Cosmic rays are high-energy subatomic particles that constantly bombard the Earth from outer space. Thousands of these particles pass through our planet, and through us, every second. This natural radiation is harmless and invisible, but the tracks that the particles leave behind can be seen using a cloud chamber.

Over the years, several experiments at CERN have used cloud chambers to detect particles. The Gargamelle experiment, for example – designed to detected neutrinos – was 4.8 metres long, 2 metres in diameter and weighed 1000 tonnes. The large CLOUD experiment at CERN today also uses a cloud chamber, to investigate the effects of cosmic rays on cloud formation.

Though the cloud chambers at CERN each took many years to plan and build, you can make your own cosmic-ray detector in the classroom providing you have access to the right materials. Although make sure your teacher or guardian is there to help you – you will need to be careful handling dry ice and isopropanol which can be dangerous.

In the video above, US / LHC communicator Sarah Charley shows us how to make a “Continuously Sensitive Diffusion Cloud Chamber”. Originally developed at UC Berkeley in 1938, this type of cloud chamber uses evaporated alcohol to make a “cloud” that is extremely sensitive to passing particles. It is based on the same principles that determine the formation of clouds in the sky. If air is saturated with water vapor and then cooled, tiny droplets of mist form around floating bits of dust or other material. They also form readily around ions; electrically charged atoms or groups of atoms. When a charged particle, such as a proton, passes through the chamber it leaves behind a trail of ions as it strikes molecules in the air along its path and tears away electrons. Mist droplets form around these ions, creating a cloud track.

By studying the track, or a photograph of it, we can determine the energy and electric charge of the particle that produced it. Every kind of particle has a characteristic cloud track, which varies in shape, length, and width. Many elementary particles have been discovered through their characteristic cloud track.

For more detailed instructions and a full tutorial, see here.