Basics of cloud chambers

A tabletop cloud chamber is a simple home project that can be built, and can detect and display condensation trails that develop in the presence of ionizing radiation. It was invented by Charles Wilson, who perfected a working model in 1911. For many decades it was used by researchers and it played an important role in the discovery of elementary particles and atomic ions.

cloud chamber

Wilson’s original cloud chamber was a glass cylinder, about 16 cm across and about 3 cm deep. Its walls were coated in gelatin, with the base dyed black for a photographic dark background. The floor of the chamber was fixed to the top of a brass plunger. It all stood in a shallow trough of water that kept air in the chamber saturated with water. A diaphragm was used to expand the air in the chamber for adiabatic expansion, cooling the air and starting to condense water vapor. This kind of chamber is called a pulsed chamber because the conditions for operation are not continuously maintained.

Around 1960 the cloud chamber was superseded by the more sensitive bubble chamber, which uses liquid hydrogen. Because bubble chambers are filled with much-denser liquid material, they reveal the tracks of much more energetic particles. Finally, the spark chamber came along, where voltage is applied to a grid of electrical wires. The ionized trails cause electrical sparks to jump from wire to wire, forming the display that indicates the presence of ionizing radiation. The location of the sparks is generally stored for later analysis by computer.

A traditional cloud chamber must incorporate a steep temperature gradient. This is created by means of dry ice (frozen CO2) and methyl alcohol inside a sealed container made transparent so the particle trails can be viewed from outside.

The dry ice sits at the bottom of the chamber, and an absorbent pad saturated with methanol is placed at the top. To see the particle trails it is necessary to shut off the lights. The chamber is illuminated from the side–home project cloud chambers can use a strong flashlight. You will see an incredible fast-moving display of particle tracks.

The way a chamber works is when a particle passes through, it accumulates condensation, in essence a visible cloud. What you are seeing, of course, is not the infinitesimal particle, but the large vapor trail that has attached to it.

More specifically, the trail arises because of the many ions produced along the path of the charged particle. These tracks have distinctive shapes. For example, an alpha particle has a broad track and shows more evidence of deflection by collisions. A track made by an electron is thinner and straight. Applying a uniform magnetic field across the cloud chamber curves positively and negatively charged particles in opposite directions, as described by the Lorentz force law with two particles of opposite charge.

In particle and nuclear physics research facilities, particle detectors have evolved considerably from Wilson’s simple cloud chamber. Besides gaseous ionization detectors, there are semiconductor detectors including CCDs and scintillation detectors. At CERN, for the Large Hadron Collider, dedicated detectors have been built for specific projects, most notably in the great quest for the Higgs Boson.

An excellent demonstration including instructions for building a cloud chamber using readily available materials may be seen at https://www.youtube.com/watch?v=xky3f1aSkB8.

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