22 BIOPHYSICALLY ACTIVE X-RAYS 



dent energy into scattered radiation. If the wavelength is large as 

 compared to the diameters of the atoms composing the absorbing filter, 

 the atomic scattering is independent of the incident wavelength and 

 we may call this an example of true scattering. Under these circum- 

 stances the scattered radiation has the same wavelength as the incident 

 radiation, and each atom acts as a source for emitting scattered rays 

 in all directions. Some of the energy is scattered forward, and back- 

 ward, as well as sideways. The scattered x-radiation may penetrate 

 and be absorbed by the filter in a manner similar to the incident energy. 

 Its intensity is additive to the unabsorbed primary x-radiation at every 

 point in the absorbing substance so that the total intensity at any point 

 in the absorber is increased by the scattering process. 



The scattering occurs within the medium; the smaller the density 

 of the absorbing medium, the greater is the possibility that the scattered 

 ray emerges. The scattered radiation which escapes from an absorbing 

 medium of great density is less than that escaping from a less dense 

 medium. 



It has been found that the intensity of a beam of hard rays is reduced 

 by scattering more than by mass absorption. Very soft rays are reduced 

 in intensity more because of mass absorption than because of the scatter- 

 ing effect. 



Table 1-5 shows the relative importance of the mass absorption coeffi- 

 cient and the mass scattering coefficients for various wavelengths and 

 filters. As the wavelengths of the x-radiation decrease, for any given 

 filter, scattering becomes more and more the predominating factor in 

 the absorption of the energy, for the scattering coefficients become 

 increasingly great in comparison to the mass absorption coefficients. 

 For absorbing material containing elements of small atomic number 

 the scattering for any given wavelength is predominantly more effective. 

 On comparing the loss due to scattering with the loss due to absorption, 

 it is found that for light elements and soft rays (0.7 A) the absorption 

 loss is roughly three times as great as the scattering loss. For hard 

 rays (0.1 A) the loss is about equally distributed between the two effects. 

 For the heavy elements the loss due to scattering is nearly neg- 

 ligible for soft rays, but it is about 20 per cent or less for the hard 

 rays. 



Finally, the scattering is smaller in proportion to the absorption, the 

 greater the density of the absorbing medium. 



This loss explains why tissue, owing to the low density and small mass 

 of its constituent atoms, produces so much scattered radiation when 

 subjected to x-ray examination. 



