CHAPTER 12 

 IONIZATION CHAMBERS 



12.1. Description. The ionization chamber may be described in the most 

 general terms as a gas-filled chamber in which a constant electric field is 

 maintained by a set of electrodes for the collection of ions formed by incident 

 ionizing radiation. In principle, the charge collected should exactly equal 

 the total ionization formed in the sensitive volume of the chamber by the 

 incident radiation. This linear relationship between ionization and collected 

 charge is the most important distinguishing characteristic of the ionization 

 chamber. It is necessary, therefore, that the field strength be high enough to 

 collect the ions formed before appreciable recombination and diffusion of ions 

 has taken place in the gas, yet not so high as to cause the formation of addi- 

 tional ions by multiplicative processes as the charge drifts to the collecting 

 electrodes (see Fig. 78). The useful operating range of potential difference 

 (arbitrary with respect to ground potential) maintained between the elec- 

 trodes can be found by observing the ionization in the chamber as a function 

 of voltage for a constant source of radiation. Starting with zero potential 

 difference, the detected ionization increases with voltage until saturation is 

 attained. The ionization then changes relatively little with further increase 

 in voltage until the field strength becomes sufficiently high to induce ion 

 multiplication by electrons as they drift to the anode. The operating 

 voltage should, of course, lie in the plateau region. In practice, the field 

 strengths used vary from 20 to 500 volts per cm and usually are not critical. 



Two methods for the detection of the ionization in the chamber may be 

 used: either measurement of the constant voltage produced by the flow of 

 collected charge through a resistance, or measurement of the rate of change 

 of voltage as charge is collected. With a high resistance R connected be- 

 tween the electrodes, the change in potential difference between them after a 

 time t, assuming the rate of ionization is constant, is 



AV = Vo - V = neR{\ - e~ l/RC ) volts 



where V = operating voltage 



n — number of ions formed per second 



e = electronic charge 



C = capacitance of electrode system 

 When / >>> RC, the voltage change reaches a constant value AV = neR. The 

 capacitance C can usually be made very small, ~ 10 to 100 pL/^id, so that the 



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