28 



BIOPHYSICALLY ACTIVE X-RAYS 



filter thickness a straight line of slope ju is obtained. The curves in 

 Figs. I— 11 and 12 show that the complete absorption curves do degen- 

 erate into straight lines, but only at high voltages and large values of 

 filter thickness. 



Hence, absorption for thick filters and high potentials can, to a close 

 approximation, be represented by an equivalent composite absorption 

 constant (c), such that / = I§e~ cx for comparatively large values of x. 

 For copper this result is attained at 150 kv with a 2-mm filter and for 

 aluminum at 110 kv with a 10-mm filter. 



100 



80 



CO 



§E 60 

 3£ 



w »40 



0> 



a. 



20 



2.0 



1.0- 



U o 



1 2 



Copper filter thickness in mm 



Fig. 1-11. After Taylor and Singer [1930]. 



The outstanding features of these absorption curves may be inter- 

 preted quite readily. The steeper the slope, the softer and more absorb- 

 able is the radiation. The slope in the more heavily filtered portion 

 decreases symmetrically with the increase in tube voltage. Mono- 

 chromatic x-radiation gives a straight-line absorption curve. The 

 degree to which the absorption curve approximates a straight line is a 

 measure of the homogeneity of the radiation. The similarity in the 

 variation of absorption by different materials makes it possible to derive 

 simple relationships between x-ray absorption characteristics of different 

 materials. It is possible to compare the x-radiation output of two x-ray 

 machines by means of the absorption curves obtained from each at 

 various voltages. 



Effective Wavelength of a Heterogeneous X-Ray Beam 



The effective wavelength emitted by the target was defined by Duane 

 [1928] as " the wavelength of monochromatic radiation that would 

 produce the same effects (the readings of the instrument employed to 

 detect it) that the actual (heterogeneous) radiation produces." 



In order to obtain the quality of an unknown beam of heterogeneous 



