Almost Fool-Proof Cloud Chamber

Overview

This activity is a way to create a cloud chamber in the classroom. A cloud chamber allows students to view "invisible" alpha particles emitted through nuclear decay. Alpha particles have a long history in nuclear physics--they are a helium nucleus and their emission during nuclear decay was one of the first ways we knew that atomic nuclei could change.
The cloud chamber allows us to see an example of "seeing the invisible" which challenges many conceptions that students have about what can exist or not. There are a great many things in science that we understand and use to our advantage that we can't even see. While often seeing is believing, sometimes we are able to believe while not seeing because we can see the after effects of an invisible reaction or thing.

The IceCube experiment observes visible light emitted by invisible muons accelerated by invisible neutrinos. The muon creates light when it moves through a material because it moves so fast that it causes a shock wave of light called Cherenkov Radiation. Similarly, we can't see the actual alpha particle coming off of a radioactive source but we can see the disturbance that it leaves in its wake when it moves through super-saturated alcohol vapor.

How it works

Place a circle of the same radius as the can inside the tin can. Pour in about 10ml of 99% isopropyl alcohol. The alcohol will start to evaporate so put the piece of Fiestaware into the can and cover it with the Plexiglas. The can should then be positioned on the dry ice with a covering over the ice so that no one accidentally touches it. I recommend Styrofoam cut to the size of the outside of the can so that the can may sit in the Styrofoam and be positioned over the dry ice. Dry Ice Can BURN! A cloth towel would do an adequate job as a barrier to the ice but don't leave it exposed because students could touch it. You want maximum surface area touching between the can and the dry ice so that the can's interior cools quickly.

The evaporating alcohol is condensed by the low temperature and becomes a super-saturated vapor layer at the bottom of the can. When the sample emits an alpha particle you can see it move off of the sample because as it flies through the vapor it leaves a trail of condensation where it disturbed the vapor. This will look like faint white lines. They are nearly impossible to see without turning off all the lights and using directional lighting (a flashlight through the Plexi) to improve the contrast. The lines have also been describes as "wisps of smoke" but it's not smoke, it's condensed alcohol vapor. If all goes well a sample could cool down in five minutes and produce 1-2 minutes of great vapor lines. Eventually the emitting sample will get too cold and liquid alcohol will condense on it. Alpha particles are stopped by even this thin layer and so the sample needs to be removed and dried and a new piece of sample can be placed in the can immediately.