Collisions of a Particles ivith Hydrogen Nuclei, 923 



The rise-curves obtained during these investigations throw 

 much light on the heterogeneity of the 7 radiations from 

 Hadium B. These considerations are reserved for a further 

 paper. 



I am much indebted to Professor Sir E. Rutherford 

 for his suggestion of this research and for his assistance 

 in overcoming many difficulties which appeared from time 

 to time ; and to Mr. G. A. R. Crowe for his help in mani- 

 pulating the radioactive material. 



Cavendish Laboratory, Cambridge. 

 June, 1921. 



C The Collisions of « Particles ivith Hydrogen Nuclei. By 

 J. Chadwick, Ph.D., Clerk Maxwell Student of the Uni- 

 versity of Cambridge, and E. S. Bieler, M.Sc, 1851 

 Scholar of Mc Gill University, Montreal *. 



^ 1. TT/'HEN a particles pass through hydrogen gas 

 H or a substance containing hydrogen, close 

 collisions between an a particle and a hydrogen nucleus 

 occasionally take place. As a result of such a close 

 collision, the hydrogen nucleus is set in swift motion, 

 and can be detected by the scintillation it produces on 

 a zinc- sulphide screen. Assuming that both the a particle 

 and the hydrogen nucleus can be regarded as points., and 

 that the forces between them arise from their charges, 

 0. G. Darwin f calculated the number of H particles pro- 

 jected within any given angle to the path of the a particle. 

 Sir Ernest Rutherford %, however, found that the numbers 

 and angular distribution of the projected H particles did 

 not agree with the simple theory, and he attributed the 

 divergence to the complex structure of the a particle. His 

 results indicated that the field of force between the « particle 

 and the hydrogen nucleus undergoes rapid changes in mag- 

 nitude, and probably also in direction, when the nuclei 

 approach within 3*5 x 10~ 13 cm. of each other. These 

 experiments were of a preliminary nature, and were not 

 carried out to any high degree of: accuracy. Further, the 

 experimental arrangement was such that the deduction 

 of the collision relation from the observations was very 

 involved. 



Recently, improvement of the optical conditions has made 



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

 t Darwin, Phil. Mag-, vol. xxvii. p. 499 (1914). 

 \ Rutherford, Phil. Map?, vol. xxxvii. p. 537 (1919), 

 3P2 



