How Does A Geiger Counter Work?

US Army Checmical Corps technician surveys radioactive contamination with CDV-700 series Geiger counterIf there is one piece of technology that is uniquely associated with the Cold War aesthetic (apart from the atomic bomb itself), it is the handheld radiation survey meter depicted in civil defense and popular fiction of the era. The survey meter–typically a U.S.-made CDV-700 or one of its variations–is probably the first image one conjures when they think of a “Geiger counter”, and is often considered a highly technical piece of equipment (they were usually handled by specialized government officials, at least in contemporary depictions). The reality, though, is that the Geiger counter (or, more accurately, a Geiger-Müller counter) is an extremely simple device based on a circuit that’s no more complicated than a toggle switch!

CDV-700 Survey Meter
Bullet308, CC BY-SA 3.0, via Wikimedia Commons

The Geiger-Müller Tube

The technology at the heart of the Geiger counter is the specialized Geiger-Müller (GM) tube. The basic operating principle of the GM tube was developed by Hans Geiger in 1908. Geiger was developing a technique to detect alpha particles based on a principle developed by John Sealy Townsend some ten years earlier. This ionization mechanism, whereby particles are charged by their impact with another ionized particle, is known as the Townsend discharge or Townsend avalanche. In 1928, Geiger and his PhD student Walther Müller developed a sealed-tube version of Geiger’s alpha detector that could reliably detect beta and gamma particles in addition to alpha particles.

Visualization of Townsend avalanche
Dougsim, CC BY-SA 3.0, via Wikimedia Commons

How Does A Geiger Counter Work?

The Geiger-Müller tube itself is a variation on a vacuum tube that is filled with a noble gas (usually argon or xenon) at low pressure and acts as a relay switch. One of the tube’s electrodes has a high electrical potential applied to it, usually around 400 volts. Being a noble gas, it is normally resistant to the flow of electricity and does not allow current to pass through. When an ionizing (alpha, beta, or gamma) particle strikes an atom of the gas in the tube, the atom sheds an electron causing it to become electrically charged (ionized). The freed electrons collide with other atoms and ionize them, and the process is accelerated by the high voltage applied to the anode (the Townsend avalanche) until the resistance of the gas in the tube drops enough to allow the voltage through to the cathode. Once the tube discharges, the gas reverts to its original inert, highly resistive state and the process begins again when another particle enters the tube.

Cutaway illustration of typical Geiger tube and counter
Svjo-2, CC BY-SA 3.0, via Wikimedia Commons

The characteristic clicking sound of the Geiger counter is made by directing the pulsed voltage from the tube’s cathode into a speaker while an analog meter measures the frequency of the pulses, converting it into microSieverts (or röntgens, in the case of older Cold War-era units made before the adoption of the SI standard). It should be noted that the Geiger counter cannot measure the type of ionizing particle detected, only that one was detected. GM tubes are often designed with certain encasing materials to “filter out” lower energy particles such as alpha or beta, but a tube designed to detect alpha radiation cannot distinguish the different types of particles because the materials will not block higher energy particles. Dosage is inferred by knowing the source material being measured, but the energy of the particles themselves cannot be measured with the device.

 

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