Proportional counter

The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation.

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.

A key design goal is that each original ionizing event due to incident radiation produces only one avalanche.

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.

In summary, the proportional counter is an ingenious combination of two ionization mechanisms in one chamber which finds wide practical use.

Proportional counters are also useful for detection of high energy photons, such as gamma-rays, provided these can penetrate the entrance window.

Proportional counters in the form of large area planar detectors are used extensively to check for radioactive contamination on personnel, flat surfaces, tools, and items of clothing.

In the United Kingdom the Health and Safety Executive (HSE) has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned.