Building a Cloud Chamber: Why Muons Should Not Be There

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Author: Jerry Silver

The Idea

Cosmic rays are subatomic particles that stream through the universe at high speed. As you read this, dozens of these particles are passing through your body harmlessly every second. In this project, you build a device called a cloud chamber, which will make some of these particles visible. The cloud chamber contains an alcohol-vapor cloud that produces vapor trails when a charged particle passes through it. This trail reveals the path or track the particles take as they pass through the vapor cloud. You begin to learn to recognize the signature of some of those particles.

Scientists believe most cosmic rays come from the sun. These cosmic rays consist of fragments of the nuclei of hydrogen or helium atoms, resulting in particles that are either singly charged (protons) or doubly charged (alpha particles). When these cosmic ray particles strike the upper atmosphere of the Earth, they collide with the air molecules present. This results in collisions that produce new particles, called secondary cosmic rays. These are what you will detect in your cloud chamber.

The most common result of the collision is a particle called a muon. The muon is a negatively charged particle, which is bigger than an electron, but smaller than a proton. The muons that are created in the upper atmosphere decay incredibly rapidly. Because of their extremely short life, they should not be able to survive the trip through the Earth's atmosphere to be detected on its surface. However, the most common particle detected in the secondary cosmic ray stream is the muon. The only way to explain the abundance of muons you see in your cloud chamber is by turning to Einstein's theory of relativity, which says that time slows down for the very high-speed muons.

Also present in the shower of secondary cosmic rays hitting the Earth's surface are positrons and electrons. The positrons are a form of antimatter that can also be seen in your cloud chamber.

What You Need

  • small 2.5 gallon fish tank
  • small Styrofoam cooler
  • 1 liter of pure isopropyl alcohol (not 70 percent alcohol, as is more readily available). Pure methyl alcohol can also be used.
  • sheet of black felt large enough to line the bottom of the tank (note, the fish tank will be turned upside down, so what we refer to as the bottom of the fish tank will become the top of the cloud chamber)
  • metal plate the size of the top of the fish tank—aluminum is preferable because it conducts heat better than steel
  • duct tape, silicone rubber sealant, or weather stripping to make the fish tank airtight
  • black (solvent resistant) paint
  • about 1 pound of dry ice to encase the bottom of the fish tank
  • bright light source, such as a powerful flashlight
  • strong magnet
  • optional: source of low-level radiation, such as a smoke detector, mantle of old Coleman lantern, or certain old ceramic objects that contain cobalt
  • optional: a digital video camera


Building the cloud chamber

  1. Because the dry ice has a limited shelf life and, for most users, takes a special effort to obtain, a good idea is to do a dry run and assemble these parts before picking up the dry ice. Also, remember dry ice is extremely cold and should not come into contact with eyes or skin.
  2. Attach the black felt to the bottom of the fish tank. Use black electrical tape or Velcro under the felt to secure it, so it remains in place when the fish tank is inverted. You may want to devise some other way to secure the felt such as small wooden supports. Keep in mind that whatever you use to support the felt must not be degraded by the solvent vapor that will be present. A solvent resistant glue such as Gorilla Glue or Gorilla tape can hold the felt without being attacked by the alcohol vapor.
  3. Place the dry ice in the bottom of the cooler. If the dry ice is in large chunks, you will have to chop it. You can add a little alcohol to the mixture to form a slush to make better thermal contact with the metal plate. If you don't have a cooler the fish tank will fit in, use a box and line it with insulating material.
  4. Soak the black felt with alcohol. Avoid dripping that produces puddles of alcohol on the metal plate by not using too much alcohol.
  5. Cover the dry ice with the metal plate, with the side painted black facing away from the dry ice. The metal plate should be completely in contact with the dry ice. You can secure the plate to the tank first if that works better.
  6. Place the tank—alcohol-soaked felt up—with the metal plate over the dry ice. It is important to establish good thermal contact between the dry ice and the metal plate. People have used solid blocks of dry ice as well as crushed ice. Good thermal contact can be achieved by creating a slurry by mixing some isopropyl alcohol in with the dry ice. If you crush the dry ice, be sure to wear protective classes to avoid extremely cold dry ice fragments from contacting your eyes. The dry ice mixed with alcohol should be around –70 degrees C or –94 degrees F.
  7. Seal the chamber above the metal plate by wrapping electrical tape around the edge where the metal plate meets the fish tank. (Some people have found that pouring a small amount of isopropyl alcohol in the channel surrounding the metal plate helps form an airtight seal.) If air is drawn into the chamber, the vapor cloud may not from properly.
  8. It may be necessary to keep the top of the cloud chamber (where the alcohol-saturated felt is) from getting too cold. The bottom of the chamber should be near –60 degrees C (–76 degrees F) to enable the formation of supersaturated vapor. However, the top of the chamber should be maintained close to room temperature (22 degrees C or 72 degrees F) to promote evaporation of the alcohol. To accomplish this it may be necessary to warm the top of the chamber either with your hands or with some other means to maintain the proper temperature gradient. It might be helpful to measure the temperatures of both surfaces.
  9. Shine the light from the side of the tank toward the metal plate.
  10. The stack should look like this, from top to bottom:
    • Bottom of the fish tank
    • Black felt soaked with isopropyl alcohol
    • Fish tank (metal plate covering the top of the tank)
    • Building a cloud chamber. Why muons should not be here. Special relativity

    • Metal plate
    • Sheets of dry ice
    • Bottom of the Styrofoam cooler
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