MASS ABSORPTION COEFFICIENTS 17 



this absorption law may be written 



2.303 , 



n = -—- (log 7 - log I) cm 

 a 



The absorption coefficient p, or " Unear absorption," is the fractional 

 decrease in intensity of a beam of unit cross section per unit (linear) 

 path length through the absorbing medium. The " Unear absorption 

 coefficient " of a given beam of x-radiation is much greater in water 

 than in steam; hence, its value depends on the nature of the medium. 

 To get a more fundamental constant, an absorption coefficient that is 

 characteristic of the absorbing substance is required; hence a mass 

 absorption coefficient* is used defined by ix m = fx/p, where p is the density 

 of the material. The absorption law in terms of the mass absorption 

 coefficient takes the form 



I = V~ (M/p) " pd = I e~' lmpd 



Experiments have shown that the mass absorption coefficient of a 

 substance for x-radiation is independent of its physical state. Thus 

 the mass absorption coefficient of water is the same whether the water 

 is in the form of liquid or of gas. 



The independence of the mass absorption coefficient of the physical 

 and chemical states of an element sharply distinguishes x-radiation from 

 light. For instance, carbon in the form of diamond is optically trans- 

 parent, whereas in the form of graphite it is opaque. The mass absorption 

 coefficient of both forms for x-radiation, however, is the same. 



Mass Absorption Coefficients Increase with Increase in Wave- 

 length 



Figure 1-8 shows how the mass absorption coefficient of copper or 

 aluminum, common forms of therapeutic filter material, very rapidly 

 increases with an increase in wavelength. Table 1-3 gives tabulated 

 values of the mass absorption coefficients of aluminum and copper in 

 detail. Note the change in the values of fx m for copper between 1.389 

 and 1.293 X 10~ 8 cm. 



In x-ray technique an understanding of the comparative opacity of 

 common filter elements like aluminum, copper, and lead is essential 

 for the interpretation of the quality of radiation transmitted by these 

 filters. Attention should be called to the enormous difference in opacity 

 of copper and aluminum for the longer wavelengths. 



* Mass absorption coefficient is the fraction of a beam 1 cm 2 cross section absorbed 

 per gram of substance traversed. 



