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The Cloud Chamber

A Cloud Chamber is a device used to detect ionizing particles and to determine their trajectories. It does not show the particles themselves, but where they have been: particles form a condensation trail in the chamber which is visible as a fine mist, and this shows a particle's path through the chamber.

[unnumbered] page of Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, Vol. 87, No. 595, Sep. 19, 1912Wilson's original cloud chamber tracks

C.T.R. Wilson, Proc. Roy. Soc. A,Vol. 87,595, pp.277

Original positron track

The first image of a positron, taken by C.D. Anderson in 1933 using a cloud chamber

What Does it Detect?

Cloud chambers are used to detect ionising particles. These can include any electrically charged particle that passes through the chamber; and the amount of ionisation can be deduced from the tracks in the chamber and used to determine a particle's properties and identity. Cloud chambers were particularly used to study radioactive elements, as alpha, beta and gamma radiation are all ionising. Alpha particles are more ionising than beta, while electrically neutral gamma rays do not ionise and therefore do not leave visible tracks in the cloud chamber. (They may produce charged particles that can subsequently be observed, but this is much less common.) Cosmic rays can also be detected by cloud chambers, as the secondary rays formed in particle showers are ionising particles such as muons and electrons. Cosmic rays became an intense area of study for spark chambers, but most of the initial major discoveries made from them, such as the discovery of the positron and kaon, were made in cloud chambers.

How Does it Work?

The first cloud chamber used air saturated with water in a glass chamber. The bottom of this chamber could be pulled down to increase the volume of the chamber, causing the gas within it to expand as well, and as such do work. However this change is adiabatic- involves no heat transfer. The first law of thermodynamics states energy is conserved and cannot be created or destroyed, so we know that the energy for this expansion has to have come from somewhere; in this case, the internal energy of the gas. The internal energy is related to the temperature of the molecules in the gas, so if the chamber expands the temperature drops. This brings the water vapour close to condensing, making it become supersaturated (see Appendix for more details). If an ionising particle, such as alpha or beta radiation, passes through this vapour then the ions formed acts as points of condensation for the surrounding vapour, leading to the formation of visible clouds.

More modern cloud chambers (known as diffusion chambers) work differently to the original apparatus, as they use alcohol instead of water and do not change the volume of the chamber but instead use dry ice to cool the base of the chamber. The alcohol is soaked in a tissue at the top of the chamber, which is much warmer than the chamber base; the alcohol vapours therefore fall to the base of the tank, where they reach a point of supersaturation. Ionising particles that pass through the vapour shows up in exactly the same way as with the original water detailed above.

The cloud tracks can be photographed for further observation to determine the nature of the particle that caused the trail; for example, frequent changes of direction suggest frequent interactions with gas molecules, which is normally shown by alpha particles (the most ionising form of radiation). An electric or magnetic field can be applied across the chamber, which will cause charged particles to curve. Positive and negative particles curve in different directions, making them distinguishable from each other.

Development of the Cloud Chamber

Year Development
1894-5 Charles T.R. Wilson invents the cloud chamber to make small clouds in the laboratory, due to his interest in their formation and the electrical and optical phenomena associated with them
1910 Wilson realises that the cloud chamber could be used in the task of identifying and describing newly discovered sub-atomic particles emitted by radioactive materials
1924 Patrick Blackett uses the cloud chamber to observe the transmutation of nitrogen into fluorine, which then disintegrated into oxygen
1932 Blackett and Giuseppe Occhialini developed a system of Geiger counters which only took photgraphs when a cosmic ray entered the chamber. Blackett had also devised another way to speed up research work, by using a spring mounted diaphragm to quickly readjust the chamber back to the conditions required to observe a cloud trace
1933 Carl D. Anderson discovers an anti-electron, the positron, in a cloud chamber
1936 Alexander Langsdorf modified the chamber to produce its modern variant, the diffusion chamber. Using dry ice to form a temperature gradient meant there was always a supersaturated region, so particles could be detected constantly
1947 G.D. Rochester and Clifford Charles Butler publish the first cloud chamber images showing evidence for kaon

What were cloud chambers replaced by?

Cloud chambers were the main type of detector used in particle physics until the 1950's, when they were replaced by bubble chambers and spark chambers. These are more sensitive and practical devices which allow more detailed and precise measurements of particle properties to be made.

Further Reading


Some pictures of the Birmingham fish tank cloud chamber are shown here.


To find a slightly more technical mathematical explanation of why supersaturation occurs in a cloud chamber, click here.
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