750 Prof. A. H-. Compton on the 



comparatively soft secondary radiation is excited by relatively 

 hard primary radiation. 



From theoretical considerations, both scattered and 

 fluorescent radiation should undoubtedly be present in 

 secondary gamma rays. According to J. J. Thomson's 

 well-known theory*, when 1 he wave-length is so short that 

 there is no appreciable co-operation in the scattering by the 

 different electrons, the mass scattering coefficient should be 

 about 0"2 for all elements and all wave-lengths (if the 

 number of electrons per atom effective in the scattering is 

 equal to the atomic number, as seems to be the case for 

 hard X-rays). The magnitude of the scattering to be 

 expected is considerably reduced if the wave-length ap- 

 proaches the size of the electron, and may, indeed, become 

 very small if the ratio of the wave-length to the diameter of 

 the electron approaches unity f. There is, however, on the 

 basis of the classical electrodynamics, no means of eliminating 

 completely the scattered radiation. 



Fluorescent radiations of a comparatively soft type, 

 presumably the characteristic K radiations, have been detected: 

 in the secondary gamma rays from elements of high atomic 

 weights J. But, in addition to this, there should be excited 

 in all elements a harder fluorescent radiation by the impact 

 of the high-speed beta particles liberated by the primary 

 rays. The number of such electrons expelled by gamma 

 rays is known to be much the same per unit mass for all 

 elements, and it, has been shown experimentally § that gamma 

 rays in not greatly different amounts per unit mass are 

 excited when beta rays fall upon different substances. Thus 

 one would expect to find in the secondary gamma rays an 

 appreciable amount of fluorescent radiation, which, like the 

 scattered radiation, does not differ greatly according to 

 the element us^d as radiator. 



The usual method of distinguishing between scattered and 

 fluorescent radiation is by comparing the absorption co- 

 efficients of the primary and secondary radiations. It is 

 assumed that the scattered rays are of the same hardness as 

 the primary rays, whereas all known high - frequency 

 fluorescent radiations are of a less penetrating type. Gray J 

 and Florance §, however, have rejected this criterion, for 

 although they find that the secondary radiation excited by 

 hard gamma rays is of a distinctly softer type than the 



* J. J. Thomson, ' Conduction of Electricity tlirouo-h Gases ' 2nd ed 

 p. 3l>5. 



t A. H. Compton, Phys. Rev. xiv. p. 23 (1919). 



t J- A. Gray, loc. cit. 



§ D. C. H. Florance, loc. cit. 



