A cloud chamber is a simple device that makes the passage of ionizing radiation visible. Ionizing radiation is all around us in the form of background radiation, which comes from cosmic rays, elements in rocks and food, and even within living organisms. Here is how to make a cloud chamber, a look at how it works, and how to use a cloud chamber to identify types of background radiation or radioactivity from radioisotopes.
A Brief History
Scottish physicist Charles Thomson Rees Wilson invented the cloud chamber in 1911. Another name for a cloud chamber is a Wilson cloud chamber, in his honor. Wilson’s chamber traced the passage of radiation through water vapor. The discovery earned Wilson and Arthur Compton the 1927 Nobel Prize in Physics. The cloud chamber and a related device called a bubble chamber led to discoveries of the positron in 1932, muon in 1936, and kaon in 1947.
How a Cloud Chamber Works
There are different types of cloud chambers. The cloud chamber in this project is called a diffusion-type cloud chamber. It is a sealed container that is warm at the top and cool at the bottom. The “cloud” consists of alcohol vapor. Isopropyl or methyl alcohol are good choices because they readily vaporize at ordinary temperatures and are polar molecules. The warm part of the chamber vaporizes the alcohol, which cools as it descends toward the cold container base. The temperature difference forms a volume of supersaturated vapor.
When ionizing radiation passes through the vapor, it ionizes particles in its path. Because the alcohol and water vapor inside the chamber are polar, they are attracted to the electrical charge of the ionized particles. When the polar molecules move toward the ionized region, they draw closer together. The vapor is supersaturated, so moving particles closer makes the vapor condense into misty droplets. You don’t see the actual radioactivity. Rather, a cloud chamber makes radiation indirectly visible. The path of the trail points back to the origin of the radiation source.
How to Make a Homemade Cloud Chamber
A cloud chamber consists of a transparent container filled with polar vapor. The container is warm at the top and cool at the bottom.
A simple devices uses these materials:
- Clear glass or plastic container with lid
- 90%-99% Isopropyl alcohol or methyl alcohol
- Dry ice
- Insulated container for the dry ice
- Sponge or other absorbent material
- Black construction paper
- Small, bright flashlight (or cell phone)
- Small bowl of warm water
A clean peanut butter or mayonnaise jar is a good size for a cloud chamber. You can make a larger chamber using a 10-gallon aquarium.
Isopropyl alcohol or isopropanol is rubbing alcohol. It’s available at grocery stores and pharmacies. Look for the highest alcohol purity you can find. 90% alcohol works, but 95% or 99% works better. Methyl alcohol or methanol is a fuel treatment. It works great, but it’s toxic. Only use methanol if you can do the project outdoors or in a fume hood.
Either use a small LED flashlight or the flashlight app on your phone as a light source. The goal is illuminating the cloud chamber, not the whole room.
- Stuff a piece of sponge into the bottom of the jar. Make sure the sponge stays in place when you turn the jar upside-down. Alternatively, cut a circle of felt so it fits into the bottom of the jar. Stick it to the jar using modeling clay or gum (not tape or glue, because alcohol dissolves the adhesive).
- Cut a circle of black paper and fit it inside the lid. The paper is slightly absorbent and eliminates reflections. If you have a radioactive source, set it on the black paper. Set the lid aside for now.
- Pour alcohol into the jar and saturate the sponge. Flip the jar over and let any excess alcohol flow out.
- Seal the lid of the inverted jar.
- Place the inverted jar on top of the dry ice.
- Set a small dish of warm water on top of the cloud chamber (which is on the bottom of the jar).
- Turn out the lights. Shine a flashlight into the cloud chamber and see the vapor trails.
More Cloud Chamber Options
- Instead of a jar, use a large clear plastic cup. Seal the plastic cup by making a modeling clay “snake” and sticking the cup onto a metal or glass plate. Then, place the plate onto the dry ice. Warm the bottom of the cup (which is the top of the cloud chamber) with your hand.
- Use a plastic petri dish instead of a jar. Just press the sponge into the bottom of the dish. Cut a circle of dark-colored felt that fits just inside the rim of the dish. This improves viewing. Soak the sponge with alcohol and set the petri dish on dry ice (i.e., don’t flip it over). Instead of a dish of warm water, warm the top of the dish with your hand.
Fun Things to Try
- Vapor trails naturally appear in the cloud chamber from background radiation. But, you’ll get more trails if you add a radiation source. Test the effects of everyday radioactive materials, such as bananas, kitty litter, brazil nuts, ceramics, or vaseline glass. Alternatively, use a radioisotope. You’ll either need to order a source online or else harvest the source from a smoke detector (americium-241). Note: Alpha particles cannot penetrate glass or plastic, so if you want to see their trails, you need to seal the radiation source inside the jar.
- Test the effectiveness of radiation shielding methods. Place different materials between your radioactive source and the cloud chamber. Examples include your hand, a sheet of paper, and a sheet of foil. Which material shields against radiation the best?
- Apply a magnetic field to the cloud chamber. Use a strong magnet, like a neodymium magnet. Positive and negative particles curve in opposite direction.
Identify Cloud Chamber Trails
Observe the vapor trails and see if you can identify the type of radiation. Also, look for wavy or forked tracks.
- Short, thick trails: Short, thick trails come from alpha particles. You might not see many of these unless you have a radioactive item sealed within the jar.
- Long, straight trails: Long, straight trails comes from muons. Muons are subatomic particles that form when cosmic rays interact with the atmosphere.
- Curling or zig-zag trails: Electrons and their antimatter counterparts called positrons readily interact with matter. They bounce around with each interaction, leaving wavy trails.
- Forked trails: Forked trails indicate radioactive decay. When particles decay, they release smaller particles, such as electrons and neutrinos. These particles shoot off from the main track.
You may see trails you don’t expect. Keep in mind, air contains traces of radioactive tritium, radon, and other isotopes. Also, you may see condensation trails from the daughter isotopes of a radioactive source.
- Alcohol is flammable, so keep it away from a heat source or open flame.
- Both isopropyl alcohol and methyl alcohol are toxic. Do not drink them. Isopropyl alcohol or rubbing alcohol is much less toxic than methanol. If you use methanol, also avoid skin contact or vapor inhalation.
- Handle dry ice using gloves or tongs because it is cold enough to cause frostbite on contact.
- Don’t store dry ice in a sealed container because pressure build-up may burst it. Put dry ice in a paper bag or in a foam cooler with a lid that rests on top.
Difference Between a Cloud Chamber and a Bubble Chamber
A bubble chamber works on the same principle as a cloud chamber. The difference is that a bubble chamber contains superheated liquid instead of supersaturated vapor. A bubble chamber is a cylinder filled with liquid heated to just above its boiling point. The usual choice is liquid hydrogen. Applying a magnetic field makes ionizing radiation spiral according to its speed and charge-to-mass ratio. So, bubble chamber trails offer more information about the type of radiation and track more energetic particles than cloud chambers.
- Das Gupta, N. N.; Ghosh S. K. (1946). “A Report on the Wilson Cloud Chamber and its Applications in Physics”. Reviews of Modern Physics. 18 (2): 225–365. doi:10.1103/RevModPhys.18.225
- Glaser, Donald A. (1952). “Some Effects of Ionizing Radiation on the Formation of Bubbles in Liquids”. Physical Review. 87 (4): 665. doi:10.1103/PhysRev.87.665
- “The Nobel Prize in Physics 1927“. www.nobelprize.org.