Mr Kaye, The Emission of Rontgen Rays, etc. 269 



The Emission of Rontgen Rays from Thin Metallic Sheets. 

 By G. W. C. Kaye, B.A., D.Sc.i Trinity College, Cambridge. 

 (Communicated by Prof. Sir J. J. Thomson.) 



[^Received 4 August 1909.] 



Prof. Bragg and Dr Madsen* have, from the point of view 

 of the neutral-pair theory of the Rontgen and 7 rays, made 

 measurements of the secondary corpuscular radiation which goes 

 forward and backward when 7 rays are incident on thin metal 

 sheets. They find in most cases a large want of symmetry in the 

 distribution of these secondary rays — there is more " emergence " 

 than " incidence " radiation : especially is this so for metals of low 

 atomic weight. For example, the ratio {R) of the emergence 

 intensity to the incidence is, in the case of soft 7 rays, about 13 

 for carbon, 7 for aluminium, 2 for the metals of the copper group, 

 1"1 for tin and lead : with hard 7 rays these numbers become 

 20, 7, 3 and 1 respectively. 



Madsen f obtained similar results in his investigation of the 

 distribution of the secondary 7 radiation produced by 7 rays under 

 analogous conditions. For zinc R (defined as before) is about 5, 

 for lead 7. He showed also that the 7 rays in passing through 

 matter are softened as well as scattered. The distribution of the 

 scattered radiation depends on the quality of the exciting 7 rays 

 and also upon the nature of the scattering medium. The qualities 

 of the incidence and emergence radiations are not alwa37s identical. 



CoOKSEY {Nature, Ap. 2, 1908) worked in like manner with 

 the corpuscular secondary radiations from Rontgen rays, and brought 

 his results into parallelism with those for 7 rays; although the 

 asymmetry was very much less pronounced, R varying from 1*1 

 to 2 for different elements. 



It remained for Bragg and GLASSON+ to find a corresponding 

 lack of symmetry in the distribution of the secondary X rays 

 produced by the transmission of X rays through thin metal sheets. 

 The ratio R ranges from about 3 for Al to 1'3 for Pt. 



Quite recently Madsen § has extended the enquiry to /3 rays 

 and found very similar results. The greatest value of R is about 

 9 for Al and 4"5 for gold. He found that as the thickness of the 

 metal screen is increased, the total emergent scattered radiation 

 increases rapidly to a maximum (at '01 3 cm. Al and "OOOS cm. Au) 

 and afterwards decreases. 



With some of the foregoing results in mind the problem of 



* Bragg and Madsen, Phil. Mag. 16, 1908, p. 918. 



t Madsen, Ih. 17, 1909, p. 423. 



+ Bragg and Glasson, Ih. 17, 1909, p. 855. 



§ Madsen, TrauH. Hoy. Soc. S. Australia, April, 1909. 



