Degradation of Gamma-Ray Energy. 769 



At la roe angles with the primary beam the scattered 

 energy is probably less than 0*001 of that required by the 

 usual electron theory. 



The secondary fluorescent radiation is found, in accord 

 with observations by others on the whole secondary radiation, 

 to be harder and more intense at small angles with the 

 incident beam than at large angles, and tables are given 

 showing the manner of this variation. 



While at large angles the radiation from heavy elements 

 is somewhat more penetrating than that from the light 

 elements, at small angles both the hardness and the intensity 

 of the fluorescent radiation are approximately the same from 

 elements covering a wide range of atomic numbers. 



A study of the absorption coefficients of this radiation in 

 various elements shows that the softest parts of it, though 

 of shorter wave-length than the K radiation from lead, are 

 not harder than the most penetrating X-rays. The hardest 

 parts approach in penetrating power the primary gamma 

 rays from radium C. 



It is pointed out that the very small scattering observed 

 is not incompatible with the classical electrodynamics, if 

 the wave-length of the gamma rays and the diameter of the 

 electron are of the same order of magnitude. 



A satisfactory qualitative explanation of the observed 

 fluorescent radiation is found in the gamma rays produced 

 by the impact of the secondary beta particles liberated in the 

 radiator by the primary gamma rays. 



The observed asymmetry in the intensity and hardness 

 may be accounted for if the oscillators which give rise to 

 the fluorescent radiation are electrons moving in the direction 

 of the primary beam with about half the speed of light. 



The wave-length of the softest part of the observed 

 fluorescent radiation is shown to lie between 006 and 0'12 

 A.U., probably nearer the former value, while the wave- 

 length of the hardest part is probably about half as great. 

 By a comparison of absorption coefficients, the effective 

 wave-length of the hard gamma rays from radium is estimated 

 as about 2 or 3 x 10" 10 cm. 



The writer performed these experiments at the Cavendish 

 Laboratory as National Research Fellow in Physics. He 

 desires to express his appreciation of the interest which 

 Professor Rutherford has shown in the work. 



Washington University, 

 St. Louis, 

 September 24th, 1920. 



