Secondary Rontgen Radiation from Carbon. 767 



they found that the results do not agree with this simple 

 theory. 



Now it is well known that if a primary beam of Rontgen 

 rays of ordinary penetrating power falls on an element of 

 higher atomic weight than 40, then that substance emits, in 

 addition to the relatively feeble scattered radiation, a secondary 

 radiation whose penetrating power is characteristic of the 

 particular element and does not depend in any w T ay on the 

 character or intensity of the incident primary rays. Barkla 

 has called this a fluorescent radiation. Although a fluorescent 

 radiation has not yet been identified in the case of carbon, 

 yet there is good reason for predicting that if a very pene- 

 trating beam of Rontgen rays fell on it a fluorescent radiation 

 would be given off. The fluorescent radiation has entirely 

 different properties for the scattered radiation. Two laws 

 will be referred to frequently, the first governing the pro- 

 duction and the second governing the distribution of fluorescent 

 radiation. The first was proved by Barkla and Sadler and the 

 second by Barkla. 



(1) The fluorescent radiation of a particular element is only 

 excited by a radiation which is harder than itself. 



(2) The value of the ratio T - of it is unity for all directions. 

 v ' I90 J 



The laws of distribution for the fluorescent and scattered 



radiations are thus entirely different, and so it is necessary to 



consider in what way the superposition of a hard fluorescent 



radiation will affect the value of ~ for whole beam. 



ho 



If the primary beam is very soft, none of the harder fluor- 

 escent carbon radiation will be excited and the ratio should 

 be (l + cos 2 a). When the beam is harder, the fluorescent 

 radiation will then be excited, and any given atom of carbon 

 w T ill emit it uniformly in all directions. 



The total radiation, it is easy to see, will give a ratio less 

 than (1 4- cos 2 a). As the beam is made very penetrating 

 indeed, the proportion of fluorescent to scattered radiation is 

 very largely increased so that for a very penetrating beam 

 the ratio should approximate to unity. As well as the very 

 penetrating fluorescent radiation, it is highly probable that the 

 carbon emits an exceedingly soft fluorescent radiation. The 

 intensity of this per unit area of a thick radiation must be 

 extremely small compared with the intensity per unit area of 

 the scattered radiation, because it will be very readily absorbed 

 in the radiator itself, and all that emerges into the air will 

 have been generated in an extremely thin surface layer and 

 w T ill be readily absorbed by the air. The scattered radiation, on 



