754 Dr. C. G. Barkla and Mr. C. A. Sadler on 



us consider how much of the energy absorbed has been ac- 

 counted for. One of us in a previous paper * has shown 

 that when X-rays within the range of penetrating powers 

 experimented upon are transmitted through a substance of 

 atomic weight less than sulphur, the energy of the radiation 

 scattered is independent of the penetrating power of the 

 radiation, and depends merely on the quantity of matter 

 traversed. 



It was shown that if I be the intensity of a beam passing 

 through air under atmospheric conditions, 



~ - due to scattering = —'000241, approximately,. 



— -— due to scattering = —'21 

 p dx & 



for all light elements. 



If by analogy with the absorption coefficient we call the 



portion of ( — y ^~ ) due ^° scattering the scattering co- 



efficient s, we get - for elements of atomic weight less than 



sulphur to be "2. 



This law does not hold, however, for elements of higher 



atomic weight. The value of - is much more difficult to 



determine in these owing to the usual admixture of the 



characteristic secondary radiation. Only in the case of Ag 



has the scattered radiation been measured after complete 



elimination of the other type of radiation. In this case it 



was found to be about 6*5 times that found from light 



s 

 elements. This would give for - about 1*3. 



. P 

 In copper, however, an estimate of the quantity of scattered 



radiation mixed with the homogeneous secondary rays was 



made and found to be about twice as great as for the light 



elements, when a penetrating primary radiation was used. 



On comparison of the scattering coefficients with the total 



absorption coefficients it will be seen that only when the rays 



are fairly penetrating and when absorption takes place in 



light atoms does the scattering account for a large fraction 



of the loss of energy of the primary beam. Thus in the case 



of the transmission of Ag radiation through C, nearly half of 



the total loss of energy is due to scattering. Below are given 



* Barkla, "Energy of Secondary Rontgen Radiation," Phil. Mag. 

 May 1904, pp. 553-560. 



