Secondary Rontgen Radiation from Carbon, 



769 



coefficient which is the ratio o£ the energy in the secondary 

 beam excited by a primary beam traversing unit thickness 

 of material to the energy in the primary beam, supposing 

 the primary beam to be maintained in some way of con- 

 stant intensity during its passage through the substance. 

 In the case o£ copper the transformation coefficient is so 

 large that the ionization produced by the fluorescent ra- 

 diation is 300 times that produced by the scattered beam 

 when beams of ordimiry penetrating power are used. Now 

 although it is very unlikely that the transformation coefficient 

 for the hard fluorescent carbon radiation is as high as for 

 copper radiation, it is clear that if we have a primary beam 

 sufficiently hard, the proportion of fluorescent radiation to 

 scattered will be quite considerable and so the secondary 

 beam would be much harder than the primary. When the 

 primary beam becomes so hard that taken as a whole it is 

 harder than the fluorescent carbon radiation, ihen the 

 secondary carbon radiation will be softer than the primary, 

 as is the case with copper radiation when we use an ordinary 

 Rontgen-ray bulb. To sum up, we should expect that the 

 secondary beam should be just as penetrating as the primary, 

 then much harder than it, gradually approaching an almost 

 constant value, while the primary getting harder all the 

 time soon overtakes it, becoming just as hard and finally 

 much harder. 



IV. Description of Apparatus used to measure the 



Value of ~* 



-1 90 



The plan of the apparatus is shown in fig. 1. The carbon 

 radiator R was '94 cm. thick and 8»9 cm. square. It was 



