[eve] secondary y RAYS DUE TO THE y RAYS OF RADIUM C 7 



Lead is a poor secondary radiator in both cases, but iron radiates 

 well with the X rays and badly with the X rays. It is unnecessary to 

 examine these results more closely, because the secondary X rays have 

 been carefully investigated by Barkla and others, while the secondary 

 y rays will be discussed in this paper. 



As regards the secondary kathode rays from X rays, /? rays, and 

 y rays, it has been shown by J. J. Thomson, McClelland, and Kleeman, 

 respectively, that the radiation intensities follow the order of the atomic 

 weights. 



It will be seen later that the secondary y rays from a thick block 

 of iron may amount to 25 or 30 per cent of the total secondary kathode 

 and y rays from the same block. Moreover, the velocity of the 

 secondary kathode rays due to y ra5's is nearly equal to that of the 

 secondary kathode rays due to p ra3^s. The values of the so-called 

 coefficients of absorption by a screen of aluminium 0.4 mm. thick, 

 placed before the electroscope I find to be as follows: 



Primary /5 21 .5 



Kathode secondary from lead, due to /3 and y rays 24.5 



Kathode secondary from lead, due to y rays only 31 .0 



Hence, the velocities of the kathode secondary rays are a little 

 less for y than for /? rays, and both these groups have less velocity 

 than that of the p primary rays. 



On the other hand, the coefficient of absorption by lead of the 

 secondary y rays due to y primary, reflected from lead or iron, is 

 about 4, nearly equal to the coefficient of absorption found by Godlew- 

 ski for the primary rays of actinium, greater than the coefficient of 

 absorption of the y rays from uranium, and yet greater than that of 

 radium. (Phil. Mag., April, 1906) It may, therefore, be concluded 

 that y rays give rise to secondary rays of the y type, and not merely 

 to very penetrating kathode rays. This will be seen more clearly from 

 the diagrams given later. 



The fact that the kathode secondary vsljs due to ^ or to y rays 

 have velocities comparable with that of the primary y5 rays is a very 

 important one. The secondary kathode rays due to X rays have 

 much less velocity. Bragg has pointed out that it is probably more 

 than a coincidence that the secondary kathode rays, due to X rays 

 have velocities of the same order as those of the kathode rays in the X 

 ray tube, while the secondary kathode rays due to y rays approximate 

 in velocity to that of the primary fi rays. These relations, if more than 

 accidental, do not necessarih^ furnish an argument in favour of the view 

 recently advocated with so much skill by Professor Bragg as to the 

 material character of X and y rays. For it may be that ether pulses 



