

[ >669 ] 



LXXIX. The Scattering of a. and /3 Particles by Matter and 

 the Structure of the Atom. By Professor E. Rutherford, 

 F.E.S., University of Manchester * . 



§ 1. "FT is well known that the a and /3 particles suffer 

 X deflexions from their rectilinear paths by encounters 

 with atoms of matter. This scattering is far more marked 

 for the /5 than for the a. particle on account of the much 

 smaller momentum and energy of the former particle. 

 There seems to be no doubt that such swiftly moving par- 

 ticles pass through the atoms in their path, and that the 

 deflexions observed are due to the strong electric field 

 traversed within the atomic system. It has generally been 

 supposed that the scattering of a pencil of a. or /3 rays in 

 passing through a thin plate of matter is the result of a 

 multitude of small scatterings by the atoms of matter 

 traversed. The observations, however, of Geiger and 

 Marsden f on the scattering of a rays indicate that some of 

 the a particles must suffer a deflexion of more than a right 

 angle at a single encounter. They found, for example, that 

 a small fraction of the incident « particles, about 1 in 20,000, 

 were turned through an average angle of 90° in passing 

 through a layer of gold-foil about '00004 cm. thick, which 

 was equivalent in stopping-power of the a particle to 1*6 milli- 

 metres of air. Geiger % showed later that the most probable 

 angle of deflexion for a pencil of ol particles traversing a gold- 

 foil of this thickness was about 0°*87. A simple calculation 

 based on the theory of probability shows that the chance of 

 an a particle being deflected through 90° is vanishingly 

 small. In addition, it will be seen later that the distribution 

 of the a. particles for various angles of large deflexion does 

 not follow the probability law to be expected if such large 

 deflexions are made up of a large number of small deviations. 

 It seems reasonable to suppose that the deflexion through 

 a large angle is due to a single atomic encounter, for the 

 chance of a second encounter of a kind to produce a large 

 deflexion must in most cases be exceedingly small. A simple 

 calculation shows that the atom must be a seat of an intense 

 electric field in order to produce such a large deflexion at a 

 single encounter. 



Recently Sir J. J. Thomson § has put forward a theory to 



* Communicated by the Author. A brief account of this paper was 

 communicated to the Manchester Literary and Philosophical Society in 

 February, 1911. 



f Proc. Roy. Soc. lxxxii. p. 495 (1909). 



t Proc. Roy. Soc. Ixxxiii. p. 492 vl910). 



$ Camb. Lit. & Phil. Soc. xv. pt. 5 (1910). 



