The Wilson chamber or cloud chamber is a particle detector, which contributed to important particle physics discoveries.
Operation
The Scottish physicist Charles Thomson Rees Wilson developed the cloud chamber between 1895 and 1912. A container is filled with a supersaturated vapor of water or isopropyl alcohol, the latter is currently much more used, due to purity and lower freezing point than dry ice, which is carbon dioxide in the solid state. Supersaturated is a state where vapor pressure is higher than saturation pressure at a determined temperature.
The alcohol becomes a vapor through a heating source, and the dry ice reduces alcohol’s temperature below, creating a temperature gradient. Consequently, some alcohol vapor becomes supersaturated, where subatomic particles become visible. The chamber must be illuminated, to be able to observe trails left by the particles. The dry ice must be separated from the vapor by a blackboard, to improve white trail visualizations. Source: Radiation Dosimetry.
The bottom chamber temperature must be lower than -26ºC (-14.8ºF).
What’s a temperature gradient or thermal gradient? It’s the rate of temperature variation between two points and the distance that separates these two points.
Condensation inside Wilson chamber
In a supersaturated vapor, subatomic particles left condensation trails.
When an electrically charged particle, for example, an alpha (α) particle, enters the Wilson chamber, air molecules (gas atoms) that are on the particle’s trajectory are ionized and, by that, lack electrons, therefore, become electrically charged. As a consequence, ionized molecules attract polar alcohol molecules, becoming a condensation center. Therefore, appears a condensation trail on chamber. Source: AE1S.
Observing particles on Wilson chamber
The Wilson chamber can reveal secondary particles from cosmic rays and particles that come from materials. The most common particles seen in chamber are alpha particles (helium atom nuclei), electrons, protons, and positrons. While, muons, pions, and kaons are rarer.
The alpha particle (α), due to its high mass in comparison to other particles, left a short and thick trail, the source can come from decay of isotope radon-220 or other natural sources. The muon (μ) is a secondary particle that comes from cosmic rays, as the antimuon. The electron (e) can have an erratic movement due to Compton scattering, photoelectric effect, beta decay from an isotope, or muon decay. Electrons with higher energy left straight line trails. Source: symmetry.
