Cloud chambers!

After an unexpected hiatus of three months I feel my next topic needs to pretty special, and the one I have picked I have been looking forward to doing for a little while now – Cloud Chambers. Now these are not devices that create the clouds in the sky unfortunately (how cool would that be incidentally?), but they are in fact particle detectors, specifically for ionising radiation. I am fortunate enough to work in a science centre containing a cloud chamber.

A modern cloud chamber
A modern cloud chamber

Perfected in 1911 by a Scottish physicist called Charles Thomson Rees Wilson, cloud chambers are a sealed case of air which has been super saturated with either water or alcohol vapour. The air is then cooled from underneath using dry ice to the point where the water or alcohol vapour is just about to start to condense back into liquid form. When a radioactive source is introduced, it releases alpha or beta particles as it decays. When one of these ionised particles passes through the cloud chamber, the water or alcohol vapour condenses around the particle producing a visible trail in the form of a vapour cloud. An electric field can also be used to direct particles to the most sensitive areas of the chamber. Alpha particles would produce short, thick tracks, whereas Beta particles would produce long, thin tracks. Cloud chambers were also used in the discovery of several other particles, including the positron in 1932, the Muon in 1936, and the kaon in 1947.  (See my blog of particles for an explanation here:

Wilson was awarded the Nobel Prize in Physics in 1927.

Charles Thomson Rees Wilson in 1927
Charles Thomson Rees Wilson in 1927

So above is a very quick description of how cloud chambers work, and you may not have understood a single word. However, the best thing about the chambers is the patterns of radioactive decay that they produce, so I have collected several videos to demonstrate this incredible detector below. Enjoy!


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