Cloud chamber
From Wikipedia, the free encyclopedia
The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water vapour. When an alpha particle or beta particle interacts with the mixture, it ionises it. The resulting ions act as condensation nuclei, 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. These tracks have distinctive shapes (for example an alpha particle's track is broad and straight, while an electron's is thinner and shows more evidence of deflection). When a vertical magnetic field is applied, positively and negatively charged particles will curve in opposite directions. This is evidenced in the photograph above showing the discovery of the positron; an electron that curves the other way. It is possible (and essential) in this case to determine that the positron was actually moving upwards (presumably it had been deflected from below), because the curvature of the track is greater in the lower part of the figure (the photograph was upside down!). For more detailed track shape information see bubble chamber.
Contents |
[edit] Invention
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 centres 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 is used to expand the air inside the chamber (adiabatic 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. A diagram of Wilson's apparatus is given here. Wilson, along with Arthur Compton, received the Nobel Prize for 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.
[edit] Other chambers
The diffusion cloud chamber was later developed in 1936 by Alexander Langsdorf. This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation and that the bottom must be cooled to a rather low temperature, generally as cold as or colder than dry ice. Alcohol vapour is also often used due to its different phase transition temperatures.
The bubble chamber similarly reveals the tracks of subatomic particles, but as trails of bubbles in superheated liquid. Bubble chambers can be made physically larger than cloud chambers. Being filled with much denser material they will reveal the tracks of much more energetic particles. These factors rapidly made the bubble chamber the particle detector of choice so that cloud chambers were effectively superseded in fundamental research by the start of the 1960s.
[edit] Reference
Das Gupta, N. N.; Ghosh S. K. (1946). "A Report on the Wilson Cloud Chamber and its Applications in Physics". Reviews of Modern Physics 18: 225-365.
