Proportional Counter

Radiation Detectors PROPORTIONAL COUNTER 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 (� 10%) 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. Rev. 0 Page 21 IC-06