IONISATION CAUSED BY PENETRATING y RAYS. 



143 



to the cylinder a greater number of the y rays more oblique 

 to the axis of the cylinder get in. These rays have to 

 traverse a greater distance in the screen and so have their 

 quality changed more than the rays parallel to the axis. 

 The effect of screening the y rays is to increase the kd in 

 any substance relative to lead; the increase in the relative 

 kd when the radium is close to the cylinder is not more 

 than could be accounted for by this fact. 



In these experiments the radium was screened with 

 various thicknesses of lead from # 5 centimetres, (the 

 thickness of the base of the lead cylinder), to 5*4 centi- 

 metres, by steps of about 0*25 centimetres. The curves, 

 in figures 1, 2 and 3, show the relation between the kd and 

 the thickness of the screen for the substances enumerated 

 above. The increase in the value of kd, as the thickness 

 of the screen becomes greater, is attributed by Professor 

 Bragg to a change in the value for the lead only. An 

 inspection of the curves shows that the full change has not 

 taken place with the thickest screen employed, 5*4 centi- 



Fig. 3. 





ti 































C^-^^" 



^r^- 



■1-4= 1 



+ 









^£^~ 



*= — *■ + 



















Thickness of Screen. 



