62 BIOLOGICAL EFFECTS OF RADIATION 



slope of the tangent. Of course, where the curvature is very small, the 

 tangent and cord nearly coincide and the two effective wave-lengths are 

 nearly alike. The main drawback to Duane's method is that it gives a 

 different result for every filter thickness employed (33). 



Half-value Layer.— Another means of expressing quality, known as the 

 half-value method, is used extensively in Europe (8). By this, a given 

 radiation quality is described as the thickness of copper (or aluminum) 

 which placed in an X-ray beam reduces the intensity to half its incident 

 value The result is expressed in millimeters of copper (or aluminum). 

 For the higher voltages, the H.V.L. of copper is used. On dropping to 

 lower voltages the H.V.L. of copper decreases rapidly and when it 

 becomes less than 0.1 mm., H.V.L. values of aluminum are used instead. 

 To obtain the H.V.L. for a given beam, filters of gradually increasing 

 thickness are placed in the beam and the transmitted intensity read by a 

 suitable ionization chamber. The transmitted intensities are then 

 plotted against the corresponding filters and that filtration correspondmg 

 to the desired half-intensity is read from the graph. It is important to 

 note that the H.V.L. measurements should be made on the beam actually 

 used for treatment, not on the original unfiltered beam (unless the 

 unfiltered is being used). Figures 10 and 11 give half-value layers for 

 copper and aluminum for various constant voltages and various 

 filtrations (21). 



STANDARD MEASUREMENT OF X-RAY QUANTITY OR INTENSITY 



International Definition of X-ray Quantity.— As noted above, the 

 property of X-rays which permits them to ionize a gas forms the present- 

 day basis of X-ray-intensity measurements. The ionization produced by 

 a given beam is proportional to the energy absorbed from the beam by 

 the gas between X 0.1 and 1.5 A. This in turn is a function of the density 

 of the gas, or of the number of atoms encountered by the X-ray m a given 

 distance along its path. The absorption is likewise approximately pro- 

 portional to the fourth power of the atomic number of the gas and to the 

 third power of the wave-length of the X-rays. As already pointed out, 

 in detail, the wave-length dependence of the absorption is the factor 

 which necessitates the quality specification in intensity measurements. 

 The dependence upon the atomic number of the gas would be of utmost 

 importance if various gases were commonly employed in practice. This 

 undesirable complication is avoided by the arbitrary choice of a single 

 gas for all comparative purposes. 



For the ionized medium, air has been chosen for two particular reasons : 

 (a) because its effective atomic number is very nearly the same as for 

 body tissue (the X-ray absorption per gram is nearly the same for air, 



