Degradation of Gamma-Ray Energy. 761 



arrangement, the mass absorption coefficient of the primary 

 rays in lead was 0*062, which presumably means that this 

 fluorescent radiation is of very appreciably longer wave- 

 length than the primary gamma rays. On the other hand, 

 the experiments or Hull and Rice # show that X-rays of 

 wave-length 0*122 X 10" 8 cm. have a mass absorption 

 coefficient in lead of about 3*0, which indicates that even 

 the softest part of this fluorescent radiation is of shorter 

 wave-length than the critical wave-length 0"117 x 10~ 8 cm. 

 required to excite the characteristic K radiation in lead. 

 This conclusion is confirmed by the fact that the mass 

 absorption of this fluorescent radiation is greater in lead 

 •than in tin, which is the reverse of the case for wave-lengths 

 ] jet ween the K radiation from lead and the radiation from 

 tin. There can thus be no question but that the fluorescent 

 rays under examination are of a distinctly harder type than 

 the characteristic K radiation from even the heaviest 

 elements. 



In an experiment with a Coolidge tube operated by an 

 induction coil at a maximum potential of 196,000 volts, 

 Rutherford has obtained X-rays whose mass absorption co- 

 efficient in lead is as low as 075 f- This is practically the 

 same as the value observed for the fluorescent gamma 

 radiation from paraffin at 135° (Table VII. ). According to 

 the quantum relation, hv = eV,the wave-length in Rutherford's 

 experiment must have been greater than 0'063 A.U. The 

 wave-length of the softest part of this penetrating fluorescent 

 radiation must therefore lie between 0*06 and 0'12 A.U. 



It is interesting to note that these secondary gamma rays 

 bridge the gap which has existed between the hardest X-rays 

 and the very penetrating gamma rays ; for as we have just 

 seen, the softest part of this secondary radiation falls within 

 the wave-length of the hardest X-rays, while Table VII. 

 shows that at small angles it is nearly as penetrating as the 

 hard gamma rays from radium 0. 



The Origin of the Fluorescent Radiation. 



Although the secondary gamma radiation under exami- 

 nation seems, without doubt, to be fluorescent in nature, it, 

 differs in several important respects from the characteristic 

 fluorescent K and L radiations excited in matter when 

 traversed by hard X-rays. In the first place, whereas these 

 characteristic radiations differ greatly in hardness from 



* Hull & Rice, loo,, cit. 



t E. Rutherford, Phil. Mag. xxxiv. p. 153 (1017). 



