Section III., 1%9. [ 85 ] Trans. R. S. C. 



Ill, — Primary and Secondary Gamma Rays. 

 By A. S. Eve, M.A., D.Sc, McGill University, Montreal. 

 (Communicated by Dr. Barnes and read May 2.5, 1909.) 



Whatever may be the views held about Bragg's theory of the 

 nature of the Rontgen and gamma rays it is certain that his able 

 advocacy has led to a notable increase of our knowledge of the 

 properties of both types of radiation. The gamma rays in 

 particular are so little affected by physical conditions that new 

 methods of investigation are essentially valuable. Therefore, the 

 plan adopted by Bragg of examining the incident and emergent 

 secondary radiations, consisting of electrons or corpuscles, from a 

 reversible pair of dissimilar metals traversed by the gamma rays, 

 appears to be a promising line of attack. He has himself published^ 

 a few results showing the changes produced in the ionization current 

 of an electroscope, when four plates, two above and two below, of 

 dissimilar substances, are traversed by a pencil of y rays, and are 

 interchanged in the four possible arrangements. In this manner 

 he has proved the asymmetry of the incident and emergent secondary 

 radiations. 



\M^ The writer has for some months made an experimental study 

 of effects produced in a similar way with primary and secondary 

 Y rays from radium C, and with the primary y rays from Uranium 

 X. The main results are capable of considerable simplification, 

 although the underlying ultimate processes remain obscure. 



The first paper on Secondary Radiation due to the /? and y 

 rays of radium was published" in December 1904, and the results 

 then obtained have been on the whole well substantiated by later 

 observers. The work of McClelland,^ however, placed the theory 

 of absorption of the /? rays and the laws of reflected or incident second- 

 ary radiation on a sound basis, and a host of other workers in the 

 field have contributed to steady advance in various directions. In 

 particular McClelland found that the amount of incident secondary 

 radiations from various elements followed strictly the order of atomic 

 weights. Such radiations have been proved to come from a very slight 

 depth, and to consist of high velocity negatively charged particles. 

 Using relatively large masses Kleeman* and the writer have proved^ 

 the existence of secondar}' radiations of the y ray type, less penetrat- 

 ing than the primary y radiations to which such secondary rays are due. 



> Trans. Royal Soc. of S. Aust., 32, 1908. 



2 Phil. Mag., Dec. 1904. 



^ Trans. Roy. Dublin Soc, 27, 9, 1906. 



'Phil. Mag., May 1908. 



^ Phil. Mag., Aug. 1908. 



