Radiation DetectorsPROPORTIONAL COUNTERAs an example, if the 10,000 electrons produced by the gamma ray are increased to 40,000 bygas amplification, the amplification factor would be 4. Gas amplification factors can range fromunity in the ionization region to 103 or 104 in the proportional region. The high amplificationfactor of the proportional counter is the major advantage over the ionization chamber. Theinternal amplification of the proportional counter is such that low energy particles (< 10 KeV)can be registered, whereas the ion chamber is limited by amplifier noise to particles of > 10 KeVenergy.Proportional counters are extremely sensitive, and the voltages are large enough so that all of theelectrons are collected within a few tenths of a microsecond. Each pulse corresponds to onegamma ray or neutron interaction. The amount of charge in each pulse is proportional to thenumber of original electrons produced. The proportionality factor in this case is the gasamplification factor. The number of electrons produced is proportional to the energy of theincident particle.For each electron collected in the chamber, there is a positively charged gas ion left over. Thesegas ions are heavy compared to an electron and move much more slowly. Eventually the positiveions move away from the positively charged central wire to the negatively charged wall and areneutralized by gaining an electron. In the process, some energy is given off, which causesadditional ionization of the gas atoms. The electrons produced by this ionization move towardthe central wire and are multiplied en route. This pulse of charge is unrelated to the radiationto be detected and can set off a series of pulses. These pulses must be eliminated or "quenched."One method for quenching these discharges is to add a small amount (10%) of an organic gas,such as methane, in the chamber. The quenching gas molecules have a weaker affinity forelectrons than the chamber gas does; therefore, the ionized atoms of the chamber gas readily takeelectrons from the quenching gas molecules. Thus, the ionized molecules of quenching gas reachthe chamber wall instead of the chamber gas. The ionized molecules of the quenching gas areneutralized by gaining an electron, and the energy liberated does not cause further ionization, butcauses dissociation of the molecule. This dissociation quenches multiple discharges. Thequenching gas molecules are eventually consumed, thus limiting the lifetime of the proportionalcounter. There are, however, some proportional counters that have an indefinite lifetime becausethe quenching gas is constantly replenished. These counters are referred to as gas flow counters.Rev. 0 Page 21 IC-06
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