Absorption  of  a.  Rays.  139 
occurs  and  at  which  the  ionization  ceases  for  different  thick- 
nesses o£  aluminium  are  given  in  Table  I. 
Fig.  5. 
60 
j  • 
50 
* 
1 
6 
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10      12      14     16      18      20     22     24     26     28 
Ionization 
Table  I. 
Thickness  of 
Aluminium 
(L  layer =-00031  cm.). 
Distance  between 
Source  and  Gauze 
at  which  maximum 
ionization  occurs. 
Distance  between 
Source  and  Gauze 
at  which  ionization 
ceases. 
2  layers. 
4       „ 
6       „ 
8       „ 
4"8  cms. 
3-6     „ 
2-7 
1-5     „ 
5'8  cms. 
4-8     „ 
3-9     „ 
2-6     „ 
