Section III, 1922 [129] Trans. R.S.C. 



The Softening Exhibited by Secondary X-rays 



By J. A. Gray, F.R.S.C. 



(Read May Meeting, 1922) 



In this paper the term secondary X-rays has no reference to 

 ordinary characteristic radiations, but will be confined to those X-rays 

 which are given off in all directions from any substance (the radiator) 

 struck by any beam of X-rays (the primary rays) and which are 

 dependent in quality or frequency on that of the primary rays. 

 The quality of such secondary rays is, for radiators of small atomic 

 weight, independent of the nature of the radiator. Until recently, 

 it was thought that these secondary X-rays were identical in quality 

 with that of the primary and, as a rule, they have been called scattered 

 X-rays. 



In 1913,^ however, the writer showed that secondary 7-rays were 

 less penetrating or softer than primary 7-rays, a radium salt being 

 used as a source of 7-rays. It was then shown that this "softening" 

 was due to a real transformation of the primary rays and that it 

 increased with the angle between the primary and secondary rays 

 (usually called the angle of scattering). These results were confirmed 

 by Florance^ in 1914 and A. H. Compton» in 1921. In 1920," it was 

 shown that the same phenomenon was true for ordinary X-rays but 

 was not so marked and consequently had escaped attention, although 

 in 1913 Sadler and Mesham^ published results which indicated that 

 secondary X-rays were softer than primary X-rays. The first 

 experiments of the writer with X-rays were performed in 1919 at 

 University College, London. These experiments have not hitherto 

 been published in detail and a short account of them will be given 

 below. 



A beam of primary X-rays, covering a comparatively narrow 

 range in frequency, was obtained by filtering the rays from an X-ray 

 tube through a screen of tin. The tube was operated by means of an 

 induction coil and with a parallel spark gap of about 3 .6 cm. between 

 brass balls 2 cm. in diameter. Figure 1 shows that the rays were 

 comparatively homogeneous. It was obtained by first passing the 



iGray, Phil. Mag. 26, p. 611, 1913. 



^Florance, Phil. Mag. 27, p. 225, 1914. 



3A. H. Compton, Phil. Mag. 41, p. 749, 1921. 



^Gray, Journ. Frank. Inst, p. 633, 1920. 



t^Sadler and Mesham, Phil. Mag. 24, p. 138, 1912. 



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