Secondary 7 Radiation. 227 



whether the whole beam was used as in the case of the 

 hemispherical absorbers, or whether part of the beam was 

 used as in the ordinary absorption experiments. Hence 

 since this transformed radiation is always present, it is 

 evident that an exponential law of absorption cannot be 

 regarded as a definite proof that the emergent radiation is 

 homogeneous, i. <?., unchanged in type in passing through 

 matter. At the same time the exponential law of absorption 

 for the 7 rays of radium C indicates that part of the 7 

 radiation persists unchanged in type after passing through 

 great thicknesses of matter. Some of the radiation is always 

 being transformed into softer types of radiation in passing 

 through matter ; but equilibrium is soon set up between the 

 primary and the transformed radiation, so that the combined 

 primary and secondary radiations are absorbed according to 

 an exponential law. 



The following experiments were carried out to see if any 

 evidence could be adduced to explain how a primary radiation 

 like the 7 radiation from radium C, consisting essentially of 

 one type, gives rise to a very complex secondary radiation. 

 In the case of X rays we know that characteristic radiations 

 are produced under suitable conditions^ and that scattering 

 of the primary rays also takes place. Gray * showed that 

 the 7 rays of radium E produced in certain elements radiations 

 characteristic of them, The writer f, in his earlier experi- 

 ments, was unable to detect any characteristic radiation 

 excited by 7 rays of radium C, and concluded that all the 

 materials examined simply scattered the primary 7 rays. 

 But in those experiments the side of the electroscope was 

 3 mm. of lead, so that any absorbable type of radiation would 

 not have been detected in the electroscope. 



In re-investigating this question it was necessary to bear 

 in mind that since the 7 rays of radium C are characteristic 

 of that element, it might be anticipated from the analogy of 

 X rays that these 7 rays should excite characteristic radiation 

 in elements of high atomic weight and should be scattered 

 by elements of low atomic weight. 



For the sake of convenience the secondary radiation is 

 divided into two parts, the "emergent/"' i. e., the radiation 

 emitted from the radiator in the direction of the primary 

 radiation, and the "returned," i.e., the radiation emitted 

 from the incident surface of the radiator. There is no 

 essential difference in these two radiations, except that the 

 returned radiation is less penetrating than the emergent. 



* Gray, Proc. Roy. Soe. September 1912. 

 t Florence, Phil. 'Mao-. December 1910, 



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