Geiger counters

by Chris Woodford. Last updated: September 26, 2023.

Click click click! Thanks to an ingenious German physicist named Hans Geiger, we've all heard the sound of radioactivity. It's just as well we do have Geiger counters because most radiation (radioactive particles and energy) is extremely harmful to living things, completely invisible, and very difficult to detect in other ways. What are Geiger counters? How do they work? Let's take a closer look!

Artwork: The basic concept of a Geiger counter—a tube, attached to a meter, that can detect and measure particles of radiation.

What is radioactivity?

There are several different types of radiation, caused by different processes. Cosmic rays, for example, arrive on Earth from outer space, but there's plenty of naturally occurring radiation here on Earth as well. Radiation is also made by artificial processes that happen inside nuclear power plants and nuclear bombs.

What causes radiation? Atoms of a particular chemical element often exist in slightly different forms called isotopes. The metal tin, for example, has ten stable isotopes: atoms that have the same number of protons and electrons (50 of each) but different numbers of neutrons. Stable isotopes are ones that are happy enough to stay as they are indefinitely: they have nothing to gain by changing into a different form. Not all isotopes are stable, however. Carbon has lots of isotopes, the two best known being carbon-12 (ordinary, stable carbon atoms with six protons, six neutrons, and six electrons) and carbon-14 (with six protons, eight neutrons, and six electrons). Having more (or fewer) neutrons than the ideal can make an atom so unstable that it spontaneously changes into a different, more stable atom or isotope by giving off some of its unwanted, subatomic particles or energy. Thus, carbon-14 atoms spontaneously (albeit very slowly) turn into nitrogen atoms. Atoms that are unstable in this way are called radioactive isotopes and the particles they give off are radiation. The kinds of radiation we're talking about are alpha particles (two protons and two neutrons joined together, so they're like the nuclei of helium atoms), beta particles (electrons traveling at high speeds with high energy), and gamma rays (very high energy electromagnetic rays—a bit like supercharged light rays, only invisible to our eyes and much more dangerous).

Artwork: Isotopes are atoms of an element that contain the same number of protons and electrons but different numbers of neutrons. An unstable (radioactive) isotope will naturally try to make itself more stable by getting rid of some of these particles and changing into a different atom.

Ionizing radiation

Whether they come from Earth or space, radioactive particles and rays have energy. Earth is surrounded by a blanket of gas (the atmosphere) so, when radioactive particles race through it, they collide with molecules of gases such as oxygen and nitrogen, splitting them apart into electrons and positively charged ions. This is called ionization. Now radiation may be impossible to see but detecting ions and electrons is much easier. That's the job that a Geiger counter does for us: it detects ionizing radiation by detecting the charged particles that the radiation creates as it passes through gases in the world around us.

Photo: A sailor with the US Navy uses a Geiger counter to check for radiation onboard a nuclear-powered vessel. Note the detector tube at the front and the handheld meter and loudspeaker in the separate box at the back. Photo by Tracy Lee courtesy of US Navy and Wikimedia Commons.

What is a Geiger counter?

A Geiger counter is a metal cylinder filled with low-pressure gas sealed in by a plastic or ceramic window at one end. Running down the center of the tube there's a thin metal wire made of tungsten. The wire is connected to a high, positive voltage so there's a strong electric field between it and the outside tube.

When radiation enters the tube, it causes ionization, splitting gas molecules into ions and electrons. The electrons, being negatively charged, are instantly attracted by the high-voltage positive wire and as they zoom through the tube collide with more gas molecules and produce further ionization. The result is that lots of electrons suddenly arrive at the wire, producing a pulse of electricity that can be measured on a meter and (if the counter is connected to an amplifier and loudspeaker) heard as a "click." The ions and electrons are quickly absorbed among the billions of gas molecules in the tube so the counter effectively resets itself in a fraction of a second, ready to detect more radiation. Geiger counters can detect alpha, beta, and gamma radiation.

Photo: Checking canned food for possible radioactive contamination back in 1963. This is a classic Geiger counter with the detector tube wired to a separate meter and a headset the operator wears to listen to the clicks. Photo by Warren Dobson courtesy of Centers for Disease Control (CDC).

Photo: Modern Geiger counters often have the two parts (detecting tube and associated electronics) packed into a single unit, with a digital display for easy reading. This is a Mirion ADM-300 with two built-in Geiger counters for detecting beta and gamma radiation and (inset) its LCD display. Both photos by Rhonda Smith courtesy of US Air Force and DVIDS.

Artwork: A slightly different approach. This counter uses a standard Geiger tube (yellow, left) with a central wire (blue) as above. But instead of detecting electrons directly, it looks for photons of light and uses a photomultiplier tube (red, middle) to convert them into a measurable current. The results are displayed on an "indicator" (blue, right), which is typically a counter of some sort. Artwork courtesy of US Patent and Trademark Office from US Patent 2,485,586: Geiger counter by Ladislas Goldstein, International Standard Electric Corporation, granted October 25, 1949.

Who invented the Geiger counter?

Geiger counters are the most familiar of various ionizing radiation detectors that work in broadly the same way. German physicist Hans Geiger (1882–1945) developed the idea in 1912 while working with Ernest Rutherford, the New-Zealand-born physicist who "split the atom" (proved experimentally that atoms consisted of other, smaller particles). Back in Germany, sixteen years later, Geiger greatly improved the instrument with the help of a colleague named Walter Müller, which is why Geiger counters are often called Geiger-Müller counters (or Geiger-Müller tubes).

Photo: Using a Geiger counter to check cans of buried nuclear waste for possible radiation leaks. Photo courtesy of US Department of Energy (Flickr).

Can you make your own?

Odd though it might seem, there's a long tradition of amateur "radiation hunting." In my 2015 book Atoms Under the Floorboards (p.106), I reprised the story of how amateur uranium prospectors used to sneak out with their Geiger counters, under cover of darkness, to try to find lucrative uranium deposits (read more in Finding Uranium in the Dark, Popular Science, July 1955, p.71). In 2013, in the wake of the Fukushima nuclear disaster, citizen science groups equipped themselves with DIY counters called bGeigies to check the radiation. You can build your own Geiger counter if you want to—and you'll find a few in kit form in the hobbyist/maker space. The one pictured below is the battery-powered MightyOhm version from Jeff Keyzer. You can clearly see the Geiger tube at the bottom.

Photo: A DIY Geiger counter. Photo by Jeff Keyzer published on Flickr under a Creative Commons (CC BY-SA 2.0).

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Find out more

On this website

• Atomic energy
• Atoms
• Tools, instruments, and measurement
• Solids, liquids, and gases

On other sites

• Homemade Geiger Counter: A relatively complex, homemade detector project from the Instructables site.
• The Nobel Prize in Chemistry 1908: Ernest Rutherford: This biography charts how the atom's secrets were unraveled by Rutherford and his colleagues.

Books

Textbooks

• Radiation Detection and Measurement by Glenn F. Knoll. John Wiley, 2010. A guide for nuclear engineers that covers all kinds of radiation measurement, including Geiger counters, scintillation detectors, photomultipliers, and much more. Quite a complex book and not really one for beginners or general readers.
• Atoms, Radiation, and Radiation Protection by James E. Turner. John Wiley, 2008. A simpler and more general book. Chapter 10 covers "Methods of Radiation Detection."

General books

• The Fly in the Cathedral by Brian Cathcart. New York: Farrar, Straus and Giroux, 2005. An account of how Cambridge scientists, led by Ernest Rutherford, fathomed out the mysteries of the atom in the early 20th century.
• The Radioactive Boy Scout: The Frightening True Story of a Whiz Kid and His Homemade Nuclear Reactor by Ken Silverstein. Paw Prints, 2008. How an American teenager tried his own nuclear experiments, with terrifying consequences.

Articles

• Fun—and Uranium—for the Whole Family in This 1950s Science Kit by Allison Marsh. IEEE Spectrum, January 31, 2020. How a science-fair kit from a half-century ago aimed to encourage a whole new generation of nuclear scientists.
• Measuring Radiation in Fukushima With Pocket Geigers and bGeigies by Eliza Strickland. IEEE Spectrum, September 4, 2013. How citizen scientists are tracking radioactivity with their smartphones.
• What's Lurking in Your Countertop? by Kate Murphy. The New York Times, July 24, 2008. Is your new granite kitchen giving off dangerous levels of radiation? A Geiger counter will tell you.
• Me and My Geiger Counter by Fred Bernstein. The New York Times, June 27, 2002. How the 9/11 terrorist attacks rekindled interest in home radiation detectors.
• How to choose a Geiger counter by Griff Borgeson. Popular Science, January 1956. This old article from the Pop Sci archive speaks of another age when amateur uranium hunting was all the rage! It explains the basic principles of Geiger counters and compares simple counters, rate meters, and multitubes.

Patents

For deeper technical detail, try these:

• US Patent 2,485,586: Geiger counter by Ladislas Goldstein, International Standard Electric Corporation, granted October 25, 1949. A small, lightweight Geiger counter that uses a photon (light particle) detector and photomultiplier tube.
• US Patent 2,442,314: Geiger counter improvement by Allen F Reid, Atomic Energy Commission, granted May 25, 1948. This counter uses quenching with a halogen gas to detect rapid emission of particles more effectively. You'll find a detailed description of how quenching works here.