A small demonstration cloud chamber

Seven kilograms of dry ice and 250 milliliters of ethanol maintain a supersaturated alcohol vapour in the box for up to eight hours.

As an alpha particle passes through the vapour it ionises the air. Droplets of alcohol form on the ions and the path is visible as a short white streak.

Typical short thick alpha particle tracks in the cloud chamber. The alpha particles are emitted by the needle across the bottom of the photograph. The needle contains 2% Thorium. Alpha particles have a short range in air at one atmosphere, because there are many ion forming collisions per mm. 

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