Scattering of a and ft Particles by Matter. 



681 



Metal. 



Atomic weight. 



z. 



z/A? 12 ' 



Lead 



Gold 



Platinum 



Tin 



207 



197 



195 



119 



10S 



64 



56 



27 



62 



07 

 Cvs 

 34 

 27 

 14-5 

 102 

 3-4 



20S 

 2*2 

 232 

 226 

 241 

 225 

 250 

 243 



Silver 



Copper 



Iron 



Aluminium ... 



Average 233 



On the theory of single scattering, the fraction of the total 

 number of a particles scattered through any given angle in 

 passing through a thickness t is proportional to n.A% 

 assuming that the central charge is proportional to the atomic 

 weight A. In the present case, the thickness of matter from 

 which the scattered a. particles are able to emerge and affect 

 the zinc sulphide screen depends on the metal. Since Bragg 

 has shown that the stopping power of an atom for an a 

 particle is proportional to the square root of its atomic weight, 

 the value of nt for different elements is proportional to 1/ \/A. 

 In this case t represents the greatest depth from which the 

 scattered a particles emerge. The number z of a particles 

 scattered back from a thick layer is consequently proportional 

 to A or £/A 3/2 should be a constant. 



To compare this deduction with experiment, the relative 

 values of the latter quotient are given in the last column. 

 Considering the difficulty of the experiments, the agreement 

 between theory and experiment is reasonably good *. 



The single large scattering of a particles will obviously 

 affect to some extent the shape of the Bragg ionization curve 

 for a pencil of a. rays. This effect of large scattering should 

 be marked when the a rays have traversed screens of metals 

 of high atomic weight, but should be small for atoms of light 

 atomic weight. 



(c) Geiger made a careful determination of the scattering 

 of a particles passing through thin metal foils, by tlio 

 scintillation method, and deduced the most probable angle 



* The effect of chang-e of velocity in an atonnc encounter is neglected 

 in this calculation. 



Phil. Man. S. 6. Vol. 2\. No. 125. May 1911. 2 Y 



