Proportional Counter - Operation

Operation

In a proportional counter the fill gas of the chamber is an inert gas which is ionised 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 a molecule of the inert gas and ionises it to produce an electron and a positively charged atom, commonly known as an "ion pair". As the charged 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 ionising 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 ionisation 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 ionisation regions:

1. Ion drift region: in the outer volume of the chamber - creation of number ion pairs proportional to incident radiation energy.
2. Avalanche region: in the immediate vicinity of the anode - Charge amplification of ion pair currents, while maintaining localised 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 ionisation mechanisms in the one chamber which finds wide practical use.