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Cloud Chamber Experiment

Cloud Chamber Experiment

The cloud chamber should look like the figure shown below. The felt strip is glued inside the box, about  inch below the top. The bottom of the box can be painted black, or use black cloth or paper. This helps with viewing the vapor trails.

Dry ice is cold enough to freeze skin, if touched. Never touch dry ice without wearing heavy gloves. Better to handle with tongs. Sheets of dry ice work best, since the box can be placed flat on the sheet. With other forms of dry ice, make a flat surface of ice to place the box upon. Be sure to put a foam-paper plate or other protection under the ice.

This provides the gamma rays, beta rays, etc. that will show up in the cloud chamber. A small pebble containing uranium or radium works best. Also, some cloth lantern mantles, available in camping stores, contain thorium and also work well. Its best to roll up the mantle into a ball so it stands up. Secure with string or elastic. Don't put anything too large into the chamber (such as a radium dial watch). It will act as a heat-sink and prevent the necessary cloud from forming. A source of radioactivity as small as the pebble is not dangerous. Try other items you think may be radioactive.

The cloud chamber works because a cloud is formed inside. Nuclear radiation passing through the cloud precipitates liquid droplets in its path. This is also known as a "vapor trail," as seen behind high-altitude aircraft. The cloud in the chamber is not a water cloud, but an alcohol cloud. Methanol is the best alcohol to use. Denatured alcohol, sold in hardware stores, will also work. Remember, methanol is a poison.

Drip some methanol on the felt strip. Don't let it pool at the botttom of the chamber. Place the source in the chamber, close the lid tightly and place the chamber on dry ice. Darken the room. In about 2 minutes, visible tracks should begin to appear in the chamber. Use a flashlight to highlight them. The cloud chamber will work for about 10 minutes.

Question: How can you tell alpha particles from betas and gammas? Hint: use a magnet.

CLOUD CHAMBER KITS are available from the Health Physics Society

  • They include the chamber box, blotter paper and source (uranium-rich pebble.)
  • These are well tested and work great. You'll need methanol, dry ice and a flashlight.
  • Kits are sold at cost plus shipping, for $6 each. Send a check (payable to name below) or cash to:

    Joel Cehn (

     4714 Windsor Blvd.
     Cambria, CA 93428