Degradation of Gamma-Ray Energy. 75 1 



primary radiation, they conclude that the primary rays are 

 truly scattered, but in the process of scattering are so 

 modified as to become less penetrating. It is therefore 

 important to determine under what circumstances, if any, 

 the hardness of the scattered rays may differ from that of 

 the primary rays. 



If the scattering is due to electrons of negligible dimen- 

 sions which are separated far enough to act independently 

 of each other, there is no question but that the scattered ray 

 will be exactly similar to the primary ray in every respect 

 except intensity ; for since the accelerations to which each 

 electron is subject are strictly proportional to the electric 

 intensity of the primary wave which traverses it, and since 

 the electric intensity of the scattered ray (at a great distance) 

 due to each electron is proportional to its acceleration, the 

 electric vector of the scattered wave is strictly proportional 

 to the electric vector of the primary wave. Thus the fre- 

 quency, the wave-form, the damping, etc., will be the same in 

 both beams. Radiations scattered by such electrons should, 

 therefore, be identical in character with the primary waves. 



Whatever type of scattering unit be assumed, it is also 

 clear that, if the primary wave is perfectly homogeneous — 

 i. <?., if it is an indefinitely long train of simple harmonic 

 waves of constant frequency, — the scattered waves must also 

 be homogeneous and of the same frequency. If, however, 

 the scattering unit — whether a group of electrons or the 

 individual electron — is of dimensions comparable with the 

 wave-length of the incident radiation, theory demands that 

 the scattering, especially at large angles, shall be less for 

 short than for longer waves. This prediction is confirmed 

 by measurements of the scattering of X-rays and gamma 

 rays over a wide range of frequencies. If the primary beam 

 consists of very short, highly-damped pulses, or of waves of 

 some irregular form, it may, of course, be considered as the 

 Fourier integral of a large number of long trains of waves 

 of different wave-lengths. Thus, unless the primary beam 

 consists of long trains of monochromatic w r aves, the scattered 

 radiation will, in general, be softer than the primary rays, 

 and the hardness will be greater at small than at large 

 angles with the incident beam. This corresponds qualitatively 

 with the properties of the secondary gamma rays *. 



* This explanation of the difference in hardness, as well as an ex- 

 planation of the distribution of the intensity of the scattered gamma 

 radiation, lias been discussed in detail, for the special ease of scattering 

 by a ring- electron of comparatively large size, by the writer (Pins. Rev. 

 xx. p. 30 (1919)). 



3 I) 2 



