436 Ionization due to Rontgen and Gamma Rays. 



multiple reflexion of the secondary @ rays, originating at 

 the walls when the 7 rays strike them, is the principal cause 

 of the ionization within the testing vessels The effect is 

 controlled by the atomic weight of the material of the walls 

 of the vessel. 



Moreover, the 7 rays which strike the concave or further 

 side of the vessel produce " incident " /9 rays to an extent 

 also dependent on the atomic weight. 



But as Bragg has shown, this is not the case with the 

 " emergent " secondary radiation arising on the convex side 

 where the 7 rays first strike. In this case the ionization 

 plotted against the atomic weight gives a U-shaped curve, so 

 that aluminium is about equal to lead as an ionizer. 



So far as the " emergent" electrons are concerned, we 

 might expect an aluminium vessel to give as much ionization 

 as a lead vessel, with penetrating 7 rays and suitable thick- 

 ness of walls. But such a result would be difficult .or im- 

 possible to realize, because the " incident " radiation and the 

 multiple reflexion together quite overwhelm and mask this 

 emergent radiation effect. 



The influence of the thickness of the walls is not very 

 pronounced in the case of the 7 rays ; but of course very 

 thin or very thick walls alike give small results. The curve, 

 ionization plotted against thickness, rises rapidly from the 

 origin to a maximum and proceeds with a slow decrease, and 

 is represented by the difference of two exponentials. 



The results obtained then agree well with the view that 

 the Rontgen rays ionize mainly or wholly by electrons pro- 

 jected with velocities of the .same order as those of the 

 cathode rays ; and that the 7 rays from radium C also ionize 

 mainly or wholly by electrons projected with velocities of 

 £he same order as those of the /3 rays from radium C. 



With the X rays these electrons originate chiefly in the 

 &ir in the testing vessel, but with the 7 rays for the most 

 part from the walls. 



Some experiments made with very thin-walled olectro- 

 scopes indicated that with Rontgen rays the ionization within 

 iron or zinc vessels about half a millimetre thick is only 15 

 or 16 per cent, of that in the free air. 



But with 7 rays the ionization in such vessels is about 

 110 per cent, of that in the free air. The experiments in 

 each case were made in the centre of a fairly 

 with brick walls. 



McGjill University Montreal, 

 June 15, 1912. 



