Secondary Iiontgen Radiation from Carbon, 111 



stopped instantaneously will give rise to Rontgen rays o£ the 

 same penetrating power as the primary *. Since some of 

 the corpuscles will be stopped much more quickly than others 

 and those which are stopped late in their career will have 

 lost some of their initial velocity, the Iiontgen rays produced 

 by a corpuscle will in general be softer than the primary 

 pulse expelling the corpuscle. 



If we assume that this " extra " radiation is very much 

 greater in amount in the forward direction than in the direc- 

 tion opposite to the direction of the primary rays, and that 

 the ratio of the amount of extra to the amount of scattered 

 radiation increases with the hardness of the rays used, we 

 can explain all our results. These suppositions seem justified 

 by the results of Gray's experiments on the 7 radiation excited 

 by the (3 rays from radium t. 



First consider the results given in Table II. The hard 

 portions of the primary will excite in the carbon 



(a) pure scattered radiation of the same penetrating power 



as the primary, the value of J^- for which does not vary 



-L90 



with increase in hardness of the rays. 



(b) The " extra " radiation softer than the primary, the 



value of -J^- for which increases as the hardness of the rays 



I90 

 used increases, and 



(c) hard fluorescent radiation, which is only produced by 

 the very hard portions of the beam and in relatively small 

 amounts. 



As the fluorescent is only present in small amounts we 

 explain the increase in ratio when only the hard rays fall on 

 the radiator. 



When the secondary rays are absorbed the fluorescent 

 radiation will be present in much larger proportions, as both 

 the " extra " and scattered radiation, and particularly the 

 " extra "J, are softer than the fluorescent radiation, and are, 



* Whiddington, Proc. Phil. Soc. Camb. p. 329 (1911). 



t J. A. Gray, Proc. Roy. Soc. A. .vol. lxxxvi. pp. 513-529. 



X The fact that the extra radiation is softer than the primary producing 

 it is not necessary to the argument. 



The extra radiation is certainly softer when a homogeneous beam is 

 used, but it is very unlikely that this is true for a heterogeneous beam, 

 as the ratio of the energy in the " extra " radiation to that in the primary 

 increases with an increase in the hardness of the primary. All that is 

 necessary is that the extra radiation is softer than the fluorescent, which 

 is the case as the fluorescent is harder than any of the primary beams 

 used, and also that the "extra" radiation is very small in amount 

 compared with the scattered radiation, which is the case for the radiation 

 in this direction. 



