XXVIII. On the Collision of [alpha] particles with 
Hydrogen Atoms. 

To the Editors of the Philosophical Magazine. 
Gentlemen, — 

IN two papers in your Journal (June 1919) I gave an 
account of some experiments on the collision of [alpha] 
particles with hydrogen atoms, using the scintillation method 
for determining the number of H atoms set in swift motion. 
With the microscope used at that time the counting of 
scintillations was a difficult and trying experience, but 
during the last year by the use of holoscopic lenses of large 
aperture and suitable eyepieces, recently constructed in this 
country, it has been found possible to improve greatly the 
ease and certainty of counting such weak scintillations. 
With the old arrangement, using a complex beam of H 
atoms of different velocities, the observer was very liable to 
miss the weaker scintillations due to lower velocity H atoms 
in the presence of the brighter scintillations due to swifter 
ones. 

The accurate determination of the relation between the 
number and angle of ejection of the H atoms for different 
speeds of the [alpha] particles is of great importance in order to 
throw light on the nature and distribution of the forces in 



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such close collisions when the nuclei must approach within 
[formula redacted] cm. of each other. This problem is being attacked 
in the Cavendish Laboratory by two methods. The electric 
balance method, as devised by Mr. McAulay [citation redacted], has the great advantage that it does not involve 
the eyestrain of counting, and, if the electrical effect is 
sufficiently large, is practically independent of the probability 
variations which make it necessary to count so many 
particles in the scintillation method. This electrical method 
should yield approximate quantitative data and has certain 
advantages over the scintillation method for examining the 
effect of K atoms near the end of their range where their 
ionization per unit path is increased due to their reduction 
of velocity. The relation between the number of H atoms 
shot out at different angles with the primary beam of [alpha] rays 
for different velocities of the particle is under direct examination by Mr. Chadwick and Mr. Bieler using the scintillation method. The general results so far obtained by these 
new methods indicate that the number of H atoms liberated 
is much larger than the theoretical number to be expected 
from point nuclei and possibly even greater than my original 
estimate. They also confirm the observation that the H 
atoms tend, for the high velocities of the [alpha] particle, to be 
shot forward in the direction of the [alpha] particle but not to the 
same extent as the preliminary experiments showed. The 
direct method used by Chadwick and Bieler indicates that 
even for swift [alpha] particles there is present a certain proportion 
of lower velocity H atoms. The determination of the 
relation between the number of H atoms and angle of ejection 
should afford valuable data for calculation in order to throw 
light on the structure of the nuclei and forces between them. 
Unfortunately such counting experiments involve a large 
amount of time, so that there will be some delay before the 
necessary data are available for adequate comparison of 
theory with experiment. 

I would like to take this opportunity of drawing attention 
to a slip in my original paper (loc. cit. p. 546) which 
fortunately is of little importance to the main argument. 
In calculating the emergent range for H atoms scattered at 
an angle [theta] with the direction of the [alpha] particle, the correcting 
factor for absorption under the experimental conditions was 
taken as cos [theta]. As Mr. 0. Darwin pointed out to me some 
time ago, this is only a rough approximation and the average 
value for the correcting factor is a complex function of [theta] 
which can readily be calculated.