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Published on Jan 2, 2014
A sealed glass container contains liquid alcohol at the top. Emanating alcohol vapors fill the whole volume of the container until they reach the bottom of the chamber maintained to a very cold temperature (-40°C). Most of the vapour condenses on the glass surface creating a mist, but a small fraction of it stays in vapour form above the cold condenser. This creates a layer of unstable sursaturated vapour which can condense at any moment. When a charged particle crosses this vapor, it can knock electrons off the molecules forming ions. It causes the unstable alcohol vapor to condense around ions left behind by the travelling ionizing particle : the path of the particle in the matter is then revealed by a track composed of thousands droplets of alcohol.
Any charged particle is visible in a cloud chamber. The most common ones are alphas, electrons, positons, protons, nuclear charged fragment, muons (...). Theses particles come from natural cosmic and telluric background radiations or from close radioactive sources. They will all leave tracks of different shapes in the chamber, based on their charge, mass and speed. Electrons are the lightest particles and will be easily deflected by magnetic fields. Alphas and protons are much heavier and slower and will thus ionize more, causing denser track of droplets. Interactions of neutral particles like gamma rays or neutrons can be seen thanks to the charged particles they create in matter.
See you at cloudylabs.fr for more experiments !
This video shows the Cloudylabs's cloud chamber running for approx. 50 min with an Uranium mineral. After 40 min, there is not enough alcohol to make newer trails. With time, the alcohol condense on the mineral. The small thickness of liquid alcohol on the mineral is enough to absorb a part of the energy of the alpha particles (their ranges in air for 5 MeV is 3-4 cm, but in water, it's 15 micrometer), so with time, the trails are shorter than in beginning. It's preferable to make such experience during 10 minutes to have longer alpha track.