X-RAY FILTERS 



19 



It has been found that, if the mass absorption coefficients are plotted 

 as a function of the cube of the wavelength, the curves of Fig. 1-8 change 

 to straight lines. This allows us to conclude that 



Mr 



cX 3 + b 



This is shown in Fig. 1-8, where c is the slope and b, interpreted as the 

 mass scattering coefficient, is the intercept on the y axis. In this way 

 the practical absorption formulas of Table 1-4 were obtained. Note 

 that copper has a slope ten times greater than aluminum. 



CuX J 0.1 



if absorption,, 

 edge at 1.38 A 

 0.5 0.6 



// 



0.2 



0.4 0.6 0.8 1,0 

 Wavelength in A 



Fig. 1-8. These curves show the relatively greater opacity of copper as com- 

 pared with aluminum when used as therapeutic filters. The broken lines show mass 

 absorption coefficients plotted as a function of wavelength cubed. 



X-Ray Filters 



In roentgen-ray therapy it is highly desirable to remove the long- 

 wavelength radiations which, failing to penetrate the deeper tissue 

 because of superficial absorption, may give rise to superficial x-ray burn. 



Depending upon the excitation voltage aluminum, copper, and mixed 

 metal filters are used of various thicknesses, in order to remove these 

 long-wave radiations. Typical per cent transmission curves for alumi- 

 num are shown in Fig. 1-9. The aluminum filter is inserted just below 

 the x-ray tube, and the per cent transmission is obtained by means of 

 ionization-chamber measurements. 



