282 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



Fig. (5-2)1. Ionization chamber: 

 A and B, electrodes; C, chamber; D, 

 diaphragm; E, voltage source; M, 

 meter. 



each other within a chamber C. A voltage source E produces an 

 electric field between the electrodes. Ionizing radiation entering 

 the space between the electrodes causes the removal of electrons 

 from some of the gas molecules, which thereby become positive ions. 

 The electrons and the ion s are separated by the applied electric 



field; they move in opposite direc- 

 tion toward the electrodes and in 

 doing so form an electric current, 

 which is measured by a meter M. 



On their way to the electrodes, 

 the ions and electrons may recom- 

 bine to neutral molecules. If the 

 applied voltage is low, the velocity 

 of the electrons and ions is small, 

 and the electrons and ions (or nega- 

 tive and positive ions) will have time to recombine; the current is 

 small. The current increases with the applied voltage, Fig. (5-2)2, 

 and reaches a constant value (satu- 

 ration current) when all ions are col- 

 lected on the electrodes before they 

 have a chance to recombine. 



The field strength required to 

 reach saturation rises with the 

 amount of ionization, i.e., with 

 incident radiation intensity, and 

 with the pressure in the chamber. 

 Higher field strengths are also 

 needed if the ionization is caused 

 by alpha particles (because of the 

 increased recombination in col- 

 umns). If electron attachment takes 

 place in contaminated gases, satu- 

 ration cannot be reached, and the 

 current may increase steadily with increased voltage, as shown in 

 Fig. (5-2)2, curve d. 



At very high ionization density or in high-pressure chambers 

 where the voltages required to obtain saturation are impractically 

 high, operation in the range of the very beginning of the voltage- 

 current characteristic is recommended (at about 1 per cent of the 

 saturation voltage). In this region the current is proportional to 

 the square root of the incident radiation intensity. 



Es Eo 



Voltage 



Fig. (5-2)2. Voltage-current charac- 

 teristic of an ionization chamber: 

 curves a, b, c obtained with increasing 

 amount of ionization; r/, curve ob- 

 served in the presence of electron 

 attachment; E s , voltage at which 

 saturation occurs; operating range 

 between E 1 and E 2 . 



