Secondary Radiation. 



231) 



the ionization-tube T, as in fig-. 4. If now we proceed to 

 measure the absorption produced by say a screen of card- 

 board, the result we get depends very much on where we 

 place the cardboard in the path of the rays. 



l£ADz 



'IFA'D- 



Fio. 4. 



V 



fi£CrrfOM£T£# 



Cells 



Iii an experiment the distance AB, fig. 4:, was 2(3 cms., the 

 internal diameter of the tube T being 3*2 cms. The result- 

 was as follows : — 



Ionization in T. 



No screen 100 



Screen at B 71 



Screen 5*5 cms. from B (52 



Screen 10 '5 cms. from B 54 



Screen 26'0 cms. from B 36 



The screen produces a different effect in different positions, 

 because it is a source of a radiation which travels in all 

 directions from the part struck by the primary rays; and 

 therefore the nearer the screen is to the testing- vessel T the 

 greater is the observed ionization. If the secondary rays 

 were of much less penetrating power than the primary, the 

 observed ionization would of course be increased by moving 

 the screen towards the vessel T; but we have seen there is no 

 very great difference in penetrating power between the 

 primary and secondary rays, so that the chief cause of the 

 effect noted must be the fact that the screen becomes a source 

 of radial ion spreading out in all directions. Of course, we 

 have also tertiary radiation from the air and from the end of 

 the testing-tube. 



The numbers given above show that after passing through 

 the sheet of cardboard, the secondary rays are of as great 

 importance as the primary rays, for the ionization with the 

 screen close to the testing-vessel is twice what it is with 

 the screen 2G cms. away. 



Another example of the importance of the secondary rays 

 is given by results obtained in comparing the penetrating 

 power of the secondary rays with that of the primary pencil 

 consisting chiefly of $ rays. 



The radium was first placed as in fig. 4, and the primary 



