996 EXPLORATION GEOPHYSICS 



RADIATION MEASURING DEVICES 

 The Ionization Chamber 



Radioactive particles and quanta can be detected by the ionization they 

 produce in gases. If we set up an electric field in a suitable vessel (Figure 

 609) we shall sweep out most of the ions and produce an electric current 

 which will be proportional to the amount of ionization produced by the 

 radiation. 



IjOUARD RING 



toaSislnsL-, collecting 



■:>>"^ ■v;-':^;: ^;: ■:■ -1 ^ELECTRODE 



Fig. 609.- 



HICH VOLTAGE 



-Scheme of an ionization chamber. Insulators are 

 shown stippled. 



The ionization chamber will have a certain capacitance C, usually on 

 the order of a few micromicro farads, and the high resistance between the 

 electrodes will be R. The product RC is called the time constant. If we 

 now charge the chamber to a voltage Vo, the charge will leak off at the rate 



dV Vo —^ 

 W^RC' ^" 



We may handle the ionization current in two different ways. First, 

 we can make the time constant so small that the charge leaks off almost as 

 fast as it accumulates. Each ionizing event will then produce a pulse pro- 

 portional to the magnitude of the event. Pulse chambers are useful in 

 the laboratory for a counting in a strong yS-y background. Suitable dis- 

 criminating pulse amplifiers can easily distinguish the large a pulses from 

 the ripple produced by ^'s and y's. 



Integrating (or direct current) chambers have a very high time constant, 

 on the other hand, and are usually used for y detection. They require 

 powerful D.C. amplifiers but can be made very sensitive when filled to a 

 high pressure. Such chambers find their greatest use in bore-hole logging. 



The potential of the collecting electrode is usually negative with respect 

 to the case, to collect positive ions. The ionization current will be greatest 

 when the chamber is saturated, i.e., when an additional increase in voltage 

 causes no further increase in ionization current, at a given flux of ionizing 

 radiation. D.C. chambers are best operated at saturation (several thousand 

 volts), but pulse chambers will work well at much lower voltages. 



