46 BIOLOGICAL EFFECTS OF RADIATION 



measurements is rendered difficult because of time lag and hysteresis 

 in the cells. The many chemical effects produced by X-rays are all com- 

 pUcated and their relationship to the radiation intensity is seldom known 

 except by calibration. In 1902, Holzknecht (22) introduced pastiles 

 which, when exposed to X-rays, changed color to a degree depending upon 

 the quantity of radiation absorbed. Attendant color-matching charts 

 were calibrated in arbitrary units. Fluorescence in certain minerals has 

 also been used in X-ray-intensity measurements but complicated fluo- 

 rescence-excitation laws have prevented its general use. 



Lastly, and most important, is the ability of X-rays to ionize gases. 

 This property was recognized soon after their discovery, and in 1908 

 Villard (54) proposed a quantitative unit of X-ray intensity based 

 on the ionization produced in air. Essentially, this unit is the one 

 generally recognized today, and it will be discussed in detail below. 



Ionization Produced hy X-rays.— Oi the various ways of measuring 

 X-ray intensity outlined in the foregoing, all but the calorimetric method 

 depend in their indication upon the quality or spectral-energy distribution 

 of the radiation. Therefore, for intensity measurement, it is necessary 

 that the dependence on quality be known to a degree which is com- 

 mensurate with the desired overall accuracy of the determination. 

 Consequently, in measuring X-ray intensity, the method must be 

 employed in which the dependence upon quaUty is best understood, or 

 at least most readily reproduced and controlled. The power of X-rays 

 to ionize gases renders the air-ionization method most suitable for practi- 

 cal purposes. 



As already shown (cf. Darrow, Paper I), the amount, or degree, of 

 ionization produced in air is directly proportional to the quantity of 

 radiation absorbed by a given mass of air. As will be discussed in detail, 

 the unit of X-ray quantity (the roentgen) is defined in terms of the degree 

 of ionization produced in 0.001293 gm. of air. Therefore, since the 

 absorption of X-rays by matter depends upon the wave-length of the 

 incident radiation, it is evident that the unit of X-ray quantity depends 

 upon those wave-lengths composing the beam in question. 



X-ray Spectral Distribution.— Corresponding to any value of a steady 

 or instantaneous voltage applied to an X-ray tube is a particular distribu- 

 tion of energy among the wave-lengths of the resultant X-ray spectrum. 

 Likewise, this distribution depends upon the material of the target- 

 usually tungsten, in medical or biological application. Below a certain 

 potential for a given target material (70 kv. for tungsten) the spectrum 

 is continuous (Fig. 2) whereas above this critical value of the potential, 

 certain series of lines appear superposed on the continuous background, 

 which, in their spectral position, are characteristic of the target material. 

 As the exciting potential is increased above the critical value, the propor- 



