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Figure 15   Recombination and Ionization Regions
Figure 17   Minimizing Gamma Influence by Size and Volume

Instrumentation and Control 2 of 2
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Radiation Detectors IONIZATION CHAMBER The plates in an ionization chamber are normally enclosed in metal, as shown by Figure 16. Figure 16   Ionization Chamber This metal chamber serves to shield the plates from outside electric fields and to contain the air or other gas.  Gamma rays have very little trouble in penetrating the metal walls of the chamber. Beta particles and alpha particles, however, are stopped by the metal wall.   For alpha and beta particles  to  be  detected,  some  means  must  be  provided  for  a  thin  wall  or  "window."    This window must be thin enough for the alpha and beta particles to penetrate.   However, a window of almost any thickness will prevent an alpha particle from entering the chamber. Neutrons  can  also  be  detected  by  an  ionization  chamber.   As  we  already  know,  neutrons  are uncharged; therefore, they cause no ionizations themselves.  If the inner surface of the ionization chamber is coated with a thin coat of boron, the following reaction can take place. 10 1 7 4 B n    Li He 5e 5 0 3 2 A neutron is captured by a boron atom, and an energetic alpha particle is emitted.   The alpha particle  causes  ionization  within  the  chamber,  and  ejected  electrons  cause  further  secondary ionizations. Another method for detecting neutrons using an ionization chamber is to use the gas boron tri- fluoride  (BF3)  instead  of  air  in  the  chamber.   The  incoming  neutrons  produce  alpha  particles when they react with the boron atoms in the detector gas.   Either method may be used to detect neutrons in nuclear reactor neutron detectors. Rev. 0 Page 31 IC-06







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