Close-up of particle tracks in a cloud chamber. The central rod contains a sample of radioactive polonium-210, an alpha particle emitter. The surrounding chamber contains a supersaturated vapour. The passage of emitted particles of ionising radiation ionises the vapour, and the resulting ions act as nuclei for condensation, making the tracks visible as streaks of cloud. The cloud chamber was invented in 1911, and was an important tool in particle physics until it was superseded by the bubble chamber in the 1950s.