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Checking radiation with a Geiger counter.

Geiger counters

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.


  1. What is radioactivity?
  2. Ionizing radiation
  3. What is a Geiger counter?
  4. How a Geiger counter works
  5. Who invented the Geiger counter?
  6. Can you make your own?
  7. Find out more

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).

Diagram showing the protons, neutrons, and electrons in an atom.

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.

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Ionizing radiation

A man driving a buggy wearing a radiation suit and checking for radiation with a Geiger counter.

Photo: A safety technician drives around the Pantex nuclear plant in Amarillo, Texas checking for radiation with a Geiger counter. Photo by courtesy US Department of Energy.

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.

What is a Geiger counter?

Checking radiation with a Geiger counter.

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.

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.

How a Geiger counter works

Diagram showing the process by which a Geiger counter works.

In summary then, here's what happens when a Geiger counter detects some radiation:

  1. Radiation (dark blue) is moving about randomly outside the detector tube.
  2. Some of the radiation enters the window (gray) at the end of the tube.
  3. When radiation (dark blue) collides with gas molecules in the tube (orange), it causes ionization: some of the gas molecules are turned into positive ions (red) and electrons (yellow).
  4. The positive ions are attracted to the outside of the tube (light blue).
  5. The electrons are attracted to a metal wire (red) running down the inside of the tube maintained at a high positive voltage. As the electrons head for the wire, some of them collide with other gas molecules, splitting them into ions and more electrons. So we get a kind of chain reaction in which even a single particle of radiation can produce avalanches of electrons in rapid succession; this process is known as a Geiger discharge.
  6. Many electrons travel down the wire making a burst of current in a circuit connected to it.
  7. The electrons make a meter needle deflect and, if a loudspeaker is connected, you can hear a loud click every time particles are detected. The number of clicks you hear gives a rough indication of how much radiation is present (the meter gives you a much more accurate idea).
  8. Before the counter can detect any more radiation, it needs to be restored to its original state through a process called quenching, which cancels out the effects of the Geiger discharge. Sometimes that's achieved by having a second gas (called a quenching gas, often a halogen) inside the tube. Or it can be done using an external circuit with a very large resistance.

Geiger counter based on photo detection using photomultiplier.

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).

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.

Hobbyist MightyOhm Geiger counter showing yellow circuit board and Geiger tube.

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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