Radiation DetectorsPROPORTIONAL COUNTERPROPORTIONAL COUNTERA proportional counter is a detector that operates in the proportional region.EO 2.1 DESCRIBE the operation of a proportional counter toinclude:a. Radiation detectionb. Quenchingc. Voltage variationsA proportional counter is aFigure 7 Proportional Counterdetector which operates in theproportional region, as shown inFigure 6. Figure 7 illustrates asimplified proportional countercircuit.To be able to detect a singleparticle, the number of ionsproduced must be increased. Asvoltage is increased into theproportional region, the primaryions acquire enough energy tocause secondary ionizations (gasamplification) and increase thecharge collected. These secondaryionizations may cause furtherionization.In this region, there is a linear relationship between the number of ion pairs collected and appliedvoltage. A charge amplification of 104 can be obtained in the proportional region. By properfunctional arrangements, modifications, and biasing, the proportional counter can be used todetect alpha, beta, gamma, or neutron radiation in mixed radiation fields.To a limited degree, the fill-gas will determine what type of radiation the proportional counterwill be able to detect. Argon and helium are the most frequently used fill gases and allow forthe detection of alpha, beta, and gamma radiation. When detection of neutrons is necessary, thedetectors are usually filled with boron-triflouride gas.The simplified circuit, illustrated in Figure 7, shows that the detector wall acts as one electrode,while the other electrode is a fine wire in the center of the chamber with a positive voltageapplied.Rev. 0 Page 19 IC-06
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