How does a Geiger counter work?

Click, click, click. The sound of a Geiger counter is often associated with nuclear weapons and fallout. While it is useful in these situations, it is also used every day for the detection and control of nuclear waste, by-products and exposure in nuclear power plants, hospitals and even mines.

These ingenious devices allow anyone to detect potentially harmful radiation around them, using the power of electrons and the degradation of unstable radioactive atoms.

The detector is the main part of the Geiger counter. It is responsible for capturing, detecting and then signalling that a radioactive particle, known as a radioactive isotope, has passed through the detector.

In order to understand what is going on, we first must know what radioactive ionising radiation is, why it is worth detecting and what makes the distinctive clicking sound.

Atoms are one of the fundamental building blocks of matter in the Universe.

Every element is made up of atoms with their own number of subatomic particles. Hydrogen is the simplest atom, having just one positively charged proton and one negatively charged electron. Having no neutral neutrons it is extremely stable, being used today in a wide variety of applications.

Uranium, on the other hand, has 92 protons, 92 electrons and 146 neutrons. This makes it much heavier and far more unstable, very slowly breaking apart into smaller, and more stable elements.

When it breaks down, it releases energy and small particles. These by-products of the atom breaking down have the ability to knock electrons off other nearby atoms, and so are known as ionising radiation. Remember this point as it is vital to understanding how the Geiger counter works.

Radiation comes in three main types: alpha, beta and gamma.

Alpha particles are slow and can’t travel more than a few centimetres.

Beta particles are able to travel further. Gamma radiation (not a particle, but an energy wave) can travel long distances and penetrate many metres of solid concrete. It is the most dangerous type of ionising radiation as it can penetrate the body, causing massive damage to the atoms inside our cells.

That’s why a Geiger counter is usually tuned to best detect beta particles and gamma radiation as they have the greatest potential to cause harm to our bodies.

At its simplest level, the detector it is a metallic hollow tube that contains an inert gas with a conductive wire running straight through its centre. The wire is given a positive charge by a battery.

When a radioactive atom breaks down, the particles and energy it shoots off knock electrons off billions of nearby atoms. These free electrons might have a tiny amount of charge, but when combined have enough power to cause a short electrical pulse if given the opportunity.

The opportunity, in this case, is through the conductive wire which, being positively charged, attracts the negatively charged electrons.

Every time a radioactive atom breaks down, knocking off nearby electrons, this builds up enough charge to create a pulse and therefore a “click” on the detector. Each click indicates a single atom has decayed, and so more clicking means more atoms and therefore, more radiation.

A normal level of clicking is around 50 times per minute. However, at the heart of a nuclear bomb test site this may increase to more than 50 times per second!

The rest of the body of the Geiger counter houses the dial that indicates the radiation level, a speaker to transmit the clicks and the battery.