How to Make a Cloud Chamber

How to Make a Cloud Chamber Although you cannot see it, background radiation is all around us. Natural (and harmless) sources of radiation include cosmic rays, radioactive decay from elements in rocks, and even radioactive decay from elements in living organisms. A cloud chamber is a simple device that allows us to see the passage of ionizing radiation. In other words, it allows for indirect observation of radiation. The device is also known as a Wilson cloud chamber, in honor of its inventor, Scottish physicist Charles Thomson Rees Wilson. Discoveries made using a cloud chamber and a related device called a bubble chamber led to the 1932 discovery of the positron, the 1936 discovery of the muon, and the 1947 discovery of the kaon. How a Cloud Chamber Works There are different types of cloud chambers. The diffusion-type cloud chamber is the easiest to construct. Basically, the device consists of a sealed container that is made warm on the top and cold on the bottom. The cloud inside the container is made of alcohol vapor (e.g., methanol, isopropyl alcohol). The warm top part of the chamber vaporizes the alcohol. The vapor cools as it falls and condenses on the cold bottom. The volume between the top and bottom is a cloud of supersaturated vapor. When an energetic charged particle (the radiation) passes through the vapor, it leaves an ionization trail. The alcohol and water molecules in the vapor are polar, so they are attracted to ionized particles. Because the vapor is supersaturated, when the molecules move closer, they condense into misty droplets that fall toward the bottom of the container. The path of the trail can be traced back to the origin of the radiation source. Make a Homemade Cloud Chamber Only a few simple materials are needed to construct a cloud chamber: Clear glass or plastic container with lid99% Isopropyl alcoholDry iceInsulated container (e.g., a foam cooler)Absorbent materialBlack paperVery bright flashlightSmall bowl of warm water A good container might be a large empty peanut butter jar. Isopropyl alcohol is available at most pharmacies as rubbing alcohol. Make sure its 99% alcohol. Methanol also works for this project, but it is much more toxic. The absorbent material could be a sponge or piece of felt. An LED flashlight works well for this project, but you can also use the flashlight on your smartphone. Youll also want your phone handy to take pictures of the tracks in the cloud chamber. Start by stuffing a piece of sponge into the bottom of the jar. You want a snug fit so it wont fall when the jar is inverted later on. If necessary, a bit of clay or gum can help stick the sponge to the jar. Avoid tape or glue, since the alcohol may dissolve it.Cut the black paper to cover the inside of the lid. Black paper eliminates reflection and is slightly absorbent. If the paper doesnt stay in place when the lid is sealed, stick it to the lid using clay or gum. Set the paper-lined lid aside for now.Pour isopropyl alcohol into the jar so that the sponge is completely saturated, but there isnt excess liquid. The easiest way to do this is to add alcohol until there is liquid and then pour the excess out.Seal the lid of the jar.In a room that can be made completely dark (e.g., a closet or bathroom without windows), pour dry ice into a cooler. Turn the jar upside down and place it lid-down onto the dry ice. Give the jar about 10 minutes to chill.Set a small dish of warm water on top of the cloud chamber (onto the bottom of the jar). The warm water heats the alcohol to form a cloud of vapor. Finally, turn off all the lights. Shine a flashlight through the side of the cloud chamber. Youll see visible tracks in the cloud as ionizing radiation enters and leaves the jar. Safety Considerations Even though isopropyl alcohol is safer than methanol, its still toxic if you drink it and its highly flammable. Keep it away from a heat source or open flame.Dry ice is cold enough to cause frostbite on contact. It should be handled using gloves. Also, dont store dry ice in a sealed container, as pressure build-up as the solid sublimates into gas can cause an explosion. Things to Try If you have a radioactive source, place it near the cloud chamber and see the effect of the increased radiation. Some everyday materials are radioactive, such as brazil nuts, bananas, clay kitty litter, and vaseline glass.A cloud chamber offers an excellent opportunity to test methods of shielding against radiation. Place different materials between your radioactive source and the cloud chamber. Examples might include a baggie of water, a piece of paper, your hand, and a sheet of metal. Which is best at shielding against radiation?Try applying a magnetic field to the cloud chamber. Positive and negative charged particles will curve in opposite directions in response to the field. Cloud Chamber Versus Bubble Chamber A bubble chamber is another type of radiation detector based on the same principle as the cloud chamber. The difference is that bubble chambers used superheated liquid rather than supersaturated vapor. A bubble chamber is made by filling a cylinder with a liquid just above its boiling point. The most common liquid is liquid hydrogen. Usually, a magnetic field is applied to the chamber so that ionizing radiation travels in a spiral path according to its speed and charge-to-mass ratio. Bubble chambers may be larger than cloud chambers and can be used to track more energetic particles.