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Description Of The Foundational Principle Of Proportional Counting

Aug 23

The cathode and anode of the proportional counter are maintained at a certain voltage (more than 1000 V), and the device is defined by a capacitance that is set by the shape of the electrodes. A typical combination of 90% argon and 10% methane, known as P-10, is used as the fill gas of the chamber in a proportional counter; this gas is ionized by incoming radiation and acts as a quench gas to guarantee that each pulse discharge ends.

An unbounded number of ion pairs are created when ionizing radiation strikes the gas in the gap between the electrodes. In air, an ion requires an average of 34 eV of energy to form. Because of this, the detector generates about 3 x 104 pairs of ions for every 1 MeV of radiation. The fill gas's kind and pressure, as well as the electric field's potential gradient inside the gas, all have an impact on the behavior of the resulting ion pairs. Since the electric field attracts like charges, the positive ions (protons) will flow toward the outer cylinder (which is negatively charged) and the negative ions (electrons) will flow toward the positive electrode (central wire). In this area, the electric field prevents the ions from merging back into electrons. The field strength is high enough close to the anode wire to trigger Townsend avalanches. This avalanche zone happens only a few millimeters away from the anode wire, which has a very thin diameter. The goal here is to take advantage of the fact that each ion pair sets off a domino effect of increased activity. Avalanche country, as it were. One primary design consideration was to ensure that each incoming radiation ionizing event would result in a single cascade. In the ionization area, the amplification factor of a gas may be 1, but in the proportional zone, it can be as high as 103 or 104. In comparison to an ionization chamber, the proportional counter has a much higher amplification factor.

To detect the presence of these electrons, a charge must be placed across the detecting circuit's electrodes. For every gamma-ray or particle interaction, there is a corresponding pulse. The magnitude of the pulse is directly proportional to the initial yield of electrons. Nonetheless, the detector here greatly boosts the pulse height. In this scenario, the gas amplification factor serves as the proportionality factor. The incident particle's energy determines the rate at which electrons are emitted. In this way, sensitive proportional counters can both identify particles and measure their energies (spectroscopy). Since the primary ionization of various forms of radiation and energy of radiation differs greatly, the pulse height may be used to discriminate between them (low-LET vs. high-LET). A following electronic amplification is often unnecessary once charge amplification has been performed on a detector, since the signal-to-noise ratio will have been considerably improved.

The proportional counter is built differently from an ionization chamber. Ionization chambers are typically built using a flat plate shape; however, concentric cylinders may be used to integrate incoming radiation pulses. Cylinder and center electrodes are used in proportional counters and Geiger counters. This is because the electric field between the electrodes must be precisely controlled in order for the proportional counter to function, making it impractical to use in practice.