The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation. The key feature is its ability to measure the energy of incident radiation, by producing a detector output pulse that is proportional to the radiation energy absorbed by the detector due to an ionizing event; hence the detector's name. It is widely used where energy levels of incident radiation must be known, such as in the discrimination between alpha and beta particles, or accurate measurement of X-ray radiation dose.
A proportional counter uses a combination of the mechanisms of a Geiger–Müller tube and an ionization chamber, and operates in an intermediate voltage region between these. The accompanying plot shows the proportional counter operating voltage region for a co-axial cylinder arrangement.
Operation
In a proportional counter the fill gas of the chamber is an inert gas which is ionized by incident radiation, and a quench gas to ensure each pulse discharge terminates; a common mixture is 90% argon, 10% methane, known as P-10. An ionizing particle entering the gas collides with an atom of the inert gas and ionizes it to produce an electron and a positively charged ion, commonly known as an "ion pair". As the ionizing particle travels through the chamber it leaves a trail of ion pairs along its trajectory, the number of which is proportional to the energy of the particle if it is fully stopped within the gas. Typically a 1 MeV stopped particle will create about 30,000 ion pairs. The chamber geometry and the applied voltage is such that in most of the chamber the electric field strength is low and the chamber acts as an ion chamber. However, the field is strong enough to prevent re-combination of the ion pairs and causes positive ions to drift towards the cathode and electrons towards the anode. This is the "ion drift" region. In the immediate vicinity of the anode wire, the field strength becomes large enough to produce Townsend avalanches. This avalanche region occurs only fractions of a millimeter from the anode wire, which itself is of a very small diameter. The purpose of this is to use the multiplication effect of the avalanche produced by each ion pair. This is the "avalanche" region. A key design goal is that each original ionizing event due to incident radiation produces only one avalanche. This is to ensure proportionality between the number of original events and the total ion current. For this reason, the applied voltage, the geometry of the chamber and the diameter of the anode wire are critical to ensure proportional operation. If avalanches start to self-multiply due to UV photons as they do in a Geiger–Muller tube, then the counter enters a region of "limited proportionality" until at a higher applied voltage the Geiger discharge mechanism occurs with complete ionization of the gas enveloping the anode wire and consequent loss of particle energy information. Therefore, it can be said that the proportional counter has the key design feature of two distinct ionization regions:
Ion drift region: in the outer volume of the chamber – the creation of number ion pairs proportional to incident radiation energy. Avalanche region: in the immediate vicinity of the anode – charge amplification of ion pair currents, while maintaining localized avalanches. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. In summary, the proportional counter is an ingenious combination of two ionization mechanisms in one chamber which finds wide practical use.
Gas mixtures Usually the detector is filled with a noble gas; they have the lowest ionization voltages and do not degrade chemically. Typically neon, argon, krypton or xenon are used. Low-energy x-rays are best detected with lighter nuclei (neon), which are less sensitive to higher-energy photons. Krypton or xenon are chosen when for higher-energy x-rays or for higher desired efficiency. Often the main gas is mixed with a quenching additive. A popular mixture is P10 (10% methane, 90% argon). Typical working pressure is 1 atmosphere (about 100 kPa).
Signal amplification by multiplication In the case of a cylindrical proportional counter the multiplication, M, of the signal caused by an avalanche can be modeled as follows:
ln M = V ln ( b / a ) ln 2 Δ V λ [ ln ( V p a ln ( b / a ) ) − ln K ] {\displaystyle \ln M={\frac {V}{\ln(b/a)}}{\frac {\ln 2}{\Delta V_{\lambda }}}\left[\ln \left({\frac {V}{pa\ln(b/a)}}\right)-\ln K\right]}
Where a is the anode wire radius, b is the radius of the counter, p is the pressure of the gas, and V is the operating voltage. K is a property of the gas used and relates the energy needed to cause an avalanche to the pressure of the gas. The final term Δ V λ {\displaystyle \Delta V_{\lambda }} gives the change in voltage caused by an avalanche.
Applications
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