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LIIT. On the Recoil of Alpha Particles from Light Atoms. 

 By L. B. Loeb, Ph.D.* 



Introduction. 



Professor Sir E. Rutherford f has recently shown that 

 when swift alpha particles collide with light atoms [e g. atoms 

 of H, N, 0), the number of these atoms projected straight 

 forward within an angle of 10° with the original direction of 

 the alpha particles is from ten to thirty times as great as that 

 demanded by the theory of point charges worked out by 

 Darwin J. The atoms of one kind all have nearly the same 

 velocity whose value is about the maximum value to be 

 expected from the type of collisions described above. From 

 these results Prof. Rutherford concludes that in collisions 

 between an alpha particle and a nitrogen atom where the 

 distance of approach of the centres of the atoms is less than 

 7*0 xlO -13 , the magnitude and directions of the forces 

 involved are entirely different from those acting at greater 

 distances. He assumes that in collisions where the distance 

 of approach is less than the value above, one is dealing with 

 the actual impacts of the nuclei themselves. A study of 

 such encounters should obviously throw important light on 

 the nuclear structure and dimensions. 



On the writer's demobilization in February, Prof. 

 Rutherford suggested that he should attempt to verify 

 the above results, at the time unpublished, by a study of 

 the number of alpha particles recoiling after such close 

 encounters. The problem was found much more difficult than 

 was at first supposed. The work had to be abandoned in 

 June, owing to Prof. Rutherford's departure for Cambridge, 

 with only preliminary results obtained and those of an inde- 

 finite nature. Work enough was done to clearly define the 

 problem, and it seems best to publish a short account of 



ronress made. 



» 



The Problem. 



It was proposed to count the number of alpha particles 

 deflected through an angle as nearly l#0°as possible by layers 

 of atoms of light elements such as 0, 0, Al, &c, and to 



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

 f Rutherford, Phil. Mao-, vol. xxxvii. June 1019. 

 X Darwin, Phil. Mag. vol. xxvii. p. 449 (1914). 



Phil. Mag. Ser. 6. Vol, 38. No. 227. Nov. 1919, 2 P 



