30 



BIOPHYSICALLY ACTIVE X-RAYS 



which possessed an effective wavelength of 0.2 A. The table of mass 

 absorption coefficients shows that copper possesses a mass absorption 

 coefficient 1.6 for this wavelength. The broken line in Fig. 1-13 is 

 drawn with slope 1.6. This represents the relation log e (I/Io) = —ex, 

 in Which c = 1.6, or log 10 (//J ) = -cz/2.3026. 



0.5 



1.0 1.5 2.0 



Copper filter thickness in mm 



2.5 



Fig. 1-13. Taylor's method of evaluating the effective wavelength of a hetero- 

 geneous beam of x-radiation. 



A line drawn parallel to this broken line contacts the 152-kv curve 

 at 0.4 mm and the 106-kv curve at 0.9 mm. 



The conclusion is that, when an x-ray tube is excited at 152 kv and 

 a 0.4-mm copper filter is placed in the beam, the radiation passing 

 through the copper filter has an effective wavelength of 0.2 A. A similar 

 effective wavelength may also be produced by a 0.6-mm copper filter 

 introduced into a 130-kv beam or by a 0.9-mm copper filter in a 106-kv 

 beam, if all other factors remain constant. 



The converse question may arise: what effective wavelength is trans- 

 mitted by a 1.0-mm copper filter? Let us assume the radiation whose 

 quality is to be determined as that produced by 120 kv. A tangent is 

 drawn to the curve at the 1.0-mm thickness. The slope of the tangent 



