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XCVIII. The Recoil of Hydrogen Nuclei from Swift 

 ol Particles. By A. L. McAtTLAY*. 



1. Objects of the Research and Experimental Method. 



SIR ERNEST RUTHERFORD t has shown that when 

 hydrogen is bombarded by « rays the number o£ atoms 

 shot forward in the direction of motion of the a particles is 

 greatly in excess of what would be expected on the simple 

 theory of two charged points approaching and repelling each 

 other according to the inverse square law. It is obvious 

 from a general survey of his curves showing the absorption 

 of the recoil radiation, that there is a rapid change in this 

 number, and probably in the type of the collision, as the 

 velocity of the incident a particle approaches that of the 

 particle of range 7 cm. from RaC. 



The present research was undertaken with the object of 

 obtaining the "number of atoms recoiling at various angles 

 from a homogeneous a ray beam, and thence of deducing 

 information bearing on the nature of the collision, the size 

 of the colliding particles, etc. 



An ionization method was used, similar in principle to one 

 described in a recent paper in the Philosophical Magazine J, 

 but with greatly improved apparatus. The following is an 

 outline of the method. The recoil radiation is produced in a 

 wax film close to a source of a rays. The source is placed 

 between the poles of a powerful electromagnet about 45 cm. 

 distant from a small ionization chamber, into which the recoil 

 radiation penetrates (see fig. 1). The electrode of the ioniza- 

 tion chamber passes at one. end into a second chamber, where 

 the current from the first is balanced. The face of the second 

 chamber is of brass, sufficiently thick to exclude the recoil 

 radiation. At the other end, the electrode passes into an 

 electroscope where the difference betw r een the ionizations in 

 the two chambers is measured. Between the source and the 

 chambers and close to the former is the paraffin film in which 

 the recoil atoms are produced. This film is fairly thick, corre- 

 sponding in absorbing pov/er to about 8 cm. of air, and is 

 covered on one side by a thin aluminium foil equivalent to 

 069 cm. of air. 



The experiment consists in observing the change in 

 ionization in the chamber which the recoil atoms enter when 

 the wax-aluminium film is turned so that first wax and then 



* Communicated by Prof. Sir E. Rutherford, F.R.S. 

 t Rutherford, Phil. Mag-. June 1919. 

 t McAulav, Phil. Mag. Dee. 1920. 



