Cloud chamber

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 supersaturated 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, 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 shows more evidence of deflection by collisions, while an electron's is thinner and straight.). When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge. For more detailed track-shape information.

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 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, 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. 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. Developments were made by Patrick Blackett who utilised a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. The cloud chamber was the first radioactivity detector.

Cloud chamber consists

Cloud chamber consists of the parts sealed environment, radioactive source (if you want to), dry ice or a cold plate and some kind of alcohol source (it has to allow easy evaporation)

Lightweight methyl alcohol vapour 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. The alcohol vapour 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 because the alcohol vapour is supersaturated it condenses onto these trails. Since the tracks are emitted radially out from the source, their point of origin can easily be determined.

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, little convection, and very 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. 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 obscured by constant precipitation. The black background makes it easier to observe cloud tracks.

Before tracks can be visible, a tangential light source is needed. This illuminates the white droplets against the black background. Drops should be viewed from a horizontal position. If the chamber is working correctly, tiny droplets should be seen condensing. Often this condensation is not apparent until a shallow pool of alcohol is formed at the condenser plate. The tracks become much more obvious once temperatures and conditions have stabilized in the chamber. This requires the elimination of any significant drift currents (poor chamber sealing)