760 Prof. A. H. Compton on the 



the total secondary radiation (which includes any scattered 

 rays that may be present) per atom is more nearly pro- 

 portional to the atomic number than to the atomic weights 

 The amount of the fluorescent radiation excited is therefore 

 approximately proportional to the number of electrons 

 traversed by the primary gamma rays. 



It will be seen on examining Table I. that the fluorescent 

 radiation at small angles with the primary beam is con- 

 siderably harder than that at right angles. This matter was 

 examined in greater detail for a number of different elements, 

 with the results shown in Table VII. Care was taken in 

 these experiments also to eliminate any possible soft scattered. 







Table VII. 









Mass 



absorption coefficients 



in Lead 



of the 



ngle. 



fluorescent 



•adiation exci 



ted in different 



materials by 



hard gamma rays : 







Paraffin. 



Al. 



Fe. 



Sn. 



Pb. 



45° 



o-io 



0-10 



011 



0-09 



0-05 (?) 



90° 







0-21 







35° 



0-78 



50 



0-50 



0-32 



0-15 



radiation by interposing suitable absorption screens between 

 the source of gamma rays and the radiating material. As a 

 result, the intensity of the secondary radiation was so low 

 that the values of the absorption coefficients obtained can be 

 considered only approximate. The data suffice to show, 

 however, that while the radiation from all substances is 

 harder at small angles, the difference is less for elements of 

 high atomic weight, so that whereas at large angles the 

 radiation from the heavier elements is considerably more 

 penetrating, at 45° the hardness differs but little from 

 element to element*. 



Interesting information is obtained on examining the 

 absorption coefficients of this penetrating fluorescent radiation 

 in various materials. This was done for the secondary 

 radiation from iron at 135° after the primary gamma rays 

 had been filtered through 0'5 cm. of lead, with the results 

 shown in the following table. With a similar geometric 



Table VIII. 



Mass absorption coefficients in different elements of 

 the fluorescemt radiation at 135° excited in iron 

 by hard gamma rays from radium C : 

 Pb. Sn/ Fe. AL 



0-50 018 008 007 



* The constancy observed for different elements at 45° is confirmed by 

 the measurements' of Florance, Phil. Mag. xx. p. 935 (1910). 



