Why Geiger Counters Make That Iconic Clicking Noise While Measuring Radiation
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Geiger counters are used to detect and measure levels of radioactivity.
Scientists invented the Geiger counter while working on experiments to prove that the center of an atom has a high-density zone, called the nucleus.
These devices make a clicking sound as ionized gas conducts electricity through a central wire to the outer shell.
One of the most haunting moments in the 2019 hit series, Chernobyl, features liquidators shoveling graphite from the roof of reactor No. 4 at Chernobyl Nuclear Power Plant, the site of the worst nuclear disaster in history. As part of the cleanup effort following the 1986 nuclear meltdown near Pripyat, Ukraine, these men were sent to remove massive chunks of graphite from the reactor casing, which once contained the radiation.
All the while, a faint clicking noise plays in the background, intensifying and picking up frequency as the workers make their way through the facility and onto the roof. The source of this ominous clicking? Geiger counters, the instruments used to detect and measure ionizing radiation—in this case, radioactive nuclear debris.
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These ticking machines appear frequently in science fiction (IMDB says there are 182 movies that feature Geiger counters), but they’re very much real. Geiger counters alert us to the presence of radioactivity that generates alpha or beta particles or gamma rays. Some naturally occurring elements, like uranium and thorium, are radioactive. Human-made technologies like nuclear bombs or fission power plants can also dangerously augment natural levels of radioactivity as the Chernobyl tragedy shows. But how exactly do Geiger counters work, and why do they make that iconic clicking sound?
A Brief History of the Geiger Counter
In the early 20th century, scientists in the United Kingdom developed the Geiger counter while working on experiments that demonstrated that the atomic nucleus is a high-density zone that exists at the center of an atom. Previously, scientists thought that there was solid material throughout the atom.
Hans Geiger, the device’s primary inventor, was the postdoctoral lab assistant of Ernest B. Rutherford, a professor of physics at the University of Manchester who was studying atoms. Geiger had found that when uranium decays, two of its isotopes—which have the same number of protons as the element, but different numbers of neutrons—emit alpha particles of two different energies. An alpha particle is made of two protons and two neutrons. A beta particle is an electron or positron that travels at a high energy and high speed. Both of these particles are emitted by elements that are undergoing radioactive decay.
So, Geiger developed a device to measure alpha radiation by shooting alpha particles through gold foil onto a screen. The researchers set up a microscope that could be rotated around the foil so they could count the flashes that occurred when the particles passed through the foil. It was difficult to watch the flashes in the dark laboratory and count accurately; They were only able to observe for up to a minute each before needing to rest their eyes.
In the experiment, some alpha particles bounced back, meaning that they had struck something dense—the nuclei within the gold atoms. This disproved the earlier model of the structure of an atom.
To remove the need for visual observation, Geiger invented a tube-shaped counter with a central high-voltage tungsten wire. Sixteen years later, he collaborated with his graduate student Walther Müller to improve its sensitivity, performance, and durability. For that reason, the device is sometimes known as a Geiger-Müller counter.
How Geiger Counters Work
Geiger counters work through ionization, a process in which atoms or molecules gain or lose electrons and become electrically charged.
In the device, an electric circuit containing a transformer and capacitors energizes a central tungsten wire to a high voltage. The transformer steps the voltage up while capacitors maintain the voltage level.
The wire is surrounded by a tube of inert gas, such as argon or xenon, that has a window. When the radioactive particles pass through the window, they ionize the gas inside. The electrons in the gas separate from their nuclei. The electrons are drawn to the positive wire. On their way, they collide with more gas molecules and ionize them. This results in many electrons traveling toward the wire.
The ionized gas conducts electricity, causing a pulse of current to travel from the wire to the outer shell of the tube. This pulse causes the Geiger counter to make a clicking sound. The sound has a frequency corresponding to the number of ions that were created in a minute. The signal can also make a needle move on a scale or generate a number on an information readout.
After the pulse travels to the outside of the tube, the ions and electrons are rapidly reabsorbed into the gas and the detector resets itself. This process is called quenching. It can be accomplished through a quenching gas that absorbs the positive ions or by electronic controls that adjust the voltages in the tube.
“A very simple Geiger counter … is in everyone’s house,” Earl Scime, a professor of physics and astronomy at West Virginia University, tells Popular Mechanics. “Every smoke detector is a Geiger counter, essentially. It detects alpha particles that are coming from a radioactive source in the smoke detector, and when smoke gets in the way of those particles getting to the detector … the signal goes away and the alarm sounds.”
Building Your Own Geiger Counter
It’s easy to find extensive instructions for homemade Geiger counters for people who are curious about DIY approaches for measuring radiation at home.
“There are many kits you can buy for all different kinds of prices, depending on how sophisticated you want to be,” Lawrence Greenwood, a lab fellow of nuclear chemistry and engineering at Pacific Northwest National Laboratory, tells Popular Mechanics. “For less than $100, you can buy a kit and put it together. Of course, it’s on you to make sure it’s working right. They’re not calibrated and checked.”
A few apps promise to turn one’s cell phone into a Geiger counter, Greenwood says. “They call it a Geiger counter, but it’s not exactly right. Basically, what you do is cover up your camera and turn your camera on, but tape over the lens. [The apps] will show you flashes.”
Organizations are watching for radiation throughout the United States to protect the public, Greenwood says. The Nuclear Regulatory Commission website says sophisticated radiation data is available from the U.S. Environmental Protection Agency’s system, RadNet, which tracks data from the air. Its researchers sample pasteurized milk, outdoor precipitation, and drinking water.
Greenwood says Popular Mechanics readers should “study up” on what radiation means rather than being unnecessarily fearful. “Just because you see a little bit of radiation doesn’t mean it’s going to be harmful to you.”
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