MEASUREMENT OF QUALITY OF X-RADIATION 27 



structural details rather indistinct. The photograph lacks contrast and 

 detail. In a similar way the lack of contrast and detail in a radiograph 

 is attributable to the scattering of the x-radiation into the geometrical 

 shadow of the object. 



Measurement of Quality of X-Radiation by Absorption 



The radiation emitted by an x-ray tube is heterogeneous. It consists 

 of a continuous spectrum overlaid with a line spectrum characteristic 

 of the metal target, provided that the potential across the tube is suffi- 

 ciently high to excite the characteristic radiation of the target. The 

 " quality " of the radiation, especially with regard to its therapeutic 

 applications, depends both upon the wavelength content of the spectrum 

 and upon the distribution of the energy over its whole range of wave- 

 lengths as measured on the outside of the glass housing of the x-ray 

 tube. 



The direct determination of this spectral composition is beset with 

 considerable experimental difficulties, as we have seen. Accordingly, 

 an indirect way of obtaining the same results has recently been developed 

 by W. S. Taylor and his associates at the Bureau of Standards, by using 

 a so-called filtration method. 



The specification of x-radiation quality, with the aid of an absorption 

 curve, has been recommended by the x-ray standardization committee 

 [1934] of the Radiological Society of North America. The committee 

 reported: " For most 'practical purposes the quality of x-radiation may be 

 satisfactorily specified in terms of the copper or aluminum absorption curve 

 combined with a statement of the initial filtration. In lieu of an absorption 

 curve, the equivalent constant potential applied to the tube terminals to yield 

 the same curve may be stated as a single numerical magnitude. Up to 

 100 kv (constant) aluminum absorption curves and above 100 kv (constant) 

 copper absorption curves shall be used to establish the equivalent potential. " 



In order to obtain the complete absorption curve, filters in the form 

 of successively thicker sheets of a metal are placed in the diaphragmed 

 beam of an x-ray tube driven at constant potential and predetermined 

 constant milliamperage. The transmitted intensities are obtained by 

 means of an ionization chamber (spectral distribution of energy assumed 

 constant) placed below the absorber. 



Typical results as obtained by Taylor and Singer [1930] are shown 

 in Figs. I— 11 and 12, for copper and aluminum filters of increasing thick- 

 ness for radiations from a tube with tungsten target excited at various 

 potentials. 



If the absorption of filters were of the simple exponential type 

 (/ = I e~ fix ), then plotting log per cent transmission as a function of 



