The Cloud Chamber – detecting radioactive particles at home

Have you ever notice the vapour trail left behind after airplanes jet through the air?

These trails, called contrails, are caused by aircraft travelling through the cold air at high altitudes. The trails are caused either by condensation of water vapor from the engine exhaust as it is ejected into the cold air, or by condensation of water vapor already in the air around pressure changes due to vortices formed as air passes over the aircraft’s wing. .

Well in 1932 Carl David Anderson announced his discovery of the positron, a positively charged electron. A discovery that was made using a cloud chamber, a relatively simple device, that can produce contrails when particles pass through the vapour sealed within the cloud chamber container.

Here’s Brian Cox building a rudimentary version of a cloud chamber and using it to demonstrate cosmic rays.


And here’s a cloud chamber in operation


Anderson’s Discovery

Particles that enter a cloud chamber can be distinguished by their relative size, due to the thickness of their trails, and also by how much they interact with the vapour, by the length of their trails. For example electrons typically have a long thin trail compared to radioactive alpha particles (helium nuclei), which are heavier and are more likely to interact with matter.

While analysing particles that Anderson believed to be electrons (the long, thin trails) he applied an electric field and discovered that the particles were deflected as a positively charged particle would be if it had the same mass as an electron. He concluded that this particle was a positron, or a positive electron.

Anderson’s discovery was important because it was the first verification of Paul Dirac’s prediction of anti-particles and anti-matter.


[Note: This is a simplified version of what Anderson achieved and how he achieved it. Feel free to research and learn more about him and his research and discovery]

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