Mi^ Growther, On the secondary Rontgen radiation, etc. 101 



On the secondary Rontgen radiation from air and ethyl 

 bromide. By J. A. Crowther, M.A., St John's College. 



[Read 25 January 1909.] 



In a previous paper* I have shewn that certain gases and 

 vapours, and notably compounds containing arsenic or bromine, 

 give off amounts of secondary Rontgen radiation enormously 

 greater than that given off by air under similar conditions. Thus 

 the ionization produced in air by the secondary rays from ethyl 

 bromide is some 540 times greater than that produced by the 

 rays from air itself under similar circumstances. The rays from 

 ethyl bromide are much softer than those from air, and in a 

 recent paper Prof Braggf has siaggested that in this fact may 

 lie a possible explanation of what he describes as the " startling 

 results obtained in the case of arsenic and bromine." He points 

 out that the rays from an ordinary X-ray bulb are hard to sub- 

 stances of low atomic weight such as aluminium or air, but soft to 

 the elements of higher atomic weight such as copper or iron. The 

 secondary rays from air are of the same type as the primary rays. 

 The secondary rays from bromine however are of a much softer 

 character, and so, to quote Prof Bragg's paper, " are brought 

 within reach, so to speak, of air, which rapidly converts them into 

 cathode rays, so that there is a very large ionization." He 

 concludes, finally, " that the very large secondary radiations which 

 some substances appear to give, therefore owe their magnitude 

 largely to the fact that the air in which they are measured is 

 sometimes ten to twenty times as favourable to them as to the 

 primary rays which produced them." 



I was, of course, not unaware of the increased ionizing power 

 of the softer rays from ethyl bromide as compared with those 

 from air, and in my original paper, I endeavoured to correct for 

 this fact, on what seemed to be, in the absence of direct evidence, 

 the reasonable assumption that the ionization produced in air by 

 rays of differing hardness, was proportional to the amount of 

 absorption undergone by the rays in the secondary ionization 

 chamber. The final values given in Column HI, Table 1, of my 

 earlier paper are reduced on this assumption. With this correc- 

 tion the relative intensity of ethyl bromide compared with air 

 became 217, as given in the table. 



The assumption on which this calculation is made has never 



* Crowther, Phil. Mag. [6], Vol. xiv. p. 653, 1907. 



t Bragg and Glasson, Proc. Roy. Soc. Suzith Australia, Vol. xxxi. Oct. 1908, 



