Degradation of Gamma-Ray Energy. 759 



In common with these experimenters, it is found that the 

 secondary radiation, which in the writer's work consisted 

 almost wholly of fluorescent radiation, is very much more 

 intense at small angles than at large angles with the incident 

 ^amnia rays. 



The relative amount of fluorescent radiation excited in 

 different substances per unit mass is shown in Table V. 



Table V. 



. , Relative fluorescence per unit mass* 



Angle - Paraffin. Al. Fe. 



Sn. 



Pb. 



0-78 



. 074 



0-8 



0:9 



135° 1-12 104 (POO) 



45° 1-7 0-9 (1-0) 



The readings at 135° were taken for primary rays filtered 

 through 0*5 cm. of lead, and those at 45° were with a 4*1 cm. 

 lead filter. Thus it was made certain that practically all 

 of the secondary radiation was of the fluorescent type. 

 Sufficiently thin plates of the radiating materials were 

 employed that the necessary corrections for the absorption 

 of the primary and secondary radiation in the radiator were 

 not large. The values here given therefore represent the 

 amount of the fluorescent radiation excited in unit mass 

 of the different radiators, which penetrates the 0*15 cm. 

 lead window of the ionization chamber. It will be seen that 

 the values do not differ greatly over a wide range of atomic 

 weights. 



The constancy is even more marked when the fluorescence 

 per electron is calculated by multiplying each of the above 

 values by the ratio (atomic weight)/(atomic number), as is 

 -done in the following table. At the angle 135° the constancy 



Table VI. 



. 1 Relative fluorescence per electron : 



^ ngie - Paraffin. Al. Fe. Sn. Pb. 



135° 0-91 1-01 (100) 0-87 0"87 



45° 1-4 09 (1-0) 0-9 1-1 



■of these values is somewhat accidental, since, as we shall 

 .see, the absorption coefficient of the fluorescent radiation at 

 this angle is considerably greater for the radiation from the 

 light than for that from the heavy elements. At 45°, how- 

 ever, the hardness of the fluorescent radiation is practically 

 the same for the different radiators, so the constancy of the 

 values at this angle is of real significance. The result 

 expressed by this table is confirmed by the more quantitative 

 experiments of Ishino *, who found that the magnitude o[ 

 * M. Ishino. Joe. cit. 



