Cloud chamber

The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation.

In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle (for example, an alpha or beta particle) interacts with the mixture, the fluid is ionized. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and straight, while an electron's is thinner and shows more evidence of deflection by collisions). 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 with two particles of opposite charge.

Cloud chambers played a prominent role in the experimental particle physics from the 1920s to the 1950s, until the advent of the bubble chamber. In particular, the discoveries of the positron in 1932, the muon in 1936, both by Carl Anderson (awarded a Nobel Prize in Physics in 1936), and the kaon in 1947 (discovered by George Rochester and Clifford Charles Butler) were made using cloud chambers as detectors. Anderson detected the positron and muon in cosmic rays.

...
Wikipedia