page_title: Cloud chamber
section_title: Structure and operation
hierarchical_section_title: Cloud chamber / Structure and operation
caption_attribution_description: English: Rutherford scattering of an alpha particle from a Pb210 source in a diffusion cloud chamber.
context_page_description: A cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation.
  A cloud chamber consists of a sealed environment containing a supersaturated vapor of water or alcohol. An energetic charged particle interacts with the gaseous mixture by knocking electrons off gas molecules via electrostatic forces during collisions, resulting in a trail of ionized gas particles. The resulting ions act as condensation centers around which a mist-like trail of small droplets form if the gas mixture is at the point of condensation. These droplets are visible as a "cloud" track that persists for several seconds while the droplets fall through the vapor. These tracks have characteristic shapes. For example, an alpha particle track is thick and straight, while an electron track is wispy and shows more evidence of deflections by collisions.
  Cloud chambers played a prominent role in 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 and the muon in 1936, both by Carl Anderson, used cloud chambers.
context_section_description: Diffusion-type cloud chambers will be discussed here. A simple cloud chamber consists of the sealed environment, a warm top plate and a cold bottom plate (See Fig. 2).  It requires a source of liquid alcohol at the warm side of the chamber where the liquid evaporates, forming a vapor that cools as it falls through the gas and condenses on the cold bottom plate.  Some sort of ionizing radiation is needed.
  Methanol, isopropanol, or other alcohol vapor saturates the chamber. The alcohol falls as it cools down and the cold condenser provides a steep temperature gradient. The result is a supersaturated environment. As energetic charged particles pass through the gas they leave ionization trails. The alcohol vapor condenses around gaseous ion trails left behind by the ionizing particles. This occurs because alcohol and water molecules are polar, resulting in a net attractive force toward a nearby free charge.  The result is a misty cloud-like formation, seen by the presence of droplets falling down to the condenser.  When the tracks are emitted radially outward from a source, their point of origin can easily be determined. (See Fig. 3, for example.)
  Just above the cold condenser plate there is a volume of the chamber which is sensitive to ionization tracks. The ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation. This sensitive volume is increased in height by employing a steep temperature gradient, and stable conditions. A strong electric field is often used to draw cloud tracks down to the sensitive region of the chamber and increase the sensitivity of the chamber. The electric field can also serve to prevent large amounts of background "rain" from obscuring the sensitive region of the chamber, caused by condensation forming above the sensitive volume of the chamber, thereby obscuring tracks by constant precipitation. A black background makes it easier to observe cloud tracks.  Typically, a tangential light source is needed. This illuminates the white droplets against the black background. Often the tracks are not apparent until a shallow pool of alcohol is formed at the condenser plate. 
  If a magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law;  strong-enough fields are difficult to achieve, however, with small hobbyist setups.
caption: Fig. 3: In a diffusion cloud chamber, a 5.3 MeV alpha-particle track from a Pb-210 pin source near Point (1) undergoes Rutherford scattering near Point (2), deflecting by angle theta of about 30 degrees.  It scatters once again near Point (3), and finally comes to rest in the gas.  The target nucleus in the chamber gas could have been a nitrogen, oxygen, carbon, or hydrogen nucleus. It received enough kinetic energy in the elastic collision to cause a short visible recoiling track near Point (2).  (The scale is in centimeters.)
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