Thanks to Dr Aidan Robson from Glasgow University I was able to borrow a set of cloud chamber kits which I have used with my Higher physics classes this week. This facility is now available to any physics teachers in the central belt, via an on line booking facility, with kits available from both Glasgow and Edinburgh University Physics departments. (Teachers interested in borrowing kits from the scheme can register by contacting Aidan via the email given in the link above).

Like many commercially available cloud chamber kits, these use dry ice in order to cool the alcohol vapour in which the trails form due to ionisation caused by energetic particles. Both the dry ice and the alcohol are provided with the kits. However, unlike other 'home made' cloud chambers, which often use small fish tanks, these kits are very compact, allowing pupils to easily build them and observe interactions within a single 50 minute period. Their size also allow pupils to get in very close, making the whole experience more personal for them.

The kits use very simple items to construct the cloud chambers - small aluminium pie cases, plastic pint tumblers, a few strips of black tape, expanded polystyrene and a small torch. The instructions are clear and easy to follow, so that with a little preparation (pre-cutting lengths of tape speeds things up a great deal) a class of 20 pupils could build 10 cloud chambers in as many minutes.

The booking website also links to resources which include a short presentation explaining a little about the invention of cloud chamber by CTR Wilson, who won a Nobel prize for its invention, and an overview of how it operates. It includes a number of annotated images of cloud chamber interactions, which lead on to information about particle accelerators and their detectors, including the Stanford linear accelerator and Large Hadron Collider.

Without the need for a source of radiation, the working cloud chambers can be used to detect particles created by the interaction of cosmic rays with the upper atmosphere. These particles include muons, protons, alpha particles, pions, electrons, and neutrons.

The majority of particles detected at ground level are muons, which are an excellent example of the relativistic effect of time dilation. Muons travel at velocities very close to the speed of light (0.999 c) but due to their very short lives (~2.2 µs), and according to classical mechanics should only travel around 500 m through the atmosphere. However, because they travel at such high velocities, special relativity allows this time to be increased for stationary observers (us on the surface of the Earth, some 10 km below the upper atmosphere) so that in our frame of reference the muon has a significantly longer life, allowing it to reach us.

Cloud chamber detecting cosmic ray muons

Although my school no longer has any sealed radioactive sources, we do have a small collection of radioactive minerals. Ensuring it was handled appropriately, a small piece of Cornish pitchblende was placed inside one of the cloud chambers.


Cloud chamber with pitchblende source

This video clearly shows the tracks created by particles which stream from the small piece of rock. These are alpha particles, which though highly ionising are very easily absorbed in air (and in the alcohol vapour in the cloud chamber), travelling only a few centimetres. Their short range and careful handing ensures that the risks from exposure to the radiation from this source is minimised.

[It should be noted that mineral sources should not normally be removed form their packaging. This ensures that any small fragments which fall off and may be active, will remain with the main sample. A readily available alternative is a thoriated TIG welding rod - these can be bought from many hardware and DIY stores relatively cheaply and should produce visible trails when placed inside a cloud chamber.]

If in any doubt concerning the safe handling of radioactive materials, refer to the SSERC website in Scotland or CLEAPSS in the rest of the UK.