Cloud chamber

Alpha particles from a Radium source in a cloud chamber

Radon220 decay in a cloud chamber

track of subatomic particle moving upward through cloud chamber and bending left (an electron would have turned right)

Cloud chamber photograph of the first positron ever observed

A cloud chamber is a sealed box with a supersaturated vapour of water or alcohol. When a charged particle (e.g. an alpha or beta particle) goes through it, the vapour is ionized. The resulting ions act as nuclei, around which a mist forms from the vapour.

The high energies of alpha and beta particles mean that a trail is left by ions produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and shows more evidence of deflection by collisions, while an electron's is thinner and straight). When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law.

The cloud chamber was invented by Charles Wilson (1869–1959), a Scottish physicist who received the Nobel Prize in Physics in 1927 for this work. The first cloud chamber was made in 1911. The invention was used in the early days of atomic physics. It is often called the 'Wilson chamber'. Since then a number of other types of particle detectors have been invented.

Cloud Chamber Media

Radium 226 source in a cloud chamber.
Fig. 3: A diffusion-type cloud chamber. Alcohol (typically isopropanol) is evaporated by a heater in a duct in the upper part of the chamber. Cooling vapor descends to the black refrigerated plate, where it condenses. Due to the temperature gradient, a layer of supersaturated vapor is formed above the bottom plate. In this region, radiation particles induce condensation and create cloud tracks.
Fig. 4: How condensation trails are formed in a diffusion cloud chamber.
Thorium rod in a cloud chamber.
Radon gas within a cloud chamber.
A home-made cloud chamber.thin: β-particles). See also animated version
Example of watercooled thermoelectric cloud chamber
Radioactivity of a thorite mineral seen in a cloud chamber
Diffusion cloud chamber with 4.6kV ion scrubber