, half the initial 

 velocity of RaC. For velocities lower than this the agree- 

 ment breaks down completely. This is only to be expected, 

 for experimental data of other kinds show that the previous 

 homogeneity of the beam of a particles begins to disappear 

 at about this velocity. 



In the foregoing calculation no account has been taken of 

 probability variations in the beam, while from a velocity of 

 •5Vo downwards these variations become marked. Neither 

 has account been taken of the orbital velocities of the 

 electrons which may become appreciable for low a particle 

 velocities. Experiment shows that the beam becomes 

 anhomogeneous in velocity. Straggling becomes very large 

 at this point. Further, recent results obtained by the writer 

 give evidence to show that the charge on the a. particle does 

 not remain invariable for low velocities, though further 

 discussion of this point must be reserved. All things 

 considered it seems clear that the behaviour of a beam of a 

 particles becomes much too complicated at low r velocities to 

 be dealt with by simple treatment. 



Summary. 



Tn this paper the equation of motion of an a. particle 

 passing through matter is developed, making use of the 

 concepts of resonance and ionization potentials. 



The equation found is shown to give good agreement with 

 experiment in the case of air, but does not furnish a decisive 

 test when compared with other solutions which have been 

 proposed. 



Cavendish Laboratory, 

 July 1922. 



