Secondary y .Radiation. 211 



the whole sphere a small portion of which had been explored 

 with the electroscope. It was thus found that 13 per cent, 

 or! the total energy of the primary beam falling on this 

 radiator appeared as secondary radiation. A correction of 

 course is necessary for the absorption of the secondary 

 radiation in the material of the radiator. This correction 

 factor would be of the order 1*5, so that the total secondary 

 radiation would probably be not more than 20 per cent, of 

 the primary beam. 



Let us for the present assume this value, so that the total 

 energy of the secondary radiation is 0*20 of the energy of 

 the primary beam. Consequently, for a radiator of unit 

 mass per square centimetre we have 



Energy in secondary radiation A ^ 



-^ — ^ — = -. r^— -. =0*05 approx. 



liiiergy in primary radiation x l 



Comparing this with the value 0'20 found by Barkla * it 

 appears that aluminium scatters the y rays to a much less 

 extent than the X rays. 



If this aluminium radiation had been used simply as an 

 absorbing screen for y rays from radium C, then not more 

 than 20 per cent, of the rays would have disappeared from 

 the primary beam. But it has been shown that this 20 per 

 cent, absorption can be approximately accounted for by the 

 production of secondary y radiation. We have still to account 

 for the energy of the ft rays emitted during the passage of 

 y rays through matter. If, as is generally supposed, the /3 

 rays receive their energy from the y rays, then in the above 

 calculation we have overestimated the energy contained in 

 the secondary y radiation. From the results of experiments 

 carried out with methyl iodide it is clear that these ionization 

 experiments do not express accurately the intensity of the 

 radiation in different directions ; and before the distribution 

 of the energy can be definitely determined it will be necessary 

 to know what correction must be made for this greater 

 liberation of energy the less penetrating the radiation 

 becomes. 



In all investigations on the secondary y radiation it has 

 been definitely shown that it is less penetrating than the 

 primary radiation. The writer has shown that there is no 

 sudden change in character from the primary to the secondarv 

 y radiation. Consider first of all the case when aluminium 

 or zinc is used as a radiator and the emanation tube is 

 surrounded with lead 1 cm. thick. The penetrating power 



* Barkla, Phil. Mag. May 1904 & May 1911. 

 Phil. Mag. S. 6. Vol. 27. No. 158. Feb. 1911. R 



