X-RAY SPECTRAL DISTRIBUTION 7 



Of the total energy incident on the target, less than 1 per cent is con- 

 verted into x-radiation even under a driving potential of 100,000 volts. 

 The remainder, or 99 per cent, of the incident energy of the impinging 

 electrons can raise the temperature of the target even to its melting 

 point. This rise in temperature necessitates a rapid cooling of the 

 target by circulating oil or by radiating fins. Thus, regarded as a 

 machine for producing x-rays, a Coolidge x-ray tube has a very low 



TABLE 1-1 



Hardness of Radiation 

 (As used in this book.) 



efficiency, but the radiant energy emitted has a remarkably high pene- 

 trating power because of its high frequency (v) or short wavelength (X). 



Soft and Hard X-Rays 



Soft x-radiations and hard x-radiations correspond to long and short 

 wavelengths, respectively. The soft rays are produced by compara- 

 tively low electron-accelerating potentials and the hard by high poten- 

 tials. Soft x-rays are readily absorbed; those not so readily absorbed 

 by the same substance are qualitatively designated as hard x-rays. 

 These differences are only descriptive and have no quantitative signifi- 

 cance. 



For all practical purposes the radiations emitted by a tungsten- 

 target x-ray tube in terms of a scale of hardness may be roughly classified 

 as shown in Table 1-1. 



X-Ray Spectral Distribution 



Corresponding with a given applied voltage, a definite distribution 

 of intensity at different wavelengths is found, depending on the material 

 of the target. Since in medical or biophysical work ordinarily tungsten 



