Sec. 12.1] 



IONIZATION CHAMBERS 



345 



time required to reach the steady state is conveniently short, < 1 sec, even 

 for the high resistances necessary to make AV easily measurable. For 

 ionization chambers in which pulses are detected, it is usually necessary to 

 make RC less than 0.001 sec 



On the other hand, if R is removed or, more 



io electrometer 



Fig. 95. Schematic diagram of the parallel-plate ionization chamber. The essential 

 components of these chambers are A, low-leakage insulators between the electrodes; B, 

 guard ring, maintained at the same potential as the collecting electrode; C, collecting 

 electrode (anode); D, high-voltage electrode for maintaining the charge collecting field 

 (cathode); E, chamber which may be designed to withstand high or low pressures or may- 

 serve merely to shield against external fields. 



exactly, made very large, ~ 10 13 ohms, it is convenient to measure the time 

 rate of change of AV since 



d(AV) ne 



dt 

 or in an interval of time / « RC 



= ^e 



-t/RC 



C 



AT' ^ 4 



AT = r l 



volts sec 



volts/sec 



and the linear increase in AV with time is measured. This method is usually 



used only when the smallest ion currents, ne < 10r u amp, are to be detected. 



The details of the construction of an ionization chamber depend to some 



extent on the type of radiation to be detected, but more often its design and 



