Secondary y Radiation. 445 



10 mm. of Pb, placed in the path of the primary, reduces 

 the secondary effect to 35 per cent., so that it appears that 

 very nearly all the secondary incidence y radiation from Zn 

 and such substances is derived from the soft bundle of 

 primary y rays. 



In the case of the radiation from Pb the measurements 

 could not be performed with any great accuracy. However, 

 it was found possible to place a 2 cms. Pb plug over the Ra 

 without cutting off all the incidence radiation ; the reading 

 was reduced to about 30 per cent. 



The experiment is accurate enough to enable us to say 

 that the secondary incidence radiation from Pb is derived 

 to a large extent from the original hard bundle of y rays ; 

 its quality, as tested by Pb domes, shows also that it is of 

 much the same nature as the emergence radiation, which 

 appears to be the product of the hard y rays. 



In all these experiments, and especially in those which 

 deal with the incidence radiation, it must be borne in mind 

 that the radiation has to penetrate a thickness of Pb, cor- 

 responding to 2 mm. before its effect is measured, and con- 

 sequently the effect of very soft y radiation may have been 

 missed. 



The lack of symmetry in quality of the emergence and 

 incidence radiation from some materials may be explained 

 if we suppose that the distribution of the scattered radiation 

 which is produced from the hard y rays is not the same as for 

 that produced from the softer rays. The secondary radiation 

 produced from the hard 7 rays appears to move on more in 

 the direction of the original stream than do the secondaries 

 produced from the softer rays. 



The distribution of the secondaries may also to some extent 

 depend upon the nature of the medium in which scattering 

 has taken place, and it does not seem at all unlikely that the 

 atomic structure of the radiator should determine to some 

 extent the nature and result of a collision between the 7 

 particle and the constituent part of the atom. 



It has already been stated that these experiments give 

 little or no support to the theory of selective absorption 

 advanced by Kleeman ; the effects seem much simpler than 

 we might be led to suppose from that theory. It seems 

 possible, also, to suggest probable causes for the effects which 

 Kleeman has observed. In some cases the absorbing screens 

 used by Kleeman seem not to be of sufficient thickness to 

 preclude the possibility of a certain amount of ft rays, given 

 off by the radiator, penetrating the screen. Secondly, it has 



Phil. Mag. S. 6. Vol. 17. No. 99. March 1909. 2 H 



