The Wave-Length of Hard Gamma Rays. Ill 



wave-length of the " incident " secondary gamma radiation, 

 and calculating from this the wave-length o£ the "emergent" 

 secondary radiation on the hypothesis that the difference in 

 wave-length is a Doppler effect due to motion or! the particles 

 emitting the secondary radiation, the wave-length of the 

 primary gamma rays can be estimated, since the absorption 

 coefficient of the primary and the "emergent" secondary 

 radiation is nearly the same. This method leads to a value of 

 between 0*02 and O03 A.U. for the effective wave-length 

 of the hard gamma rays from radium *. 



In the present paper a new method of measuring the wave- 

 length of high frequency radiation will be proposed, and the 

 method will be applied to the determination of the wave- 

 length of gamma rays. Instead of studying the spectrum 

 lines reflected by a grating composed of regularly arranged 

 atoms in a crystal, this method consists in observing the 

 diffraction pattern due to the individual atoms. To consider 

 an optical analogy, if the reflexion of X-rays from a crystal 

 is compared with the spectrum from a ruled grating, the 

 method of atomic diffraction corresponds to a study of 

 the diffraction pattern due to a large number of parallel lines 

 ruled at random distances. The distance between the different 

 order lines in the spectrum is determined by the grating- 

 space between the lines ruled on the grating, while the 

 distance between the bands of the diffraction pattern is 

 determined by the breadth of the individual lines. The 

 advantage of the method as applied to gamma-ray measure- 

 ments lies in the fact that the effective diameter of the atom 

 is much smaller than the distance between two atoms in a 

 crystal, so that the effective width of the diffraction band is 

 much greater than the distance between two spectrum lines. 

 Thus, whereas the spectrum of hard gamma rays from a 

 crystal grating would have to be studied at angles less than 

 1/2 degree, atomic diffraction measurements may be made at 

 angles in the neighbourhood of 10 degrees. In order to use 

 the method quantitatively, it is of course necessary to know 

 the effective diameter of the atom. This may be determined, 

 in a manner that will be described below, by measurements 

 with X-rays of known wave-length. 



Deby e has shown that if an atom is composed of N electrons, 

 and if at any instant the distance between the with and the 

 nth electron is s mn , the probable intensity of the X-rays 



* A. H. Compton, Phil. Mag. supi'd, p. 749. 



