We build a cloud chamber

What is it?

The fog chamber, also known as a Wilson chamber, is a device used to detect ionizing radiation particles. In its simplest form, it is a closed environment containing supercooled, supersaturated water vapor.

When a charged particle of sufficient energy interacts with the vapor, it ionizes it. The resulting ions act as condensation nuclei, around which liquid droplets form into a fog.

As the particles pass through, a trail or trace is produced, due to the numerous ions produced along their path. These traces have distinctive shapes (for example, the trace of an alpha particle is broad and straight, while that of an electron is thinner and shows evidence of being deflected).

What materials will we need?

• A transparent container that can be hermetically sealed, such as a fish tank.
• A metal plate to close the container
• Some strips of felt or carpeting
• Isopropanol (Isopropyl alcohol). It is on sale in some drugstores. If you buy 1 liter, you can do several fog chamber experiments.
• Dry ice (solid carbon dioxide or carbonic snow). It can be obtained in laboratories and it is convenient to use it pulverized, in small pieces or in flat and thin plates, we recommend that you never use big pieces.
• Insulating tape
• Illumination, a halogen lamp that can be oriented and regulate its height would be ideal.

Before starting, it is good to keep a few tips in mind

Dry ice can be dangerous if handled carelessly, since in this experiment we have it in air and it is sublimating, i.e., going from solid to vapor is at a temperature of -79°C.

Isopropanol can also be dangerous, as it is highly flammable and can be toxic and irritating if inhaled or accidentally ingested.

The only thing left to do is to get down to work

• Our fog chamber, consists of a fish tank, in whose bottom, we have glued with contact glue, some strips of felt or carpet that we have previously impregnated with isopropanol (a syringe can be used).
• Then, we will close the fish tank hermetically with a metallic plate, whose inner face is glued with adhesive tape of black color.
• So that it closes hermetically (and this is very important) we can put along the edge of the sheet a strip of rubber weatherstripping (as there is in the windows), but always of continuous section, so that there are no hollows when the edge of the walls of the fish tank rests on it.
• Finally, the chamber is closed by taping the aluminum sheet metal and the tank together. Sealing it very well and being careful with the corners is advisable.

The Experiment

• We will place the metal sheet with the dry ice, the contact must be good, so the bed must be flattened and above all, we must use CO2 in the form of powder, thick plates, or small fragments.
• On the other hand, the temperature of the classroom where we experiment is important. The aim is that the bottom of the chamber is cold, but the upper part must be warm since the alcohol soaking the felt must evaporate well. In addition, the vertical temperature gradient cannot just be any temperature gradient for the traces to form.
• We must get good illumination of the black background of the chamber, which should be in a dimly lit environment. We are going to place a halogen lamp that orients laterally to the bottom of the chamber (not from above).

We begin to observe!

• Before we can see the traces, we have to wait a few minutes. The key is to look at the black base of the chamber from the side opposite the lamp (which we will have to move to get optimal illumination) until we see a rain of very fine drops of condensed isopropanol falling.
• On this mist, in a layer of a few centimeters above the sheet, condensation will appear from time to time in the form of a white thread that will disappear quickly (like the wake of an airplane).
• These are the traces of charged particles passing through the material. When the camera is running for a long time, traces will be seen continuously.