MEASUREMENT OF X-RAYS AND RADIUM 



49 



absorption curve in its most useful form. Such a curve is shown by 

 ACB in Fig. 4, plotted on the .Y-, F-axes for X-rays excited by 150 kv. 

 (constant) in a thin-walled cerium glass tube. 



Until recently, the complete copper-absorption-curve method has 

 had the disadvantage of not being expressible by a single numerical 

 magnitude. This, however, has been removed (50, 51) by finding that, 

 within reasonably satisfactory hmits, the complete composite absorption 

 curve of radiation excited by the different potential wave forms in use 

 may be matched by the instantaneous absorption curve of the same 



2.0 



15 



c 

 o 



in 



El.O 



in 

 c 

 O 



u 

 f- 

 +- 



£0.5 

 u 





o 



Copper Filter Cm m) 

 Fig. 4. — Hypothetical logarithmic absorption curves to show the effect of filtration. 



material for radiation excited by some definite constant potential. It 

 is further found that such beams having equivalent absorption curves 

 are, over a considerable range of initial filtration, closely alike as regards 

 their intensity distribution in a large body of low-atomic-number material 

 such as a water phantom. Since the spectral distribution of radiation 

 excited by constant potential is perfectly reproducible, a family of 

 complete absorption curves of constant voltage radiation would constitute 

 a very convenient and adequate standard of reference for inferring the 

 constant-potential equivalent^ of any given radiation. 



Equivalent X-ray Beams. — It is, of course, obvious that two equiva- 

 lent X-ray beams might be produced by voltages of very different peak 



1 By "constant-potential equivalent" is meant the constant potential necessary 

 to apply to an X-ray tube to yield an absorption curve of the same form as the com- 

 posite absorption curve in the same material for the unknown radiation in question 



(51). 



