21 



The following co-ordinates have been used in drawing the 

 curve : — 



DjR -067 -110 -167 -250 -333 -443 -568 -067 -833 

 ijl -773 -657 -532 -378 -262 -148 -069 030 -010 



CASE (b). 



Thin layer of radio-active material. 



The limits for 6 are now ^, and 6-2 where 



D sec $,-^r=R, and {D+D') sec e^^r=R. 



D' being the air equivalent- of the layer of radio-active 

 material. Hence, the total number of particles whose ranges 

 lie between r and r + dr, is 



'^[ \Y^) ~ XR^rj J ~ " 2.« " (R-rf 

 where D'- is neglected. 



