40 



BIOPHYSICALLY ACTIVE X-RAYS 



possibility of exciting the gas to emit its characteristic radiation through 

 electron collisions. The characteristic radiation emitted under these 

 circumstances, however, is of such long wavelength that it is reabsorbed 

 before traveling very far. 



Let us next introduce electrodes, in the form of two parallel metal 

 plates, into the gas. Then connect these electrodes to the terminals of 

 a battery having a difference of potential of several hundred volts, and 

 insert a very sensitive ammeter in the circuit. If the beam of x-radia- 

 tion is now passed through that volume of gas lying between the two 

 plates, the ionized condition of the gas manifests itself by a current 

 flowing through the ammeter. Such an ionization current is very small 

 and for best results must be measured by a sensitive electrostatic instru- 

 ment in the form of an electrometer. 



Potential in volts applied to plates 



Fig. 1-15. A typical saturation curve obtained by means of an ionization 

 chamber. The extended curve becomes asymptotic to the potential axis. This asymp- 

 totic value is the saturation value I a attained when the applied potential is V a . 



The variation of the current with successively greater potentials 

 applied to the electrodes is shown in Fig. 1-15. It will be noted that, 

 although the intensity of the x-ray beam is kept constant, the ionization 

 current increases with the voltage applied to the plates, but reaches a 

 flat maximum, where it remains despite further increase in plate voltage. 

 This maximum value to which the current rises is called its " saturation ,; 

 value 7 S . This saturation value of current is attained when all the ions 

 are removed from the gas as fast as they are formed as a result of absorp- 

 tion of energy from the x-ray beam. The removal of all the electrons 

 and ions from the gas is accomplished by applying a minimum voltage 

 V s across the ionization chamber. The minimum voltage is called the 

 saturation voltage, and the resulting ionization current is said to have 

 reached its saturation value. 



