366 Secondary y Radiation. 



The penetrating power of the scattered radiation was 

 measured in lead screens 0*04 cm. and 0*084 cm. thick 

 placed against the side of the electroscope. The amount 

 of scattered radiation from thin radiators is very small and 

 accurate results were not easily obtained, but from a large 

 amount of observations there appears to be no doubt that the 

 penetrating power of the scattered radiation is independent 

 of the material of the radiator. This has also been confirmed 

 by Gray (loc. cit.) . It is generally agreed that the scattered 

 X rays are also independent of the material of the radiator, 

 and that they are of the same penetrating power as the 

 primary X rays when the penetrating power is measured 

 in aluminium. In the case of y rays, the penetrating power 

 of the scattered radiation has been shown to be less than 

 that of the primary radiation when measured in aluminium, 

 and consequently the character of the radiation has been 

 modified. This " softening " of the 7 rays is most marked 

 when the absorbability is measured in lead. But it is well 

 known that lead acts as a powerful absorber for the softer 

 radiation, and this is probably due to the fact that the 

 radiation is producing a radiation characteristic of the 

 absorbing screen. 



It has already been stated that there is evidence of a 

 characteristic radiation, and this evidence is supported by 

 the fact that there is an increase in the number of /3 rays 

 produced when lead is used as an absorber. This agrees 

 with the observations of Barkla that, besides the increase in 

 absorption of X rays, there is an increase in the production 

 of ft rays when the rays characteristic of the element are 

 excited. It was stated in a previous paper by the writer 

 that the penetrating power of the scattered radiation depended 

 chiefly on the angle through which scattering takes place, 

 rather than on the fortuitous circumstance of the radiation 

 being either "emergent 3 ' or "returned." Further experi- 

 ments have been carried out with the radiator so arranged 

 that whether the returned or the emergent radiation is being 

 examined, the angle of scattering remains approximately the 

 same. The former results have been completely confirmed. 



The scattering appears to be simply proportional to the 

 number of scattering electrons, each electron acting as a 

 distinct scattering centre and acting independently whether 

 it exists amongst a small number of electrons, as in the atom 

 of aluminium, or amongst a large number of electrons, as in 

 the atom of lead. At the same time, a certain amount of 

 energy is given up to the electron in the process of scat- 

 tering, and the amount of this energy increases with the 



