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.
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