Skulls in the Stars

Physics demonstrations: Geiger counter


Update: Fixed a couple of incorrect statements regarding cosmic rays and the radiation of uranium.  Thanks to encephalartos for the corrections!

In recent months, I’ve been diving wholeheartedly into learning how to build and design electronics.  My ultimate goal is to build a Tesla coil, but before I do, I’ve been warming up with a variety of kits and designs online.

Not too long ago, I learned that it is possible to buy a kit to build a basic Geiger counter, for only around $100!  I jumped at the opportunity and, after some minor modifications, started checking for radioactivity!


If you compare this with the image of the original kit below, you can see that I’ve protected all of the circuitry in a plastic case.  I also added an external switch as well as a spiffy drawer handle from Lowe’s.

So you probably know that a Geiger counter detects radioactivity, but how does it work — and what sort of things can you detect?  I thought I would write a short post discussing this, ending with a video showing my Geiger counter in action.

Geiger counters are, in fact, surprisingly simple in design and operation, though they take advantage of some pretty cool physics in the process.  The main component is the Geiger-Müller tube, named after its two developers.  The GM tube is filled at low pressure with an inert gas, such as argon or nitrogen, that does not react strongly with other substances.  A large electrical potential difference is then induced between the inner metal of the cylinder — the cathode — and a wire threaded through the center of the tube — the anode.  Since the gas is inert, no current flows between the cathode and the anode most of the time.  However, when a high energy radioactive particle passes through the tube, it can ionize the neutral atoms, i.e. knock electrons off of them.  These electrons are strongly attracted to the positively-charged cathode and, along the way, they collide with other atoms and liberate more electrons, producing an “avalanche” of electrons.  This sudden burst of electrons produces a short current spike, and these current spikes can be counted or converted to a flash of an LED or a click of a speaker.  This is illustrated below.

Image of a Townsend avalanche in a GM tube, by Wikipedia user Dougism under CC BY-SA 3.0.  I normally draw my own pictures, but this one was hard to beat.

This picture can be a little hard to follow at first, so it is worth a little explanation.  A high-energy radiation particle (purple) passes through the tube.  Along the way, it smacks into an atom (or more than one) and ionizes the atom, i.e. liberates an electron.  This liberated electron gets accelerated towards the anode, and builds up enough kinetic energy to ionize another atom.  Then the two electrons, the one that caused the ionization (blue) and the one just liberated (orange) each can ionize again, and so forth, until a huge flow of electrons arrive at the anode.  Each ionization even has a chance of also generating an ultraviolet photon (green), and each of these photons can cause another ionization, creating additional avalanches.

In other words, each “click” of a Geiger counter typically represents a single radioactive particle passing through the Geiger-Müller tube.  The faster the clicks come, the more radioactivity is present.

So what types of radioactive particles are there?  There are 3 types of radioactive particles that can be detected:

Of these three, alpha particles are very bad at penetrating matter, even getting stopped by a piece of paper!  This is bad news for our sealed Geiger tube, as few alpha particles  penetrate inside to make a detection.  We can only detect beta and gamma particles efficiently with the model I built.  Geiger tubes are also made with thin “windows” of mica, though which alphas can pass.

The biggest challenge in building a Geiger counter is, in fact, finding efficient radioactive sources of radioactivity to test it on!  As a first test of a new counter, simply turn it on: you will find it clicks every few seconds.  These clicks are mostly due to natural radiation in earth-bound sources, such as the building material in your home. A small number of these clicks are the result of high-energy particles produced in the upper atmosphere via radiation from outer space: cosmic rays.  (For a history of cosmic rays, and more details, see my old guest post on Scientific American.)  My counter clicks roughly about every 10 seconds, though keep in mind it is a random process and the count will sometimes be a little faster or a little slower.

Other sources are harder to come by.  You may have heard that bananas are radioactive — they contain a small amount of potassium-40, which can decay via beta or gamma radiation.  However, a single banana has only about 14 radioactive decays per second!  The radioactive particles go in all directions, and only a very small fraction will hit a simple Geiger counter.  The same goes for most other radioactive foods: Brazil nuts, which contain small amounts of radium, also will not trigger the counter (I’ve tried).

Smoke detectors provide more success.  Many of them use radioactivity in what might be considered an “inverse Geiger counter” configuration.  The radioactive element (often americium-241) ionizes the air in a chamber of the detector, allowing a small current to pass through it.  When smoke enters the chamber, however, it binds to the ions, causing the current to drop and thus triggering an alarm.

Americium produces primarily alpha radiation, again making it hard to detect.  However, I seem to be able to get a slight increase in ticks when I bring my Geiger counter nearby.

Much better sources can be purchased online.  A variety of radioactive disk sources can be purchased from United Nuclear that are sealed and therefore safe to handle for short periods of time.

A little bit more showy is a sample of uranium ore, which can also be purchased from United Nuclear. Uranium is an alpha emitter, but its decay products are also radioactive, and they produce a nice strong signal in a closed Geiger tube.  The ore is safe to handle, but be sure to wash your hands thoroughly afterwards!  (Accidentally eating bits of uranium isn’t a good idea.)

Certificate, schmertificate. The real certification comes in Geiger clicks.

The best source to test a Geiger counter on, though?  Orange Fiesta dinnerware made between 1936 and 1944.  To get the bright orange color, the Fiesta company used uranium oxide, which has a significant amount of radioactivity.  The original run of “Fiesta red” ended in 1944, when all uranium was being snapped up for the war effort.  The company reintroduced the uranium based color in 1959, using depleted uranium.  It was finally discontinued completely in the early 1970s when the popularity of the dinnerware faded.

Despite the radioactivity, there does not seem to have been any danger to the public or any reported illnesses associated with the radioactive Fiestaware.  Today, it can be purchased online from antiques dealers, and I scored a 1939 piece to try out.

Vintage 1939 Fiesta red Fiestaware.

In the short video below, I test out my uranium ore and my innocent-looking Fiestaware.

People used to eat off of such Fiestaware!  Nowadays, it is not recommended.

So, in short: Geiger counters are fun, relatively easy kits to build.  Just be sure you’ve got a decent radioactive source on hand to test it!