IONIZATION CHAMBERS 



type of gas used to fill the chamber, the pressure, and the dimensions of the 

 system. The effects of these factors are discussed by Rossi and Staub^° and 

 Wilkinson^^. 



IONIZATION CHAMBERS 



Ionization chambers are operated in the plateau region (B) of Figure 31.1, 

 where the total charge collected is ideally equal to the total ionization within 

 the sensitive volume, the potential being high enough to achieve saturation 



To electrometer 



Figure 31.2 Essential features of a parallel-plate ionization chamber 



but not so high that there is any gas multiplication. Figure 31.2 shows the 

 essential features of a parallel-plate ionization chamber, the only one needing 

 further description being the guard electrode, maintained at the same 

 potential as the collecting electrode and serving both to define precisely the 

 sensitive volume of the chamber and to prevent leakage of current across 

 the insulator on which the collecting electrode is mounted. In order to 

 measure the charge collected the chamber must be connected to some type 

 of electrometer (i.e. to a high impedance device for measuring small potential 



Transparent 

 insulator 



♦— ' Illumination 



Gitded^quartz Charging pin 

 fibre 



Figure 31.3 Pocket quartz-fibre dosimeter 



changes), the most satisfactory being the vibrating reed electrometer, in 

 which a form of variable capacitor serves to convert the steady potential 

 change across the capacitance of the collecting electrode and the rest of the 

 system into an oscillating voltage which can be amplified by a conventional 

 a.c. amplifier. An alternative method of measuring the charge is to combine 

 the chamber with a quartz-fibre electroscope of the type developed by 

 Lauritzen. In this instrument the metal-coated quartz fibre (see Figure 31.3) 

 is initially displaced by charging it to a potential of the order of +100 V 



425 



