Bubble chamber

While bubble chambers were extensively used in the past, they have now mostly been supplanted by wire chambers, spark chambers, drift chambers, and silicon detectors. Notable bubble chambers include the Big European Bubble Chamber (BEBC) and Gargamelle.

Function and use

The bubble chamber is similar to a cloud chamber, both in application and in basic principle. It is normally made by filling a large cylinder with a liquid heated to just below its boiling point. As particles enter the chamber, a piston suddenly decreases its pressure, and the liquid enters into a superheated, metastable phase. Charged particles create an ionization track, around which the liquid vaporizes, forming microscopic bubbles. Bubble density around a track is proportional to a particle's energy loss.

Bubbles grow in size as the chamber expands, until they are large enough to be seen or photographed. Several cameras are mounted around it, allowing a three-dimensional image of an event to be captured. Bubble chambers with resolutions down to a few micrometers (μm) have been operated.

It is often useful to subject the entire chamber to a constant magnetic field. It acts on charged particles through Lorentz force and causes them to travel in helical paths whose radii are determined by the particles' charge-to-mass ratios and their velocities. Because the magnitude of the charge of all known, charged, long-lived subatomic particles is the same as that of an electron, their radius of curvature must be proportional to their momentum. Thus, by measuring a particle's radius of curvature, its momentum can be determined.

Notable discoveries

Notable discoveries made by bubble chamber include the discovery of weak neutral currents at Gargamelle in 1973, which established the soundness of the electroweak theory and led to the discovery of the W and Z bosons in 1983 (at the UA1 and UA2 experiments). Recently, bubble chambers have been used in research on weakly interacting massive particles (WIMP)s, at SIMPLE, COUPP, PICASSO and more recently, PICO.