10 BIOPHYSICALLY ACTIVE X-RAYS 



The overall intensity may also be increased by an increase in the 

 electron current flowing from filament to target. The latter increase 

 may also be obtained by raising the temperature of the filament. 



Since the impinging electrons must be decelerated through collision 

 with the atoms of the target, one may conclude that, the greater the 

 density of the atoms in the target, the more frequently will these colli- 

 sions occur. Broadly speaking, the density increases with the atomic 

 number; hence an increase in the atomic number of the target material 

 should parallel an increase in emitted x-ray intensity. 



It has been shown by Nicholas [1930] that the total amount of con- 

 tinuous spectral energy emitted per second 



I = constant V 3/2 zi 



where V is the potential across the tube, z the atomic number of the 

 element used as a target, and i the current passing from filament to 

 target in the form of the stream of bombarding electrons. This relation 

 is limited to values of V extending from 40 to 150 kv. 



Characteristic X-Ray Emission Spectra 



The peaks superimposed on the general radiation curves shown in 

 Fig. 1-5 compose the X-line spectrum. They make their appearance 

 at certain exciting potentials characteristic of the element out of which 

 the target is constructed. The K series of tungsten makes its first 

 appearance at the characteristic exciting potential of 69.3 kv. Below 

 this voltage it never appears, but at this voltage and above it these 

 radiations are always present. Table 1-2 shows the most prominent 

 of the tungsten and molybdenum spectral emissions in angstrom units, 

 classified into their appropriate series with an indication of their relative 

 intensities. The spectrum of tungsten, plotted at the bottom of the 

 intensity-wavelength curves of Fig. 1-4, shows the relative wavelength 

 positions of these characteristic emissions with respect to the general 

 radiation. 



This so-called i£-series radiation of tungsten is caused by a disturbance 

 in the configuration of the innermost K ring of electrons. Suppose that 

 one of the electrons about to collide with a tungsten atom possessed 

 sufficient energy to penetrate to the innermost group of K electrons and 

 succeeded in removing one of them by a collision process. To re-estab- 

 lish the undisturbed normal state an electron from the nearest outer 

 group or L configuration will replace the missing electron. This L 

 configuration, having lost an electron, will have a replacement drop 

 in from the M configuration, etc., until all other electron adjustments 

 have taken place. 



