[ Intro].

Say you’ve settled in for a day of gaming glory, and you’re busy cruising the radioactive wasteland of Fallout 4, when suddenly your speakers start emitting crackling, clicking noises. Gamers aren’t the only ones who will recognize that sound: it’s the characteristic noise of a Geiger counter detecting radiation.

But it turns out that what you’re really hearing is the sound of rogue electrons. The Geiger counter was developed in 1908 by Hans Geiger and perfected 20 years later by his student Walther Müller. So the radiation detector inside it is called, surprise surprise, a Geiger-Müller or G-M tube.

The tube is made of metal so it conducts electricity, and it's filled with a gas that’s normally inert, or nonreactive, like neon. A long metal rod runs through the middle, with a power source charging it. Everything is stable because the inert gas between the rod and the walls doesn’t conduct electricity—at least not yet.

That’s where radiation comes in. At the most basic level, radiation is how energy travels through space and is transferred between things. Geiger counters specifically measure ionizing radiation: radiation with enough energy to strip electrons from atoms, leaving behind positively-charged ions.

That also makes it powerful enough to damage DNA, which is why ionizing radiation is dangerous for living things … and why we have a vested interest in detecting it. When a radioactive atom decays, it produces ionizing radiation in the form of subatomic particles. And with a Geiger counter, that's what you're looking to detect.

When the ionizing radiation passes through the gas inside the G-M tube, it knocks electrons off the gas atoms inside. The newly-freed electrons are attracted to the positively-charged rod at the center, so they zoom towards it. And as they do, they knock electrons off other atoms, in what’s called a Townsend avalanche.

This cascade of atoms being ionized creates enough charged particles inside the tube to briefly conduct electricity between the rod and the walls. So there’s a momentary surge of current. When the tube is connected to a speaker, that current causes a rapid, jerking movement of the speaker components—which is that crackling or popping noise.

While the electrons move toward the middle, the positive gas ions head the other way. When they hit the walls of the tube, they pick up an electron from the metal, returning to their original state and resetting the detector. The same crackling can also happen with any abrupt change in the current running through your speakers—like if you accidentally roll your gaming chair over your audio cables.

Thankfully, though, that type of crackling doesn't mean you need to go find a lead shield. Now, tweaks to the tube design and the gas inside can make different Geiger counters slightly more or less sensitive to different kinds of ionizing radiation. But they’re only really useful for detecting somewhat low levels of radiation.

That’s because any radiation that hits before the tube finishes resetting isn’t detected, so they actually underestimate the danger in high radiation areas. So this probably goes without saying, but if you hear a Geiger counter crackling, stay away! Unless you’ve got some mutant hound chops or a refreshing beverage to flush your rads, that is.

Thanks for watching this episode of SciShow, brought to you by our patrons on Patreon. If you’d like to propose questions like this for us to answer, or gain access to other awesome rewards like blooper reels, you can pledge your support at patreon.com/scishow. [ Outro ].