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ROYAL SOCIETY OF CANADA 



in terms of Y^, the velocity of the « particles when the screens are 

 removed. 



The velocity of the a particle is thus reduced only 36 per cent 

 after passing through 12 layers of aluminium. 



Since the a particle produces about the same number of ions per 

 cm. of path in air over its whole range, the simplest assumption to 

 make is that the energy of the « particle is diminished by a constant 

 amount in passing through each layer of aluminium foil. After passing 

 through 13 layers, the kinetic energy of the a: particle is reduced to 

 41 per cent of the maximum. Each layer of foil thus absorbs about 

 4.9 per cent, of the maximum energy. The observed kinetic energy 

 of the a particle after passing through different thicknesses of 

 aluminium foil, and the value calculated on the above assumption are 

 shown in the following table: — 



The experimental and calculated values agres within the limit of 

 experimental error. We may thus conclude, as a first approximation, 

 that the same proportion of the total energy is abstracted from the a 

 particles in passing through successive layers of aluminium foil. 



