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XC. A Iheory of the Absorption and Scattering of the 

 u Bays. By C. G. Darwin, B.A., Lecturer in Mathe- 

 matical Physics, Manchester University *. 



Part I. — Absorption. 



THE a particles from radioactive substances have been 

 very thoroughly investigated, so that the main features 

 of their motion are now well known. On account of their 

 great mass they go straight or nearly straight through 

 matter until their energy is exhausted. It is for this reason 

 principally that an experimental formulation of their law of 

 motion is comparatively so much easier than for the /3 rays. 

 Braggt showed that the number of a rays remained constant 

 after traversing matter and that the absorptive effect was to be 

 attributed to changes of velocity ; and Geiger % found the 

 form of the velocity curve §, which for mica he empirically 

 represented by the equation f 3 = V 3 (1 — #/B,), where V is the 

 initial velocity and R is the " range. " The present paper is 

 concerned with the theoretical reason for this curve and the 

 deductions which may be made from it. It also considers, 

 but less completely, the scattering of the a particles. 



§ 1. The Mechanism of Absorption. 



It is known that the ionization produced by an a particle 

 is proportional to the rate at which it loses its energy. It is 

 thus necessary to adopt a structure for matter such that the 

 a particle pulls electrons out of the atoms containing them 

 and in so doing loses velocity. I have taken the atomic 

 structure proposed by Prof. Rutherford ||. This supposes 

 the atom to consist of a cluster of electrons held by an 

 unknown field of forces round a central charge, which is of 

 such amount as to neutralize them and which is supposed to 

 be the seat of the mass of the atom. This structure presents 

 fewer analytical difficulties in the present problem than any 

 other, and there is strong experimental evidence for it in 

 the large scattering of the a rays H. The adoption of this 



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



t W. H. Bragg, Phil. Mag. vol. x. p. 318 (1905). 



X H. Geiger, Proe. Roy. Soc. A. vol. lxxxiii. p. 505 (1910). 



§ I call by the name " velocity curve " the curve whose ordinate is the 

 velocity of the a rays and whose abscissa is the distance they have travelled 

 from their source. 



|| E. Rutherford, Phil. Mag. vol. xxi. p. 669 (1911). 



51 Rutherford, foe cit., and 11. Geiger, Mane. Lit. & Phil. Soc. 1911. 



