Degradation of Gamma-Ray Energy. 151 



where I is the intensity of the primary beam at the ionization 

 chamber when the radiator is replaced by the source of 

 gamma rays, N is the number of electrons which are effective 

 in scattering, e and m are the charge and mass respectively 

 of the electron, 6 is the angle with the primary beam at 

 which the scattered beam is observed, I is the distance of 

 the radiator from the source of gamma rays, and C is the- 

 velocity of light. Taking the number of electrons per atom 

 as equal to the atomic number, and using the experimental 

 values 10'3 cm. for I and 234 g. for the mass of the iron 

 radiator, this expression gives for the ratio I /I at 90° the 

 valce 0*023. The experimental value of this ratio was 0'0017. 

 But of this we have seen that less than 3 per cent, probably 

 represents true scattering. The value of the ratio I s /I r 

 where I s is the observed true scattering, is therefore less 

 than 0*00005, only 2 per cent, of that required by theory. 

 Similar results for the scattering at 45° and 135° are given 

 in Table III. It will be seen from this table that at large 

 angles, if there is any true scattering, it is probably less than 

 a thousandth part of the amount predicted on the basis of 

 the usual electron theory. 







Table III. 







Lngle. 



observed. 



Is/I 

 observed. 



Is/I 



calculated. 



Is obs. 

 Is calc. 



45° 



0-015 



c. 0-001 



0-035 



c. 0-03 



90° 



0-0017 



< 000005 



0-023 



<0002 



135° 



0008 



< 0-00002 



0-035 



< 0-0005 



It is not impossible to account for this very low value of 

 the scattering on the basis of the classical electrodynamics^ 

 if suitable assumptions are made with regard to the wave- 

 length of the primary gamma rays and the properties of the 

 electron. Thus, for example, the writer has shown else- 

 where * that if the electron is a rigid sphere which is not 

 subject to rotational displacements by the primary beam, the 

 scattering at all angles becomes negligible when the ratio of 

 the wave-length to the radius of the electron is less than 

 about 2*4. Certain other types of electron give a similar 

 result for different values of this ratio. Jf this explanation 

 is the correct one, the wave-length or! these gamma rays 

 must be considerably shorter than that ol the hardest X-rays 

 which have yet been studied, since for these rays the 

 scattering, though somewhat smaller than that predicted by 

 the usual theory, is apparently of the proper order of 



* A. U. Compton, Phys. Rev. xx. p. 25 (1019). 



