Cloud Chamber

12 July 2010


A really classic physics experiment that helped to understand some of the most fundamental particles in the universe.

The setup is a transparent chamber made from a fish tank, at the bottom there is an aluminium plate that is cooled by dry ice (solid carbon-dioxide). This means that there is a huge temperature difference between the top  of the chamber at room temperature and the bottom at -80°C.

The cloud chamber

A piece of cloth soaked in alcohol (in this case iso-propanol), at room temperature the alcohol evaporates producing vapour. The bottom of the chamber is however very cold, this means that as the vapour falls down it cools until it eventually starts to condense, forming a cloud of alcohol droplets which fall down to the plate at the bottom.

Just above the point where the droplets are forming naturally there is an area of supersaturated vapour. This means that it is cool enough for droplets to grow, but not cool enough for the droplets to form spontaneously. This region is very sensitive to anything which might make forming a droplet slightly easier, so as soon as a droplet is started it will grow and grow.

If a charged particle of radiation moves through the air, it will tend to crash into a series of air molecules leaving a trail of charged ions. This can then form the nuclei of droplets which then grow. This means that in the wake of the charged ions a line of droplets - a cloud trail - is formed allowing you to see their paths.

Particle track
A charged radiation particle moves past air molecules knocking electrons off them creating charged ions.
Condensation trail
it is much easier for a droplet of alcohol to condense around these ions than normal air, so a trail of rapidly growing droplets is formed, showing the track of the original particle.

 The effect is really quite addictive to watch, and it is incredible that something so small can have such a large effect on the real world.

Cloud Chamber

Exactly this principle was used to detect some of the sub-atomic particles. If you apply a magnetic field, a particle will bend and the amount of bending will depend on its mass and charge so you can distinguish different particles this way. 

Cloud chamber positron
Cloud chamber photograph of the first positron ever observed © Carl D. Anderson, APS

Hints if you want to build one yourself

Dry ice is cold, so handle with appropriate gloves and care.

If you can get hold of the dry ice, this is not too difficult to build but here are some tips.

Don't entirely seal the fish tank, or when you cool it down the air inside will shrink and you tank will implode.

It takes a while for the right conditions to be achieved, so be patient.

The most difficult part is not making the cloud tracks but seeing them. Painting the bottom plate helps, as does arranging for illumination in a horizontal plane just where the tracks are forming, and viewing it in a dim room.

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