Cloud Chamber

Cavendish HEP cloud chamber

The group has recently built a desktop diffusion cloud chamber that allows the paths of charged particles to be seen by the unaided eye.

It is built from inexpensive and readily available materials and is suitable for classroom demonstrations. In operation it consumes small quantities of dry ice (a couple of kg provides several hours of operation if care is taken with insulation) and small quantities of isopropanol (a few cc are enough for an hour of operation).

The chamber is made from a fish tank that can be purchased cheaply from a toy shop or pet shop. The open side of the tank is sealed with a closely-fitting thin metal plate. Before closing the tank with the plate, attach a paper tissue to the inside face opposite the plate and moisten the tissue with isopropanol. Now place the tank so that the metal plate is at the bottom and in contact with a layer of dry ice in an insulating container. Illuminate the region inside the tank, just above the metal plate, from the outside using a bright light source.

The assembled chamber can be seen in the photo. Note the use of black paper to reduce annoying reflections.

Principle of operation

Cloud chamber sketch

The principle of operation is illustrated in the sketch (click on it for a higher resolution version). Vapour falls from the isopropanol-soaked tissue towards the very cold metal plate. After a short while you should see a mist of isopropanol droplets just above the plate where the vapour is cold enough to condense. Just above this region the vapour is super-saturated - it would like to condense but there are no nucleation centres to seed the droplets. A charged particle travelling through the super-saturated layer can cause nucleation along its path by polarising the vapour molecules and this allows droplets to form along the paths of the particles and so make them visible.

You may find that your apparatus works straight away without problem. More likely, however, you will find that it may need a little optimisation. At the bottom of this page we provide some hints that you might find helpful.

For the impatient, here are a few movies (QuickTime). Movies 1 and 2 are without any radioactive source added (just natural background radiation). The lighting is not ideal in these and the tracks are a little indistinct but you may see a few traces. For the second two movies we inserted a radioactive source (actually a thoriated welding rod) vertically into the cloud chamber. You can see the tip protruding from the bottom of a metal sheath. Thorium is an α-source (see below) and the α-particles produce quite bright traces. Two or three should be visible in each of movies 3 and 4.

Movies 5 and 6 are of particle trails in a commercially available cloud chamber by the company PHYWE. The physics department has one of these in the Cavendish museum area.

Movie 1 (.mp4,1MB) - No added source, viewed towards light.
Movie 2 (.mp4,1MB) - No added source, viewed towards light.
Movie 3 (.mp4,3MB) - Thorium source, viewed towards light.
Movie 4 (.mp4,3MB) - Thorium source, angled view.
Movie 5 (.mp4, 100MB) - The PHYWE cloud chamber.
Movie 6 (.mp4, 160MB) - The PHYWE cloud chamber.

A few words on what you can see

So, after a couple of minutes to allow the conditions inside the chamber to stabilise, you should begin to see filament-like trails of droplets of various lengths. Some will be straight, some irregular, some clear and bright and others more tenuous.

Most of the droplet trails are caused by particles produced in the radioactive decay of naturally occuring elements. These are classified as α-decays, β-decays and γ-decays. α-decays produce α-particles (they are the same as the nuclei of Helium atoms), β-decays emit electrons (and neutrinos, but these are not detected by the cloud chamber) and γ-decays emit high energy photons known as γ-rays (they are also not detected).

α-particles lose their energy very quickly and stop after travelling a few centimetres in air and are easily stopped by a piece of paper. Therefore, only α-particles produced inside the cloud chamber will be visible and they will typically produce a short, but usually bright, trail of droplets.

Electrons lose their energy more slowly but the ones produced in natural radiation are unlikely to be able to penetrate through the walls of the tank. Therefore these too can only be seen if they are produced inside the chamber but they may travel much further and produce trails that span the entire width or length of the chamber.

γ-rays are neutral particles and do not polarise the vapour so they are not observed except in the rare case that they interact with an atomic nucleus. Even then, the products of the interaction may not travel far enough to be seen.

It may also be possible to see droplet trails caused by particles known as muons that are generated when high energy cosmic rays strike the top of the Earth's atmosphere. They tend to be mostly travelling vertically downwards so only leave short trails because the super-saturated vapour layer is not very deep.

Hints on the cloud chamber design

Cloud Chamber Calendar and Bookings

When not "on tour" our PHYWE cloud chamber is on permanent display in the Cavendish Museum area.

Future events highlighted in green.