What is a Cloud Chamber?

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Cloud chamber

The Cloud Chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. Ionizing radiation consists of particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, therefore ionizing them. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated (a solution that contains more of the dissolved material) vapor of water or alcohol. When an alpha or beta particle interacts with the mixture, it ionizes it. The resulting ions act as condensation nuclei (are small particles typically 0.2 micrometer), 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. A cloud chamber is a device that makes visible the paths of particles emitted as a result of radioactive decay. One of the cloud chamber's greatest claims to fame is that it was used to discover the positron, the first observed form of antimatter.


What is the History of the Cloud Chamber?

A Scottish physicist, Charles Thomsan Rees Wilson (1869–1959), invented the Cloud Chamber. Inspired by sightings of the ‘Brocken spectre’, in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Brocken spectre or mountain spectre is the apparently enormous and magnified shadow of an observer, cast upon the upper surfaces of clouds opposite the sun. Soon, 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, the air was made to expand inside the chamber by adiabatic (process that occurs without heat transfer) expansion. This cools the air and water vapor starts to condense. 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, an American physicist, received the Nobel Prize for Physics in 1927 for his work on the Cloud Chamber.


What are the components of a Cloud Chamber?

A simple cloud chamber consists of the following parts:

  • sealed environment,
  • radioactive source (if needed),
  • dry ice or a cold plate and
  • any kind of alcohol source (which allows easy evaporation)


How does the Cloud Chamber work?

  • The chamber is saturated by a lightweight methyl alcohol vapor. The alcohol falls as it cools down and the cold condenser offers a steep temperature gradient. The outcome is a supersaturated environment.
  • The alcohol vapor condenses around ion trails left behind by the travelling ionizing particles. The result is cloud formation, seen in the cloud chamber by the presence of droplets falling down to the condenser.
  • As particles pass through the chamber they leave ionization trails and since the alcohol vapor is supersaturated it condenses onto these trails.
  • Just above the cold condenser plate there is an area of the chamber which is sensitive to radioactive tracks. At this height, most of the alcohol has not condensed. This means that the ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation.
  • This sensitive area is increased in height by employing a steep temperature gradient, convection (the transfer of heat through a fluid caused by molecular motion) and very stable conditions.
  • A strong electric field is often used to extract cloud tracks down to the sensitive region of the chamber and increase the sensitivity of the chamber.
  • While tracks from sources can still be seen without a voltage supply, background tracks are very difficult to observe.
  • In addition, the voltage can also serve to prevent large amounts of "rain" from obscuring the sensitive region of the chamber, caused by condensation forming above the sensitive area of the chamber. This means that ion trails left by radioactive particles are hidden by constant precipitation. The black background makes it easier to observe cloud tracks.
  • If a tangential light source which is the light beams striking the surface with a very small angle, is used, then the white droplets against the black background is illuminated. Drops should be viewed from a horizontal position. If the chamber is working correctly, tiny droplets should be seen condensing.


What are some Particle-detection chambers other than Cloud Chamber?

  • The diffusion cloud chamber: This chamber varies 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 or colder than dry ice. Alcohol vapor is also often used due to its different phase transition temperatures. The most common fluid used in them is isopropyl alcohol, though methyl alcohol can be used as well. There are also water-cooled diffusion cloud chambers, using ethylene glycol.
  • The spark chamber: This is an electrical device that uses a grid of un-insulated electric wires in a chamber, with voltages applied between the wires. Microscopic charged particles cause some ionization of the air along the path of the particle, and this ionization causes sparks to fly between the associated wires. The presence and location of these sparks is then registered electrically, and the information is stored for later analysis, such as by a digital computer.
  • The bubble chamber: Bubble chambers can be made physically larger than cloud chambers, and since they are filled with much-denser liquid material (liquid hydrogen, the liquid state of the element hydrogen), they disclose the tracks of much more energetic particles. These factors quickly made the bubble chamber the major particle detector for a number of decades, so that cloud chambers were effectively outdated in fundamental research by the start of the 1960s. This was invented by Donald A. Glaser of the United States in 1952, and for this, he was awarded the Nobel Prize in Physics in 1960.


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