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A proportional counter is a detector that operates in the proportional region. EO 2.1DESCRIBE the operation of a proportional counter to include: a. Radiation detection b. Quenching c. Voltage variations A proportional counter is a detector which operates in the proportional region, as shown in Figure 6. Figure 7 illustrates a simplified proportional counter circuit. To be able to detect a single particle, the number of ions produced must be increased. As voltage is increased into the proportional region, the primary ions acquire enough energy to cause secondary ionizations (gas amplification) and increase the charge collected. These secondary ionizations may cause further ionization.
Figure 7 Proportional Counter In this region, there is a linear relationship between the number of ion pairs collected and applied voltage. A charge amplification of 104 can be obtained in the proportional region. By proper functional arrangements, modifications, and biasing, the proportional counter can be used to detect 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 counter will be able to detect. Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and gamma radiation. When detection of neutrons is necessary, the detectors 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 voltage applied. Figure 8 illustrates how the number of electrons collected varies with the applied voltage.
Figure 8 Gas Ionization Curve When a single gamma ray interacts with the gas in the chamber, it produces a rapidly moving electron which produces secondary electrons. About 10,000 electrons may be formed depending on the gas used in the chamber. The applied voltage can be increased until the amount of recombination is very low. However, further increases do not appreciably increase the number of electrons collected. This region in which all 10,000 electrons are collected is the ionization region. As applied voltage is increased above 1000 V, the number of electrons becomes greater than the initial 10,000. The additional electrons which are collected are due to gas amplification. As voltage is increased, the velocity of the 10,000 electrons produced increases. However, beyond a certain voltage, the 10,000 electrons are accelerated to such speeds that they have enough energy to cause more ionization. This phenomenon is called gas amplification. As an example, if the 10,000 electrons produced by the gamma ray are increased to 40,000 by gas amplification, the amplification factor would be 4. Gas amplification factors can range from unity in the ionization region to 103 or 104 in the proportional region. The high amplification factor of the proportional counter is the major advantage over the ionization chamber. The internal 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 KeV energy. Proportional counters are extremely sensitive, and the voltages are large enough so that all of the electrons are collected within a few tenths of a microsecond. Each pulse corresponds to one gamma ray or neutron interaction. The amount of charge in each pulse is proportional to the number of original electrons produced. The proportionality factor in this case is the gas amplification factor. The number of electrons produced is proportional to the energy of the incident particle. For each electron collected in the chamber, there is a positively charged gas ion left over. These gas ions are heavy compared to an electron and move much more slowly. Eventually the positive ions move away from the positively charged central wire to the negatively charged wall and are neutralized by gaining an electron. In the process, some energy is given off, which causes additional ionization of the gas atoms. The electrons produced by this ionization move toward the central wire and are multiplied en route. This pulse of charge is unrelated to the radiation to 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 Summary Proportional counters are summarized below. Proportional Counters Summary When radiation enters a proportional counter, the detector gas, at the point of incident radiation, becomes ionized. The detector voltage is set so that the electrons cause secondary ionizations as they accelerate toward the electrode. The electrons produced from the secondary ionizations cause additional ionizations. This multiplication of electrons is called gas amplification. Varying the detector voltage within the proportional region increases or decreases the gas amplification factor. A quenching gas is added to give up electrons to the chamber gas so that inaccuracies are NOT introduced due to ionizations caused by the positive ion.
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of an organic gas, such as methane, in the
chamber. The quenching gas molecules have a weaker affinity for electrons than
the chamber gas does; therefore, the ionized atoms of the chamber gas readily
take electrons from the quenching gas molecules. Thus, the ionized molecules of
quenching gas reach the chamber wall instead of the chamber gas. The ionized
molecules of the quenching gas are neutralized by gaining an electron, and the
energy liberated does not cause further ionization, but causes dissociation of
the molecule. This dissociation quenches multiple discharges. The quenching gas
molecules are eventually consumed, thus limiting the lifetime of the
proportional counter. There are, however, some proportional counters that have
an indefinite lifetime because the quenching gas is constantly replenished.
These counters are referred to as gas flow counters.