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Machines designed to reveal subatomic particles operate on huge scales. Detectors at the Large Hadron Collider weigh thousands of tons, contain millions of sensors, and record particles moving at close to the speed of light. But they don’t have to be so complex. In fact, some are small and simple enough to operate in a living room.

Take the cloud chamber, which uses evaporated alcohol to make vapor that’s extremely sensitive to passing particles. The DIY version is an empty fish tank with a ceiling of alcohol-soaked felt and a floor of dry ice. The room-temperature alcohol at the top evaporates into the air and sinks toward the ice at the bottom. As it cools, it will condense on anything it can.

Cosmic rays are constantly crashing into Earth from space, and when they hit the atmosphere, they release a shower of particles. If one of these particles zips through the DIY cloud chamber, it will bump into molecules that make up the air and knock off some of their electrons. This leaves a trail of charged ions, and tiny droplets of airborne alcohol cling to them. The ghostly track that results marks an individual particle’s path through the chamber.

Watch a DIY cloud chamber in action. Then build your own!

Stats

Time: 20 minutes

Cost: $70

Difficulty: 1/5

Materials

  • Clear plastic or glass tank
  • Felt
  • Superglue
  • Isopropyl alcohol, 90% concentration or more
  • Dry ice
  • Wide, shallow container for the dry ice
  • Dark metal baking sheet (larger than the tank’s mouth)
  • Flashlight

Tools

  • Scissors
  • Latex or nitrile gloves
  • Oven mitts
  • Safety goggles

Instructions

  1. Cut the felt to match the tank’s footprint, and glue it onto the bottom (where the sand and fake treasure chests would normally go).
  2. Put on your safety goggles and gloves, and pour enough isopropyl alcohol into the tank to saturate the felt. Drain off any excess.
  3. While wearing oven mitts, place the dry ice in a shallow container. Cover it with the baking sheet.
  4. Flip the tank upside down onto the baking sheet, so that the felt-covered bottom is at the top. You can use extra felt to block the seam between the tank and the tray. Now remove the goggles and mitts (but put them back on when disassembling the cloud chamber).
  5. Wait about 10 minutes for the alcohol vapor to fill the tank. Then turn off all the lights and shine a flashlight into the tank.
  6. If you see a track that looks like the path of a lost tourist in a foreign city, you’re looking at an electron or a positron, the electron’s antimatter twin. The lightweight particles bounce around when they hit molecules in the air, leaving zigzags and curlicues.
  7. A short, fat track appears when an atmospheric radon atom spits out a bulky, low-energy alpha particle (a clump of two protons and two neutrons). Radon is a naturally occurring radioactive element, but its concentration in the air is so low that its airborne presence is less radioactive than peanut butter.
  8. A long, straight track indicates that you’ve probably got muons, heavier cousins to the electron. Muons are produced when a cosmic ray bumps into a molecule high in the atmosphere. Because they are so massive, muons bludgeon their way through the air and leave clean lines.
  9. If a track suddenly forks, congratulations! You just saw a particle decay. Many particles are unstable and will transform into more stable particles. You are watching physics in action.

Warning: Wear safety goggles and gloves while handling isopropyl alcohol, and goggles and oven mitts when grabbing dry ice.

This article appeared in the June 2015 issue of Popular Science, under the title “Build Your Own Particle Detector.” It was adapted from an article in Symmetry Magazine.

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