Physics

Diffusion cloud chamber

Particle physics · Detectors

Invisible particles are zipping through you right now — how can you actually see their trails?

Radiation and particles from space pass through us all the time, too small to see. A cloud chamber fills a tank with a cold mist that is just about to turn into droplets. When a particle zooms through, droplets form along its path, leaving a short-lived trail like the white line behind a jet plane.

A diffusion cloud chamber makes the paths of invisible charged particles visible: alcohol vapour diffusing from a warm top to a plate at −78 °C becomes supersaturated, and the ions a particle leaves behind seed lines of droplets. Track shape reveals the particle — short and thick for alphas, thin and wandering for betas, long and straight for cosmic muons — and a magnetic field adds momentum and charge sign.

Key facts

Cold plate temperature

≈ −78 °C (dry ice sublimes at −78.5 °C)

Working fluid

isopropyl alcohol (≥ 99%) evaporating from felt at room temperature

Sensitive layer

≈ 1–2 cm thick, just above the plate

Alpha track length in air

≈ 3.5–7 cm (4–8 MeV alphas)

Cosmic muon flux at sea level

≈ 1 per cm² per minute (horizontal surface)

Track lifetime

≈ 1–2 s before the droplets fall and diffuse

Parts you can explore (13)

• Glass tank
• Lid
• Alcohol-soaked felt
• Alcohol vapour
• Supersaturated layer
• Cold plate
• Dry ice
• Insulated box
• Light source
• Alpha tracks
• Beta tracks
• Muon tracks
• Magnet

In the 3D model

Rotate and take apart the model, follow a guided tour of what each part does and why it’s needed, change it with a live slider, and test yourself with a short quiz. Free, no sign-up.

Open Diffusion cloud chamber in 3D →

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