How does a cloud chamber detect radioactive particles?

A cloud chamber detects radioactive particles by making their paths visible as trails of condensed vapour.

A cloud chamber, also known as a Wilson cloud chamber, is a simple device used to detect and visualise the paths of ionising radiation, such as alpha and beta particles, and gamma rays. It operates on the principle of supersaturation: the air inside the chamber is so saturated with alcohol vapour that any disturbance, such as the passage of a charged particle, causes the vapour to condense into tiny droplets, forming a visible trail.

The chamber itself is a sealed environment containing a supersaturated vapour of water or alcohol. At the bottom of the chamber, a piece of metal, often lead or uranium, emits radioactive particles. When these particles pass through the vapour, they ionise the gas molecules along their path. This ionisation forms a trail of charged particles, which act as condensation nuclei. The supersaturated vapour condenses onto these nuclei, creating a visible trail that corresponds to the path of the radioactive particle.

The trails differ depending on the type and energy of the radiation. Alpha particles, for example, are heavy and carry a double positive charge, so they plough through the vapour, creating thick, straight trails. Beta particles are lighter and carry a single negative or positive charge, so they create thinner, more meandering trails. Gamma rays, being uncharged, generally leave no trail, but can occasionally cause a secondary electron to be ejected from a gas molecule, which then leaves a trail.

The cloud chamber is a powerful tool for studying radioactive decay and cosmic radiation. It allows us to see the invisible, to track the paths of particles that are otherwise undetectable. By analysing the trails, we can determine not only the presence of radiation, but also its type and energy, giving us valuable insights into the nature of radioactive substances and the fundamental particles of matter.