MEASUREMENT OF X-RAYS AND RADIUM 



59 



Standard Absorption Curves from 225 to 550 Kv. — To permit extension 

 of the complete absorption-curve method of specifying radiation quality 

 to tube potentials above 250 kv., the copper absorption data for voltages 

 up to 550 kv. (constant) are given in Table 6. The initial filtration in this 

 case is 6.4 mm. of steel plus 13 mm. of water, which together are equiva- 

 lent to about 6 mm. Cu. Absorption curves obtained with greater 

 initial filtration can be readily correlated with curves plotted from 

 Table 6, in the same manner as those of Fig. 6 with Fig. 5; but not so with 

 curves of lower initial filtration, because of the uncertainty with which the 

 magnitudes can be extrapolated toward the higher transmissions. In 



Table 6. — Absorption of General X-radiation in Copper 

 (Tube wall, 6.4 mm. steel + 13.0 mm. water) 



other words, although these curves can be of value as standards, they do 

 not have the range of application reserved to basic curves, i.e., of radiation 

 subjected to zero filtration. 



SECONDARY METHODS OF EXPRESSING X-RAY QUALITY 



Effective Wave-length.— From the same absorption data necessary for 

 the determination of the equivalent voltage, and plotted as in Figs. 

 5 to 7, there may be determined an effective absorption coefficient and a 

 corresponding effective wave-length, both of which have found some use 

 in practice. The slope of a semilogarithmic absorption curve at any 

 point gives directly the absorption coefficient He in copper of the hetero- 

 geneous radiation corresponding to the particular thickness for which the 

 point is chosen. 



This follows from the absorption law 



where h is the incident intensity, / the transmitted intensity through a 

 filter of thickness x, and He is the efifective absorption coeflicient. 



