The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation.
Charles Thomson Rees Wilson (1869–1959), a Scottish physicist, is credited with inventing the cloud chamber. Inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centers for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911. In Wilson's original chamber the air inside the sealed device was saturated with water vapor, then a diaphragm was used to expand the air inside the chamber (adiabatic expansion), cooling the air and starting to condense water vapor. When an ionizing particle passes through the chamber, water vapor condenses on the resulting ions and the trail of the particle is visible in the vapor cloud. Wilson, along with Arthur Compton, received the Nobel Prize in Physics in 1927 for his work on the cloud chamber.This kind of chamber is also called a Pulsed Chamber, because the conditions for operation are not continuously maintained. Further developments were made by Patrick Blackett who utilised a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. A cine film was used to record the images.
The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf.This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation, and in that the bottom must be cooled to a rather low temperature, generally as cold as ?26 °C (?15 °F). Instead of water vapor, alcohol is used because of its lower freezing point. Cloud chambers cooled by dry ice are a common demonstration and hobbyist device; the alcohol used in them is commonly isopropyl alcohol or methylated spirit. There are also water-cooled diffusion cloud chambers, using ethylene glycol.