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Building a Cloud Chamber (Cosmic Ray Detector)


Warning: Dry ice and isopropanol can be dangerous!


The Rose Center's cloud chamber in operation.

Summary:


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:

Construction:


Make sure the aquarium is clean and dry before you begin.


Image by Andy Foland (http://w4.lns.cornell.edu/~adf4/cloud.html).

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).

2.The cover


   

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.

3.The base


   

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.

Other Ideas

Troubleshooting


Should you find yourself with difficulties, check these solutions:

Further Information


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:
http://helios.gsfc.nasa.gov/cosmic.html