Degradation of Gamma-Ray Energy. 763" 



direction. Indeed, "the results indicate that the beta 

 particles initially escape in the direction o£ the gamma rays,. 

 and with the same speed for all kinds of matter"*. Thus 

 the hypothesis that the fluorescent gamma radiation is due to- 

 the impact of the secondary beta particles accounts quali- 

 tatively for the observed asymmetry in the hardness and 

 intensity of the secondary gamma rays. 



With regard to the relative intensity of the fluorescence 

 excited in different materials, attention may be called to the 

 fact that the number of the beta particles excited by hard 

 gamma rays is- approximately the same per electron in 

 different elements |. There is a somewhat larger number 

 produced in the very heavy elements such as mercury and 

 lead, which appears to be connected with the excitation of 

 the characteristic K radiation in these elements. But since 

 such radiation is too soft to have an appreciable effect in the 

 present investigation, it seems probable that the number of 

 effective beta particles excited in these elements does not 

 differ greatly from that for the lighter elements. If, there- 

 fore, the simple assumption is made that the amount of 

 secondary gamma rays excited depends only upon the' 

 number of electrons traversed by the secondary beta particles,. 

 our h}pothesis gives a satisfactory account of the fact that 

 the amount of secondary gamma radiation per election is 

 practically the same for all elements. 



The greater scattering of beta particles by elements of 

 high atomic weight means that in these elements a relatively 

 larger number of the beta rays move in a direction opposed 

 to the primary beam. For this reason we should expect, as 

 Table VII. shows is actually the case, that at large angles 

 with the incident beam the fluorescent radiation from the 

 heavier elements will be more penetrating than that from 

 the light ones. 



An estimate of the relative energy in the secondary rays 

 can be obtained by integrating over the surface of a sphere 

 the observed relative intensity of the scattered beam at 

 various angles. A rough summation of this kind, using the 

 data of Tables III. and IV. and extrapolating by the help of 

 Kohlrausch's data for the very small angles, shows that the 

 ratio of the energy (as measured by the ionization) of the 

 secondary radiation from iron to the total energy absorbed 

 from the primary beam is about 0*69. This result is in good 

 agreement with Ishino's estimate that the " scattering " by 

 iron accounts for 62 per cent, of the total absorption. A 



* E. Rutherford, ' Radioactive Substances, etc./ \\ '270. 

 f Eve, Phil. Mag-, xviii. p. 275 (1909). 



