Secondary 7 Radiation* 437 



Pb and Hg, however, are distinctly different from the 

 other substances, o£ which we may hereafter take Zn as 

 typical. 



The curve for Pb has not been altered to such a large 

 extent by the change of arrangement of the experiment as 

 has that for Zn and such substances. 



The results for Zn are shown separately in fig. 9 (PL VIII.) 

 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 dA 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 7 radiation increases with the 

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

 responding to dA = 18 has been reached, after which it 

 gradually falls. For certain values of dA the effect produced 

 hj the secondary 7 rays may be almost as great as that 

 which is being produced by the primary '7 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 limit-. 



A conical ionization-chamber. ;is shown by the dotted 

 diagram in fig. 7 (p. 433), gave results i'or the first few 

 thicknesses of radiator almost the same as those shown by the 

 dotted curves, fig. 8 (PI. 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 j3 rays coming from the floor 

 and lid of the chamber ; these rays being the product of the 

 original 7 rays and of the secondary 7 rays. The ft ray- 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 /3 rays which come from the secondary 7 

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

 from the original 7 rays. 





