Building a Cloud Chamber (Cosmic Ray Detector)
Warning: Dry ice and isopropanol can be dangerous!
A cloud chamber is a simple device that you can construct in a classroom environment, and will powerfully demonstrate the existence of cosmic rays as they glide through the detector.
This activity is largely based on the work of Andy Foland, whose Cloud Chamber page can be accessed at http://w4.lns.cornell.edu/~adf4/cloud.html. We have added a few specifics based on our experiences building and "operating" our device.
Parts & Supplies Required:
- A small 2-gallon rectangular aquarium (6" W x 12" L x 6" H) with a transparent bottom.
- A slide projector or other high intensity light
- A metal plate or cover for aquarium, from 1/32" to 1/16" in thickness
- A piece of thin cardboard (from a notebook or cereal box) the same size as the metal plate
- Black electrical tape
- Three pieces of 12" x 12" felt, any color (available at art supply stores)
- A box that snugly fits around the base of the aquarium, about 3" in height
- A piece of styrofoam or padding just less than 3" in height
- Duct tape
- Clear Silicone Sealant
- 12 oz. of 100% pure isopropyl alcohol (check with chemistry suppliers)
- 1 lb of dry ice, cut into thin (1/4" to 1/2" thick) slices if possible
Make sure the aquarium is clean and dry before you begin.
1.The felt lining
First, we will discuss the construction and installation of the felt lining inside the aquarium.
Take two pieces of the felt and fold each of them three times lengthwise to create two 2" wide strips of felt padding (each 12" long). The width is not crucial but should be around 2 inches. Staple the folded strips (not to each other!) so they do not come undone. Using the silicone sealant liberally, glue the padded strips to the inside of the aquarium, along the longer bottom sides. Do not glue the felt to the bottom, because you will be turning the aquarium upside down and viewing the experiment through the bottom. Be careful to use only silicone sealant or a similar, non-alcohol soluble glue.
Take the last piece of felt and cut it in two. Fold it three times to create two 2" wide strips that are 6" in length. Staple and glue these to the remaining sides of the aquarium to create a 360° "soak zone". The felt will be liberally doused with alcohol just before operating the cloud chamber. Give the glued felt several hours to dry before use (check manufacturer's instructions).
The next step is to prepare a top for the aquarium so that it can serve as the bottom when you turn it upside-down. You will need the metal plate, the thin cardboard, and the black electrical tape.
Cover one side of the cardboard with black electrical tape; this will make the particle tracks easier to see. Trim the excess tape to make clean edges.
Place the cardboard, tape side up, on the sheet of metal, and then cover the container with the metal and cardboard so that the tape is facing the inside. The metal plate should be topping the aquarium at this point.
The base provides a platform for the upside-down aquarium and holds the dry ice to the metal plate.
Find a cardboard box just slightly bigger in girth than your aquarium. Trim it so its about 3" in height, and, if you wish, paint it or cover it with tape (we wrapped ours in black tape, but this was just for looks!).
Place the foam piece inside the box. If possible it should have a "springiness" that will help press the dry ice up against the metal plate later on.
4.Operating your cloud chamber
To complete the cloud chamber, we'll soak it with alcohol, flip it upside down, slip the dry ice underneath, and seal it.
Soak the felt with about 3-4 oz. of alcohol, and try to pour a thin layer on to the electrical tape layer of the cardboard as well. Working quickly, reinstall the cardboard and metal cover, and then seal them to the aquarium with the duct tape. Try to make it airtight, at least enough to keep the alcohol from running out when you flip it over.
Put a piece (or several pieces) of dry ice in the base, on top of the foam, enough to span the length of the aquarium. Handle dry ice with gloves. Now place the alcohol soaked aquarium upside-down so that the metal plate is in direct contact with the exposed dry ice. Seal the joint between the base and the aquarium glass with duct tape to help keep the cold in.
5.Watching your cloud chamber
This is the best part!
Turn on the slide projector lamp. Look towards the bottom of the chamber, near the electrical tape (the bottom inch or so) In a few minutes you will see a rain-like mist of alcohol. Why? The chamber is saturated with alcohol vapor. The dry ice keeps the bottom very cold, while the top is at room temperature. The high temperature at the top means that the alcohol in the felt produces much vapor, which falls downward in a gentle rain.
The low temperature at the bottom means that once the vapor has fallen, it is supercooled. That is, it is in vapor form, but at a temperature at which vapor normally can't exist (its as if you had made steam at 95° C). Since the vapor is at a temperature where it normally can't exist, it will condense into liquid form.
After about 15 minutes, you should start to notice the tracks of particles passing through. The tracks look a little like spider's threads going along the chamber floor. It may help to turn off any room lights. As the electrically charged cosmic rays come along, they ionize the vapor (i.e., tear away the electrons in some of the gas atoms along its path). This leaves these atoms positively charged, which attracts nearby atoms. This is enough to start the condensation process and so you see the little droplets forming along the path the particle took through the chamber.
- Place a very strong magnet underneath the chamber, or along the sides. You may see particles bend when they are near the magnet.
- Use a Polaroid or digital camera to record the events, although this takes practice.
- Place several plates of metal upright, one behind the other, in the chamber. See how many plates tracks can go through.
Should you find yourself with difficulties, check these solutions:
- "I don't see anything!" Solution: Be sure the light is well placed. Make sure the dry ice is neatly packed and in good contact with the metal plate. Try adding some alcohol.
- "I see only the mist, and no tracks." Solution: Wait. It takes about 15 minutes for the chamber to get to the right temperature.
- "I waited 15 minutes, and still nothing!" Solution: Be sure the light is well placed and shining into the bottom inch of the chamber. Check that the chamber is airtight.
- "It's airtight, and there's a good light." Solution: if you see only a very thick mist, try opening the chamber, letting some escape, then starting over. If nothing works, try a new container (aquarium) which is a little shorter or taller.
- "I see big clouds at the edges of the chamber." Solution: This probably means you have a major air leak. Be sure the chamber is tightly sealed with the duct tape.
More info can be found on Andy Foland's Cloud Chamber page, accessible on the Internet at http://w4.lns.cornell.edu/~adf4/cloud.html.
NASA has cosmic ray web pages as well. Check this link: