Bringing particle physics to life: build your own cloud chamber

The objective of elementary particle physics is to find the basic building blocks of which everything is made and to investigate the behaviour of these building blocks. Although it can be seen as a cornerstone of science, particle physics is often neglected or poorly understood in schools, partly because it is perceived as unrelated to the things with which we interact on a daily basis. However, particle physicists detect and measure electrons, photons or muons every day with the same confidence with which all of us ‘detect’ cows, tables or aeroplanes. Furthermore, particle detectors (e.g. PET scanners) are routinely used, for example, by medical physicists to detect tumours and monitor the function of internal organs.

Here we demonstrate how to bring particle physics to life in the classroom, using possibly the simplest type of particle detector: a continuously sensitive diffusion cloud chamber. This homemade version consists simply of an airtight fish tank full of air and alcohol vapour, cooled to a very low temperature, which can be used to detect charged particles, particularly cosmic ray muons, if they have enough energy.

Elementary particles

Elementary particles are the simplest elements from which everything is made. They are not just the building blocks of matter and radiation, but also give rise to the interactions between them (for more details of elementary particles, see Landua & Rau, 2008). These particles carry energy and momentum, and can thus be seen by detectors. Strictly speaking, you cannot directly see any particles – instead, their passage through detectors is inferred from the effects they cause, such as ionisation (for charged particles). That is precisely what we do when we observe the condensation trail left in the sky by an aeroplane that we cannot see – and what we can do with our homemade cloud chamber.

The continuously sensitive diffusion cloud chamber

This cloud chamber is basically an airtight container filled with a mixed atmosphere of air and alcohol vapour. Liquid alcohol evaporates from a reservoir and diffuses through the air from the top to the bottom of the chamber. Cooling the base with dry ice (solid carbon dioxide, which is at a constant temperature of around –79 ºC while it sublimates) results in a strong vertical temperature gradient, so that a zone with supersaturated alcohol vapour forms close to the bottom. This sensitive layer is unstable, with more very cold alcohol vapour than it can hold. The process of condensation of vapour into liquid can be triggered by the passage of a charged particle with enough energy to ionise atoms in its path. These ions are the condensation nuclei around which liquid droplets form to make a trail.

Assembly and operation

Materials

• Straight-sided, clear plastic or glass container (e.g. a fish tank) with a base about 30 cm x 20 cm, and a height around 20 cm (other sizes can be used, but the effects may vary)
• Aluminium sheet (about 1 mm thick, same thickness as the base of the fish tank)
• Shallow tray somewhat larger than the base area of the fish tank
• Two lamps, one of them strong
• Strip of felt (about 3 cm wide and long enough to wrap around the inside of the fish tank, e.g. somewhat more than 1 m long)
• Glue (not alcohol-soluble)
• Black insulating tape or duct tape
• Isopropyl alcohol (isopropanol)
• Dry ice

Method

1. Glue a strip of felt (the alcohol reservoir) around the insides at the bottom of the fish tank (the body of the cloud chamber). Some felt can be glued to the bottom of the tank, too.
2. Cut the aluminium sheet to fit (as closely as possible) the top the fish tank, and cover one side of the sheet with insulating tape, forming a black surface.

3. Soak the felt with isopropyl alcohol (but not so much that it drips down the sides of the chamber).
   Safety note: Do this in a well-ventilated room and remember that alcohol is flammable.
4. Turn the fish tank upside-down over the aluminium sheet. Make sure the black side of the sheet faces upwards (to make the particle tracks more visible).
5. Use insulating or duct tape to fasten the aluminium sheet to the rim of the fish tank, sealing the chamber so that it is airtight. This is the most critical step and must be carefully done, as the joint will become moist and very cold during operation.
6. Make a flat layer of dry ice in the tray and place the chamber on top of it, making sure that its base is horizontal. To ensure good thermal contact between the metal plate and the dry ice, avoid large chunks of dry ice: flat sheets or dust are best, but small grains will do.
   Safety note: Dry ice is around –79 ºC and should only be handled using thick gloves.
7. Keep the top of the chamber warm, for example by shining a lamp onto it. Avoid using the chamber in a cold environment, because this could prevent the correct temperature gradient from forming, meaning no tracks can be seen.
8. Leave the chamber undisturbed for about 10 min, until the temperature gradient is established. Shine a bright light through the chamber at a low angle, and look at the bottom of the chamber. At first you should see only an alcohol mist falling, but gradually, charged particle tracks should appear as thread-like condensation in the mist. Note: the tracks are more visible in a darkened room.

Although any charged particle with enough energy, for example from ambient radioactivity, can leave its trail in the chamber, the majority of the tracks will be made by secondary cosmic rays: particles created when other particles (mostly protons) coming from outer space hit the upper atmosphere. Secondary cosmic rays travel at close to the speed of light and are absorbed by the atmosphere or decay in flight, giving rise to new particles including muons, which can reach the surface of Earth and are easily detected. Muons are charged elementary particles very similar to electrons except for their mass (which is two hundred times larger).

What you can do with the chamber?

In order to make the chamber really useful, we cannot limit ourselves to showing it and describing how it works. To support the explanation, we have prepared a short, simply written comic strip^w1 (see below), showing how the chamber works and illustrating the origin and composition of cosmic rays through the story of a cosmic proton and its descendants.

We use this chamber at school with our 12- to 16-year-old students as part of an effort to help them see particles as real physical objects. Watching the visible trails left by invisible particles and comparing them to trails left by jet engines (in which much of the same physics is involved) is the first step in a process that we continue by introducing real data and pictures from high-energy physics into otherwise standard exercises and questions^{w2, w3} (Cid, 2005; Cid & Ramón, 2009) and that we conclude with another, more complicated, detector for school use: a cosmic-ray scintillation detector which allows students to record and study data by themselves (Barradas-Solas, 2007).

Acknowledgements

The authors would like to thank Dr Eleanor Hayes, Editor-in-Chief of Science in School, for her assistance in giving the final form to this article.