762 Prof. A, H. Compton on the 



element to element, the secondary gamma rays, especially 

 at small angles with the incident beam, are of nearly the 

 same hardness over a wide range of atomic numbers. And 

 in the second place, while the characteristic radiations are 

 found to be distributed uniformly with regard to intensity 

 and quality at all angles with the primary beam, the 

 fluorescent gamma rays show marked asymmetry in both 

 quantity and quality in the forward and reverse directions. 

 There is therefore good reason to suppose that the oscil- 

 lators which give rise to this fluorescent radiation are 

 radically different in character from those which are 

 responsible for the K, L, and M characteristic radiations *. 



An explanation of the origin of the fluorescent radiation 

 which appears to be satisfactory is that the high-speed 

 secondary beta particles liberated in the radiator by the 

 primary gamma rays excite the secondary gamma rays as 

 they traverse the matter of the radiator. On this view the 

 fluorescent gamma rays should be identical in character with 

 the so-called " white " radiation excited in the target of an 

 X-ray tube by the impact of the cathode particles. Experi- 

 ments have shown that when cathode rays or beta rays 

 strike a target which is so thin that the particles are not 

 greatly scattered and in which no considerable amount of 

 characteristic radiation is excited, the X-rays emitted are 

 more intense and harder in the general direction of the 

 cathode ray beam than in the reverse direction f . This 

 asymmetry is of the same kind as that observed for the 

 secondary gamma rays, and though not so marked, is found 

 to increase with the speed of the impinging electrons. For 

 speeds comparable with those of fast beta rays the asymmetry 

 may well become as great as that observed in the present 

 experiments. But it is also known that the beta rays 

 liberated by gamma rays are much more intense in the 

 direction of the gamma ray beam than in the reverse 



* The idea suggested itself that the secondary radiation which was 

 being studied was a fluorescent radiation excited in the lead screens 

 which surrounded the source of gamma rajs, this fluorescent radiation 

 being in turn scattered by the radiator into the ionization chamber. It 

 is obvious lhat such a radiation would not be eliminated by placing- 

 additional lead screens over the source, while the ionization would be 

 considerably reduced by placing screens over the ionization chamber. 

 Considerations of the energy invoh ed and of the characteristics of the 

 secondary radiation rendered this suggestion improbable, but the 

 possibility was definitely eliminated by removing all the lead screens 

 and replacing them with iron. The phenomenon in this case was 

 identical with that when lead screens were employed. 



f G. W. C. Kaye, Proc. Camb. Phil. Soc. xv. p. 269 (1909). J. A. 

 Gray, loc. cit. 



