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Figure 17   Minimizing Gamma Influence by Size and Volume
Ionization Chamber Summary

Instrumentation and Control 2 of 2
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Radiation Detectors IONIZATION CHAMBER Gamma  interference  can  also  be  minimized  by  reducing  the  pressure  of  the  gas  inside  the chamber.   The reduction in pressure reduces the number of atoms within the sensitive volume and has the same effect as reducing the volume. Ionization  chamber  sensitivity  to  gamma  rays  can  also  be  reduced  by  increasing  chamber sensitivity to neutrons.   This is accomplished by increasing the boron-coated area, as shown in Figure 18.   Both ionization chambers shown in Figure 18 have the same sensitive volume. Figure 18   Minimizing Gamma Influence with Boron Coating Area The ionization chamber in Figure 18(b) has twice the boron area as the ionization chamber in Figure 18(a).   The result is that more neutron-induced alpha particles are produced, and neutron sensitivity  is increased.   Ionization chambers  supplied commercially are  designed to minimize gamma  sensitivity  by  both  of  the  techniques  described  previously.   Gamma  sensitivity  can  be minimized but not eliminated.  For reactors operating near peak power, neutrons are the dominant radiation,  and  almost  all  of  the  current  is  due  to  neutrons.   These  chambers  are  used  at  high reactor  powers  and  are  referred  to  as  uncompensated  ion  chambers.   The  uncompensated  ion chamber is not suitable for use at intermediate or low power levels because the gamma response at these power levels can be significant compared to the neutron response. Rev. 0 Page 33 IC-06







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