Sec. 5-2] 



RA DIA TIOX TEA NSD UCK US 



287 



Insulation difficulties can be reduced by a guard-ring arrange- 

 ment, as shown in Fig. (5-2)5. The guard ring frequently serves as 

 an electrostatic shield of the collector; adequate shielding of the 

 collector and the leads should be provided. The charged ionization 

 chamber, in particular the parallel-plate chamber, is in its construc- 

 tion very similar to a condenser microphone. A small displacement 



Radiation 



r 



Fro. (5-2)5. Ionization chamber with 

 electrode C and guard electrode G. 



Z3. 



tn 



zz\ 



m 



Fig. (5-2)6. 

 chamber. 



Multiple -plate ionization 



Radiation 



*t 



I 



W 



C 



-o+E 



Fig. (5-2)7. Thimble 

 ionization chamber. 



Fig. (5-2)8. Ionization chamber 



for alpha particles. 



of one of the electrodes causes a large current. Counters of this type 

 must be protected from microphonic influences. 



Different constructions of ionization chambers are shown in Figs. 

 (5-2)5 to (5-2)8. The parallel-plate chamber, Fig. (5-2)5, has the 

 advantage of a uniform field, so that saturation is easily obtained. 

 The sensitive volume of the chamber, i.e., the volume where an ion 

 pair formed by ionization is recorded, is defined by the cross-sectional 

 area of the incident beam and the length of the collector C. Since 

 the field at the edges of a simple collector is not well defined, the 

 collector is frequently surrounded by a guard ring G which is kept 

 at or close to the potential of the collector. Ionization takes place 

 primarily in the gas for a rays and for low energy /? or y (X) rays; 

 high-energy incident radiations cause the liberation of secondary 



