Cloud Chamber
Definition and meaning of Cloud Chamber in chemistry.
A cloud chamber is a sealed vessel containing a supersaturated vapor that makes the paths of ionizing radiation visible as trails of condensed liquid droplets. Charged particles passing through the vapor ionize gas molecules along their track, and these ions act as nucleation sites for condensation.
In more detail
As an alpha particle, beta particle, or other charged particle travels through the chamber, it strips electrons from vapor molecules, leaving a line of ions in its wake. Because the vapor is held in a supersaturated (metastable) state, it condenses preferentially around these ions rather than uniformly, producing a visible fog trail that traces the particle's trajectory. Supersaturation is maintained either by sudden adiabatic expansion (the original Wilson chamber) or by a continuous temperature gradient over dry ice (the diffusion chamber). This let physicists and chemists photograph and measure individual particle tracks directly, providing early evidence for nuclear reactions and enabling the 1932 discovery of the positron.
Key facts
| Field | Physical Chemistry |
|---|---|
| Inventor | Charles Thomson Rees Wilson, 1911 |
| Working fluid | Supersaturated water or isopropyl alcohol vapor |
| Recognition | Wilson won the 1927 Nobel Prize in Physics for the invention |
In a diffusion cloud chamber cooled with dry ice and filled with isopropyl alcohol vapor, alpha particles from an americium-241 source leave short, thick, straight trails of droplets, while faster beta particles leave thinner, longer, more erratic tracks.
Frequently asked questions
What actually causes the visible trail in a cloud chamber?
Ionizing radiation knocks electrons off vapor molecules, creating a line of ions that serve as condensation nuclei, so the supersaturated vapor condenses into tiny droplets along the particle's exact path.
How does a cloud chamber differ from a bubble chamber?
A cloud chamber uses a supersaturated vapor in which droplets form along ionization trails, whereas a bubble chamber uses a superheated liquid in which vapor bubbles form instead; bubble chambers largely superseded cloud chambers for high-energy particle physics.