Particle Detectors

A cloud chamber utilizes an enclosed volume of clean air saturated with water vapor. If this volume of air is enclosed in a cylinder with a piston and the volume is suddenly expanded, the temperature of the air falls causing the mixture to become supersaturated. If a charged particle passes through the volume at this time the vapor tends to condense on the ions produced, leaving a trail of water droplets on the path of the charged particle. With proper illumination and timing these trails can be photographed. If a magnetic field is applied, the radius of curvature of these tracks can be measured and this information, combined with the density of droplets along the trail can be used to measure the energy of the particle. The cloud chamber was first used by C.T.R. Wilson around the turn of the century and was useful in the early days of nuclear physics but suffered from several disadvantages such as the long time required to recycle and the low density of air. In 1932 Carl D. Anderson discovered the positron, the antiparticle of the electron while using a cloud chamber to observe cosmic rays.

A rather similar device called the bubble chamber was developed using liquids rather than a gas. Liquefied gases such as hydrogen, xenon, and helium have been used. Pressure is applied to the liquid to keep it a liquid above its normal boiling point at atmospheric pressure. If the pressure is suddenly reduced the liquid is superheated but will not boil spontaneously, at least for a short time. In order to boil, the liquid must have small irregularities on which bubbles of vapor form and they can be provided in the bubble chamber by the ions left by charged particles passing through the chamber. Thus tiny bubbles form along the tracks of particles passing through the chamber just after the pressure has been reduced. The bubbles grow very quickly but if the tracks are photographed at just the right time after expansion they are revealed as a thin trail of tiny bubbles. An electron in a magnetic field spirals some 36 times in a cloud chamber at the Lawrence Berkeley Laboratory in California. The track of the electron, which starts at the bottom of the picture, is some 33 ft (10 m) long. Its spiral becomes tighter half way up the picture because the electron loses energy by radiating a photon. The small irregularities in the spacing between the loops are due to scattering when the electron is deflected slightly by collisions with atoms in the chamber's gas. © Lawrence Berkeley Laboratory/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
Bubble chambers work very well with particle accelerators that are pulsed. The expansion of the chamber can be timed so that particles from the accelerator pass through just after the chamber is expanded. As with the cloud chamber, application of a magnetic field permits measurement of the curvature of the tracks and when this information is combined with the density of bubbles along the track the energy, momentum, charge (sign) and mass of the particle can be determined. The bubble chamber was invented in 1953 by the American Physicist Donald Glaser who used a small glass device containing about 30 cubic centimeters of diethyl ether. The use and size of bubble chambers grew during the following decades culminating in the discovery of the omega minus particle in the 80 in (203 cm) bubble chamber at Brookhaven National Laboratory in 1964 and the construction of the 3168 gal (12,000 l) "Gargamelle" chamber at the CERN laboratory in Geneva Switzerland in the early 1970s. In recognition of the great importance of this device to particle physics research, Glaser was awarded the Nobel Prize for physics in 1961.