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How Do Geiger Counters Work?

Geiger counter
Geiger counters used to be fairly big and bulky devices, but today can be handheld like this one or even downloaded as an app on a smartphone. Sean Gallup/Getty Images

If you're a fan of vintage science fiction and horror flicks such as the 1984 film "C.H.U.D.," you're used to getting a chill down your spine whenever characters point a boxy gadget into the darkness and are alerted to the presence of some yet-unseen radioactive monster by an ominous clicking sound.

Geiger counters are such a familiar prop in old movies, in fact, that you'd be forgiven for assuming that they're either an invention that exists only in the minds of screenwriters, or else that they're an obsolete technology that went out of fashion when people stopped building atomic bomb shelters in their backyards.

In reality, though, the Geiger counter — which in the scientific and engineering world is known as the Geiger-Muller counter — is an actual device that's still frequently used to detect radiation in various settings. The Nuclear Regulatory Commission says that it's the most commonly used portable radiation instrument.

The origins of the technology date back to the early 1900s, when German physicist Hans Geiger worked as an assistant to Ernest Rutherford, winner of the 1908 Nobel Prize in chemistry. Rutherford developed the concept that an atom contains a nucleus of even smaller particles, and that a nucleus can disintegrate and give off those particles. Geiger worked with Rutherford to develop a radiation-measuring device — a counter to detect the alpha particle, a combination of two protons and two neutrons that's given off by an atom during radioactive decay.

geiger counter
Hans Wilhelm Geiger (1882-1945), left, and Ernest Rutherford (1871-1937) with their apparatus for counting alpha particles. Together they worked on the radiation detector invented by and named after Geiger.
SSPL/Getty Images

Geiger's career was interrupted World War I, in which he served as an artillery officer in the German army, but afterward, he returned to teaching and research. He eventually teamed with Walther Müller, one of his graduate students at the University of Kiel, to improve his counter, so that it could detect other types of radioactive particles as well as the alpha particle. The device they created is pretty much the same technology that's used today.

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How Does a Geiger Counter Work?

A Geiger counter is a relatively simple and inexpensive device, consisting of a low pressure gas — typically argon or xenon — in a sealed chamber containing two electrodes. When radiation reaches the counter, it ionizes the gas, freeing negatively charged electrons from atoms and creating positive ions from the part of the atom that remains. A high-voltage electrical charge is applied to the electrodes, which causes the free electrons to travel to the positive electrode, which is called an anode.

Because of the high voltage applied to the electrodes there is a region near the anode where the resulting electrical field is so strong that it creates "avalanches" of secondary electrons when the primary electrons approach the anode, explains Marek Flaska, an assistant professor of nuclear engineering at Penn State University, in an email.

"This 'gas amplification' results in a very large number of charges produced no matter how much primary charge is created by radiation," Flaska writes. "Where this charge is collected at the electrodes, a large electric pulse is created by the detector. These pulses are large — several volts — so no additional signal amplifier is needed."

geiger counter
In this early model of a Geiger counter, a low pressure gas is held in a copper cylinder, which an electrical current runs through. The radioactive particle entering the cylinder causes a burst of electric current which is registered on a counter. This particular Geiger counter was used by James Chadwick, the discoverer of the neutron.
SSPL/Getty Images

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Why Does a Geiger Counter Click?

Those pulses create the clicks that you hear when radioactive particles enter the device and cause ions and electrons to split apart. The number of clicks that you hear indicates how many times this is happening in a minute. In addition, there's usually a display that indicates the count.

The level of radiation counted by a Geiger counter is expressed in a unit called microSieverts, per hour of exposure. (One sievert is 1,000 milliSieverts and 1 million microSieverts.) So if the Geiger counter reads 0.25 microSieverts per hour, that means it detected .25 microSieverts of radiation in an hour's time. Some basic numbers to use as a guide: A single-organ CT scan gives a radiation dose of about 6,900 microSieverts, according to Reuters, while 2,000,000 microSieverts of exposure would indicate severe radiation poisoning leading to possible death, according to Pure Earth.

When you turn on a Geiger counter, you'll usually hear some clicks right away, no matter where you are, according to the NRC. That's because of naturally occurring background radioactivity that comes from the sun, natural uranium in the soil, certain types of rock, and radon, a naturally occurring radioactive gas, among other sources.

Though there are a number of other radiation detection technologies available, the Geiger counter is a fairly simple technology that's been around for a while and they're fairly inexpensive today, with some low-end consumer versions on the market that cost less than $100, according to Google Shopping.

"GMs [Geiger-Muller counters] are used elsewhere, especially when a low cost solution that doesn't require distinguishing radiation type or energy is desired," the Los Alamos National Laboratory (LANL) explains in an email.

Many industries still use Geiger counters today for such things as monitoring radioactive contamination in laboratories. Law enforcement personnel may use sophisticated versions of the devices to detect the transportation of illicit radioactive materials, and many emergency responders carry them as well, according to the Department of Homeland Security. In Japan, personal Geiger counters became hot sellers after the 2011 accident at the Fukushima Daiichi nuclear power plant. They're also used by prospectors to find uranium and other minerals.

But Geiger counters also have limitations. "A GM counter is inherently unable to discern what type of particle triggered the pulse or even the energy of a particle," LANL writes. "Because every interaction produces the same pulse strength – think of a set mousetrap; when sprung it produces the same response regardless if a mouse or human foot is responsible, though the consequences may be much different."

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