Figure 18 Minimizing Gamma Influence with Boron Coating Area

Figure 17 Minimizing Gamma Influence by Size and Volume

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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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