182 



The results for Zn are shown separately in fig. ix. by 

 curves A and B, in which the values of thickness of absorb- 

 ing screen multiplied by its density are shown horizontally 

 and the corresponding currents vertically. The difference 

 between the ordinates for any value of <i A represents the 

 amount of secondary radiation which it was possible to cut 

 out by the Pb screens L,L in the first experiment. This 

 difference is shown by the dotted curve C. It will be seen 

 that the amount of secondary y radiation increases with the 

 thickness of absorbing screen or radiator, until a value cor- 

 responding to ^ A =18 has been reached, after which it grad- 

 ually falls. For certain values of d A the effect produced by 

 the secondary y rays may be almost as great as that which 

 is being produced by the primary y rays. 



It is thus seen that the effect produced in an ionization 

 chamber in an ordinary absorption experiment depends 

 largely upon the position of the absorbing plate relatively 

 to the ionization chamber, and the shape of an absorption 

 curve may consequently be made to vary between consider- 

 able limits. The shape of the chamber, however, appears 

 to make very little difference to the shape of the absorption 

 curve, within of course certain limits. 



A conical ionization chamber, as shown by the dotted 

 diagram in fig. vii., gave results for the first few thicknesses 

 of radiator almost the same as those shown by the dotted 

 curves, fig. viii. ; while, again, with the plates placed at p,p 

 results similar to those shown by full-line curves in the same 

 figure were obtained, without of course the necessity of using 

 Pb screen such as L,L. 



This is readily explained if we remember that probably 

 most of the effect produced in the ionization chamber is due 

 to emergence and incidence 13 rays coming from the floor and 

 lid of the chamber ; these rays being the product of the ori- 

 ginal y rays and of the secondary y rays. The j8 rays as a 

 result of the scattering they undergo emerge in all direc- 

 tions from the floor and lid of the chamber, and any altera- 

 tion in the shape of the chamber will affect the ionization 

 produced by the $ rays, which come from the secondary y 

 rays to much the same extent as it will those which come 

 from the original y rays. 



Returning to fig. viii. it will be found that the ordinary 

 absorption curve, corresponding to the dotted curve Pb 1, 

 2 6., in which as much secondary radiation as possible is in- 

 cluded, can be represented very accurately by the expres- 

 sion : 30e - -32^ + 70e-i-35d. 



Again, the full -line curve Pb 2, from which as much as 

 possible of the secondary radiation has been excluded, is re- 



