EXPERIMENT 5: Observing Radioactivity with a Cloud Chamber

THINGS YOU NEED: A piece of dry ice (see paragraph four below). A large glass jar with lid; the jar should be about five inches high and about four inches wide (an old peanut butter jar would be fine). A piece of thick blotting paper. Glue. A piece of black velvet cloth. An old towel. Some rubbing alcohol. A powerful flashlight or high-intensity desk lamp. Alpha ray source from Experiment 3.

A cloud chamber is a device that allows scientists to see the trails made by nuclear particles. It was invented in 1912 by Charles T.R. Wilson, a pioneer atomic physicist.

Wilson discovered that water vapor can condense on ions just as it does on bits of dust to form raindrops. Now since nuclear particles, such as alpha rays, produce ions as they streak through water vapor, the vapor that condenses on these ions shows up as fine whitish trails.

The cloud chamber in this experiment is a modificationof Wilson’s original design. It is called a diffusion-type chamber, and it uses alcohol vapor instead of water vapor.

Look up Dry Ice in your Yellow Pages telephone directory to find a dealer in your area. You need a piece about six inches by six inches by two inches (roughly two pounds in weight). Dry ice is frozen carbon dioxide. It is cold enough to cause severe burns to unprotected skin and must be handled very carefully. Wrap the dry ice in a towel; never touch the exposed ice. For safety’s sake, it is a good idea to wear leather gloves when you handle the block.

Make the cloud chamber as follows:

Cut a circle of black velvet cloth to fit inside the metal jar lid. Cement it in place with a few dabs of glue.
Cut a circle of blotting paper to fit inside the bottom of the jar. Cement it in place with a few dabs of glue.
After the glue has dried fully, drip some rubbing alcohol onto the blotting paper. Keep dripping until the blotting paper is completely saturated, but does not show an excess of alcohol on its surface.
Screw the lid on the jar. Then place the jar, lid side
down, on top of the wrapped block of dry ice.
Darken the room completely, then position the flashlight as shown in the drawing.
Wait patiently and keep looking at the path of the light beam. After several minutes (it takes the chamber severalminutes to form its cloud) you will see occasional white tracks near the metal lid. These are produced by the passage of cosmic rays (radiation from outer space) through the cloud chamber.