A bubble chamber is an early type of particle detector used in a variety of high energy physics experiments in the 1960s and 70s. Particles left visible tracks in the fluids that filled bubble chambers by causing the fluid to boil at the specific points where the particle had passed. The bubbles left in the wake of this spot-boiling 1) helped give the chambers their name, and 2) could be captured on cameras set up outside the bubble chamber, allowing scientists to view the precise trajectories of these unstable, invisible particles that happened to be moving at nearly the speed of light -- at their leisure.

The chambers were typically filled with a liquid such as liquid hydrogen, propane or freon. Heated past its boiling point and held quiescent by a high external pressure, the liquid was rapidly and adiabatically expanded just prior to the entrance of the incoming particles. While electrically neutral particles would pass by undetected, charged particles would ionize the atoms of the liquid, creating nucleation sites for the super-heated liquid to boil. These strings of bubbles became the tracks.

In general, a beam of one specific kind of particle was focused into the bubble chamber by a series of magnetic and electric fields that weeded out particles of different charge or momentum. Applied magnetic fields within the chamber itself supplied scientists with information about the charge and momentum of the particles traversing the chamber: according to the standard Lorentz force rule of

< >< >< >< >< >F = q v x B,

charged particles traced out a curved path as a result of the magnetic field, with the lowest momentum (or smallest v) particles showing the greatest degree of curvature; those with positive charge would curve in one direction, and those with negative charge in the other.

In general, one needed a complicated and expensive projector system with which to view the data from bubble chamber experiments. Initially, the films were analyzed by hand, although physicists were soon able to pass much off the burden off to computers as they became more commonly used.

It would appear that the bulk of old bubble chamber slides are currently enjoying a second life, tormenting and frustrating new generations of undergraduate physics students who are condemned to peruse them for their senior lab courses, because their home institutions can’t afford real particle accelerators. Major drag.

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