How Do Geiger Counters 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."

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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