Radiation DetectorsIONIZATION CHAMBERThe plates in an ionization chamber are normally enclosed in metal, as shown by Figure 16.Figure 16 Ionization ChamberThis metal chamber serves to shield the plates from outside electric fields and to contain the airor 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 betaparticles to be detected, some means must be provided for a thin wall or "window." Thiswindow must be thin enough for the alpha and beta particles to penetrate. However, a windowof 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 areuncharged; therefore, they cause no ionizations themselves. If the inner surface of the ionizationchamber is coated with a thin coat of boron, the following reaction can take place.10 1 74Bn LiHe5e5 0 3 2A neutron is captured by a boron atom, and an energetic alpha particle is emitted. The alphaparticle causes ionization within the chamber, and ejected electrons cause further secondaryionizations.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 particleswhen they react with the boron atoms in the detector gas. Either method may be used to detectneutrons in nuclear reactor neutron detectors.Rev. 0 Page 31 IC-06
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