to enlarge imagesThe basic gist of a cloud chamber is this:
What you’ll need to build your chamber:
- An adult who can safely use isopropyl alcohol and dry ice
- A crystal clear plastic or glass container with a wide, tight-fitting lid to be your cloud chamber. You’ll want something at least as big as a peanut butter jar or a deli container. If you don’t have a lid, a cookie sheet larger than the mouth of the container will work.
- A durable, absorbent material that you can squish into the bottom of the container. Try felt, wool, or a sponge
- Bubble gum or modeling clay (optional)
- Black paper cut to fit inside the lid of your container
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Dry ice and insulating gloves for safe handling
(find a dry ice distributor here) - A plastic or foam container with a rim that can hold both the dry ice and your cloud chamber. It should be about four times the size of your cloud chamber
- A bottle of 90+% isopropyl alcohol (available at most pharmacies)
- A room, closet, or large box that you can make completely dark for conducting your cloud chamber observations in
- A very bright flashlight (LED flashlight works best)
- A small bowl of warm water that can sit on top of your clear plastic or glass container
- A clock, timer, or stopwatch
- Safety glasses and lab apron
- Optional: a digital camera or cell phone camera
- Isopropyl alcohol is toxic to ingest and also highly flammable. Make sure there are no hot surfaces or open flames in your work area, clean spills promptly, and avoid contact with skin or clothing.
- Direct skin contact with dry ice can cause burns. Avoid direct contact with dry ice by using dry, insulating gloves when you handle it. Since dry ice will produce large amounts of gas on contact with liquids, never store in a glass container and always wear safety goggles and a lab apron.
- Because isopropyl alcohol is flammable, it is important to safely allow any of the remaining alcohol in the cloud chamber to evaporate in a well-ventilated area away from open flame at the end of your experiment.
- Stuff the bottom of your clear container with your absorbent material. If you need help getting it to stick, try using a small piece of modeling clay or chewed gum to stick it to the bottom. Note: isopropyl alcohol dissolves most adhesives, so you may have to troubleshoot other ways of sticking the absorbent material to the top.
- Set the piece of black paper on the inside of the tightly fitting lid and trim it so that the container can still close with the paper inside.
- Pour just enough isopropyl alcohol into your container so that the absorbent material becomes saturated but is not standing in any liquid. Carefully pour any excess isopropyl alcohol down the sink.
- Turn the container upside-down onto the lid and black paper and secure the lid, making sure that your alcohol-soaked material stays stuck to the bottom of your container. If you don’t have a tight-fitting lid, you can turn your container upside-down onto a cookie sheet, covered with black paper, and tape it around the edges to the cookie sheet. The vapor of the isopropyl alcohol will begin to fill your jar immediately.
- In a room or large box that you can make completely dark, set up your rimmed container on a solid work surface. Using insulated gloves to protect your skin, pour some dry ice into the rimmed container. Place your cloud chamber lid-side down onto the dry ice and keep it there until the lid appears frosty, about 10 minutes. (If you used a cookie sheet, rest the bottom of the sheet on the dry ice.)
- Fill a small bowl or dish with warm water, and set it on top of your cloud chamber. This warms the isopropyl alcohol so the chamber fills with vapor more quickly. You now have a totally cool cloud chamber.
- Turn off all the lights, and shine your flashlight across the bottom of your container through the side. Look inside, what do you see?
When ionizing radiation enters a cloud chamber, it interacts with atoms in the atmosphere -- like hydrogen, nitrogen, and oxygen--by violently knocking off their electrons. Those atoms turn into positively charged ions, which are very attractive to the gaseous alcohol molecules in the cloud chamber! Chilling the cloud chamber on dry ice causes those gaseous alcohol molecules to crowd so close together that no matter where in the chamber ionizing radiation strikes, there will be many alcohol molecules ready to stick to the trail of positive ions it produces. The result is visible trails of condensed alcohol mist wherever ionizing radiation comes into contact with atoms in the air.
Use your cloud chamber to measure background ionizing radiation.
Using a clock or timer, try to count how many streaks of ionizing radiation you see in your chamber in a minute. Repeat this count two more times, and calculate an average “ionizing interactions per minute” for your cloud chamber. If it helps, you can print out this cloud chamber observation sheet to record your observations and do calculations.
The trails you are seeing in your chamber are a tiny window into the radiation that is buzzing in, through, and around you all of the time. Pretty cool, right?! Well, not if you’re trying to study dark matter.
Physicists have evidence that in addition to the known subatomic particles that make up most of the things we can see and touch, there is an entirely separate class of very small, potentially weakly interacting particles that make up the majority of our universe called dark matter. Though it comprises over 90 percent of our galaxy, dark matter is poorly understood.
Dark matter is difficult to study because it’s made of unimaginably small particles that we can't see, and it interacts with other atoms very rarely. Detecting dark matter interactions that are so minute and rare is made especially difficult because they are grossly overshadowed by the background radiation that is constantly pouring down on our planet from cosmic rays. Our planet’s background radiation makes the search for dark matter like trying to hear a shy, whispering child in a party of shouting adults. Science Friday's video producer, Luke Groskin, visited with scientists looking for dark matter, who describe this conundrum in the video “4850 below.”
Science Friday Video: "4850 Below"
Engineer your own cosmic ray shielding, and then test it.
Now that you have a cloud chamber that works as a particle detector, and a baseline rate of ionizing radiation (“ionizing radiation interactions per minute”), you can make your own radiation shielding and test it by monitoring whether ionizing interactions are less frequent. What will you build around your cloud chamber to shield it from background radiation from sources like the sun?
- Allow you to observe and count the number of ionizing interactions in your cloud chamber, even when the shielding is installed
- Avoid contact with the chamber itself, the dry ice, and the remainder of your experimental setup
- Lower the number of ionizing interactions per minute in your cloud chamber from your initial measurement
To test out your shielding design, first refresh the alcohol in your chamber, and make sure you have sufficient dry ice to keep it cold. As your cloud chamber cools, assemble your radiation shield around it. Once everything is in place, count how many streaks of ionizing radiation you see in your chamber in one minute, and record the count. Count the number of ionizing radiation interactions in a minute two more times, and calculate an average “ionizing interactions per minute” for your cloud chamber with it’s radiation shielding. Is this average different from the rates you observed before you installed the shielding? If you had unlimited funds and space, how would you improve your design to make your shielding more effective?
Related Links:
- Petri-dish cloud chamber from Thomas Jefferson National Accelerator Facility
- Particle detector from Symmetry Magazine
- Compressed air cloud chamber featured at Physicsworld.com
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