Deflexion of a. Particles through Large Angles. 605 



In an earlier paper *, however, we pointed out that a 

 particles are sometimes turned through very large angles. 

 This was made evident by the fact that when a particles fall 

 on a metal plate, a small fraction of them, about 1/8000 in 

 the case of platinum, appears to be diffusely reflected. This 

 .amount of reflexion, although small, is, however, too large to 

 be explained on the above simple theory of scattering. It is 

 •easy to calculate from the experimental data that the proba- 

 bility of a deflexion through an angle of 90° is vanishingly 

 small, and of a different order to the value found experi- 

 mentally. 



Professor Rutherford \ has recently developed a theory to 

 account for the scattering of a particles through these large 

 angles, the assumption being that the deflexions are the 

 result of an intimate encounter of an a. particle with a 

 single atom of the matter traversed. In this theory an atom 

 is supposed to consist of a strong positive or negative central 

 charge concentrated within a sphere of less than about 

 3xl0 -12 cm. radius, and surrounded by electricity of the 

 opposite sign distributed throughout the remainder of the 

 atom of about 10~ 8 cm. radius. In considering the de- 

 flexion of an a particle directed against such an atom, the 

 main deflexion-effect can be supposed to be due to the central 

 -concentrated charge which will cause the ol particle to describe 

 an hyperbola with the centre of the atom as one focus. 



The angle between the directions of the ol particle before 

 .and after deflexion will depend on the perpendicular distance 

 of the initial trajectory from the centre of the atom. The 

 fraction of the a particles whose paths are sufficiently near 

 to the centre of the atom will, however, be small, so that the 

 probability of an a particle suffering a large deflexion of this 

 nature will be correspondingly small. Thus, assuming a 

 narrow pencil of ol particles directed against a thin sheet of 

 matter containing atoms distributed at random throughout 

 its volume, if the scattered particles are counted by the 

 •scintillations they produce on a zinc-sulphide screen dis- 

 tance r from the point of incidence of the pencil in a direction 

 making an angle <f> with it, the number of ol particles falling 

 on unit area of the screen per second is deduced to be equal to 



Qntb 2 eosec 4 <j>/2 

 16? ' 



where Q is the number of ol particles per second in the 



* H. Geiger and E. Marsden, Rov. Soc. Proc. vol. lxxxii. p. 495 

 .(1909). 



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



