Cloud Chambers — Making Radiation Visible

⚙️ Cloud Chambers — Making Radiation Visible

A cloud chamber is a remarkable device that makes the normally invisible tracks of radioactive particles visible to the naked eye — you can literally see individual alpha and beta particles flying through the chamber.

How a Cloud Chamber Works

1. The chamber is filled with air saturated with alcohol vapour (usually isopropanol or ethanol)
2. The bottom of the chamber is cooled to about −40°C (using dry ice)
3. This creates a layer of supersaturated vapour — vapour that would normally condense, but has not yet done so
4. When a charged particle (alpha or beta) passes through, it ionises air molecules along its path
5. These ions act as condensation nuclei — the supersaturated vapour condenses on them
6. This creates a white trail of tiny droplets — a visible track showing where the particle went

What Different Tracks Look Like

Alpha and beta particles leave very different tracks that can be used to identify them:

• Alpha particles — thick, straight, short tracks. Alpha particles are heavy and doubly charged (+2), so they cause intense ionisation in a straight line. They stop abruptly after a fixed short distance (their range in air).
• Beta particles — thin, wiggly, longer tracks. Beta particles are much lighter and have only single charge. They are deflected more easily by collisions, causing curved or wiggly paths. They travel further before stopping.
• Gamma rays — gamma radiation is electromagnetic (uncharged), so it does not directly ionise air. Gamma rays leave no visible track in a cloud chamber, though they can occasionally cause secondary ionisation events that show as isolated spots or short tracks.

Cloud chambers were historically crucial in particle physics — the positron (the first antimatter particle) was discovered in a cloud chamber by Carl Anderson in 1932.